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A thymidine kinase UL23 gene (EC 2.7.1.145) from an acyclovir-sensitive strain L2 of herpes simplex virus type 1 was cloned and expressed in E. coli. Enzyme was purified by chromatography to a homogeneous state controlled by PAG electrophoresis. The Michaelis constants for the reactions with thymidine and an acyclovir were determined. It was found that enzyme phosphorilate some modified nucleosides such as d2T, d4T, d2C, 3TC, FLT, BVDU, GCV. A comparison of the purified enzyme properties and properties ofthymidine kinase of other strains of herpes simplex virus, previously published was carried out. It is shown that enzyme is inhibited by acyclovir H-phosphonate.

The primary structures of DNA-polymerase (ul30) and thymidine kinase (ul23) genes from several herpes simplex virus type 1 (HSV-1) clinical isolates which differed in sensitivity for a number of antiherpetic drugs were determined and compared with those for two laboratory HSV-1 strains one of which was ACV-sensitive (L2), while the another was resistant (L2) to ACV. The phylogenetic analysis of the sequences showed that conserved regions of ul30 gene of HSV-1 clinical isolates and L2 strain were homologous with the exception of point mutations and degenerated substitutions. Several new mutations in the HSV-1 DNA-polymerase and thymidine kinase functional domains were established and identified as the substitutions associated with the strain-resistance to ACV and other drugs.

An important problem in the development of gene therapy approaches in oncology is the necessity of using promoters providing specific and high level of gene expression in tumor cells. To solve this problem, we used inducible system of gene expression regulation (Tat-TAR-system), which is utilized by human immunodeficiency virus (HIV). tat and tk-HSV genes, as well as a fragment of LTR HIV-1, were cloned in the retrovirus vector, tk-HSV gene was under control of the LTR HIV-1 fragment. Potential capacity of these constructions for transactivating tk-HSV gene transcription was studied. Basal expression level of this gene was defined in transient transfection of HEK293 cells. It was shown that specific transactivation of the tk-HSV gene was controlled by the LTR HIV-1 fragment in lung carcinoma cells Calu-1, permanently transfected by the tat gene construction. The effect of transactivation of tk-HSV transcription in Tat-TAR-system was demonstrated in Calu-1 cells in conditions of control of cancer-specific tat gene over BIRC5 promoter.

Levels were assayed and compared of metabolic enzymes--thymidine and adenosine--in gastric mucosa tissues and blood serum from patients with gastric ulcer disease and gastric cancer (age--30-60 yrs). Enzymatic levels matched to the highest degree in cases of ulcer disease aggravated by chronic atrophic gastritis and precancerous alterations (dysplasia, metaplasia). Elevated concentrations of thymidine kinase and phosphorylase and adenosine desaminase were identified both at foci of ulceration (p > 0.001, p > 0.05, respectively) and in blood serum in all age brackets. Moreover, blood serum assay confirmed the view that changes in serum enzyme levels are suggestive of those in tissue enzymatic profile. Our data established a direct correlation between accumulated effect of level changes, on the one hand, and risk of neoplastic transformation of gastric mucosa in ulcer patients, on the other.

Enzyme capable of catalyzing the phosphorylation of thymidine and uridine was isolated from Y. pseudotuberculosis cells by fractionation with the use of ammonium sulfate, ion exchange and affinity chromatography. The degree of purification of thymidine- and uridine-kinase was approximately 350 times, and at all stages of isolation the activity of both nucleoside-kinases was detected in the same peaks. The purified enzyme was capable of the phosphorylation of thymidine and uridine at temperatures of 8-10 degrees C to 50 degrees C and exhibited the maximum enzymatic activity at pH 8-8.5 and 45 degrees C in the presence of 0.5-1.0 mM MgCl2 and 2 mM ATP. The enzyme was found to have no strict substrate specificity and transferred the phosphate group from ATP to radiolabeled thymidine, uridine and desoxycytidine with different effectiveness, but did not use thymidine-monophosphate as phosphate acceptor.

The delivery of "suicide" herpes simplex virus type-1 thymidine kinase gene (tk) into tumor cells, followed by treatment with synthetic nucleotide analogues (gancyclovir, acyclovir), is a perspective approach to cancer therapy. Serious limitations in employment of the existing means of gene delivery into target cells constitute the main obstacle for cancer gene therapy development. In the present work a possibility to use a nonviral gene delivery system is shown based on the employment of lysine rich peptide K8 and amphipathic peptide JTS-1 for transferring tk gene into human hepatoma HepG2 cells. Cationic peptide K8 forms compact complexes with plasmid DNA, and JTS-1 acts as a pH-dependent endosomal releasing agent. Transfection of HepG2 cells by tk expression vector coupled with K8/JTS-1 peptides, followed by acyclovir administration (50-100 micrograms/ml) for 24 h leads to cell cycle arrest in the G1/S checkpoint of some cells, which eventually die through apoptosis. Treatment of HepG2 cells with higher acyclovir concentration (200 micrograms/ml) additionally results in a nonspecific toxic effect. The above results demonstrate the efficacy of K8/JTS-1 delivery system for the "suicide" cancer gene therapy, and may be regarded as a basis for further elaboration of "suicide" cancer approaches in vivo.

The comparative study revealed thymidine phosphorylase activity in the initial strains of a plague microbe of the field variety and in thymidine-, thymine-dependent and trimethoprim-resistant mutants of the plague microbe of other varieties. The data fully conformed to the results of the microbiological investigation of the strains' ability to grow on the nutrient media with trimethoprim in the presence of thymine and thymidine. On the basis of these results it appeared possible to divide the initial and mutant strains of the plague microbe into four arbitrary groups: initial strains of the plague microbe of all the varieties except the field ones sensitive to trimethoprim under any temperature conditions of incubation on any medium with any supplements; initial strains of the plague microbe of the field variety resistant to trimethoprim at 28 degrees C in the presence of thymine or thymidine alone; Tmpr mutants whose resistance to trimethoprim at 28 degrees C did not depend on the presence of thymine or thymidine, purine and vitamins, but depended on the presence of these substances at a temperature of 37 degrees C.

Activity of replicase complex enzymes involving thymidine kinase (TK), ribonucleotide reductase (RR), DNA-polymerases alpha and beta as well as DNA synthesis and single breaks in DNA were studied during growth of P388 ascites tumor. Under these conditions the rate of DNA synthesis was distinctly decreased via salvage pathway and de novo. Single breaks were not detected in the preexistent DNA within various periods after transplantation of P338 leukemic cells. Retardation of DNA synthesis during tumor growth correlated with a decrease in TK, RR and DNA-polymerase alpha activities, while DNA-polymerase beta activity was markedly increased. Growth of melanoma B16 was accompanied by a decrease in content of ATP, ADP, NAD, phosphocreatine and phosphosaccharides as well as by an increase in the level of inorganic phosphates.

The biochemical mechanisms of resistance to CRC 680578, a new antitumour chloroethylnitrosourea alpha-amino acid derivative, were studied. Alterations in DNA, RNA and protein syntheses, SH-group content, drug efflux, activities of replicative and repair enzymes, such as ribonucleotide reductase, thymidine kinase, O6-alkylguanine-DNA-alkyltransferase and DNA polymerases alpha and beta and damages of the DNA secondary structure were investigated in sensitive and resistant to CRC 680578 leukemia L1210 cells. It was found that the total SH-group number in drug-resistant cells was increased (about 1.3-fold in comparison with sensitive cells) which seems to be due to the mechanisms of drug resistance. CHC 680578 induced less pronounced inhibition and more rapid restoration of DNA and RNA synthesis in resistant cells. No differences between the ribonucleotide reductase and thymidine kinase activities were found either in intact cells of the both strains or after drug administration. The efficiency of repair of DNA chloroethyl adducts by O6-alkylguanine-DNA-alkyltransferase in leukemia cells of various sensitivity was found to be identical. The differences in enzyme activities in intact cells of the both strains were insignificant. It was supposed that factors other than changes in the level of O6-alkylguanine-DNA-alkyltransferase in leukemia cells may be responsible for the resistance to CRC 680578. The increase in the levels of DNA polymerase alpha and, especially, of DNA polymerase beta, in sensitive (but not resistant) mouse leukemia cells 48 hours after drug administration is though to define the mechanism of resistance to the new antitumour agent CHC 680578.

Blood serum activities of thymidine kinase, thymidine phosphorylase, adenosine deaminase, and 5'-nucleotidase were measured in normal women, women suffering from mastopathies and mammary carcinomas, aged 36 to 70. Blood serum activities of the studied enzymes in mammary carcinoma patients differed from these values in healthy women and those suffering from mastopathies; these differences were age-associated. Measurements of the time course of enzymic activities before and in the course of chemotherapy may be employed as a biochemical test to monitor therapy efficacy.

Thymidine kinase was isolated and purified 1600-fold from sea urchin (Strongylocentrotus intermedius) oocytes. The molecular mass of the enzyme is 66 kDa, pI is 5.2. The enzyme activity needs Mg2+, ATP and the corresponding phosphate acceptor. The pH optimum of the enzyme activity is at 9.0-9.5. The isolated enzyme does not exhibit any strict substrate specificity and can phosphorylate thymidine, deoxycytidine and some other pyrimidine nucleosides and their derivatives, the phosphorylation rate being maximal for thymidine, ATP or dATP. The TMP formed via the enzymatic reaction does not influence the thymidine kinase activity.

Activities of thymidine kinase, thymidine phosphorylase, adenosine deaminase and 5'-nucleotidase of AMP were studied in blood serum and lymphocytes of healthy women, patients with mastopathy and with mammary gland cancer of 23-70 years old. Age-dependent alterations in the enzymatic activity were detected in blood serum of healthy women. Activity of thymidine kinase was increased simultaneously with a decrease in thymidine phosphorylase activity in 36-70 years old oncological patients, while adenosine deaminase activity was increased in patients with mastopathy and with mammary gland cancer of all the age groups. Dynamics of the enzymatic activity studied before and during chemotherapeutic treatment may be used as one of biochemical tests for evaluation of the therapy efficiency in oncological patients.

The effects of ovariectomy and estrogen treatment on the activity of thymidine kinase in estrogen-dependent and estrogen-independent mammary gland tumors (MGT) of experimental animals were studied. Thymidine kinase activity decreased 3-18 fold after ovariectomy in estrogen-dependent MGT only. It increased 3-7 fold again after estradiol treatment. These changes were significantly greater than those in hexokinase and glucose-6-phosphate dehydrogenase activity. High doses of estrogens (200 micrograms/day) resulted in an 1.8-3.5-fold decrease in thymidine kinase in estrogen-dependent MGT. The enzyme activity in estrogen-independent MGT did not change. It is suggested to use thymidine kinase activity as a marker of proliferative processes in evaluation of hormone-dependency of tumors and the effects of other factors on tumor growth.

Under the action of lmM of cyclic adenosine-3',5'-monophosphate (cAMP) the survival and the rate of reproduction of the cells of Chinese hamster in the culture decreased in comparison with control to 27 and 42%, respectively. Addition of 0.02 mM thymidine together with cAMP almost completely eliminated the cytostatic action of the latter. Thymidine also prevented the cytostatic effect of noncyclic 5'-AMR, but failed to influence the death of the cells with the action of dibutyril cAMP and theophylline. Thymidine failed to prevent the inhibitory action of cAMP on the mutant strains of mouse cells defective by thymidinekinase. A conclusion was drawn that in the concentrations under study thecytostatic action of the exogenous cAMP on the cells of mammals served as the sequence of splitting to 5'-AMP in the culture medium and was realized by blocking one of the stages of TMP synthesis.

The activities of two deoxythymidine-phosphorylating enzymes--thymidine kinase and nucleoside phosphotransferase--were found in the cytoplasmic fraction of normal and regenerating rat liver. The specific activity of nucleoside phosphotransferase appeared to be by 50% higher than that of thymidine kinase. Nucleoside phosphotransferase has a broad specificity for the phosphate donor. This enzyme is more stable to heating and prolonged dialysis as compared to thymidine kinase. The enzymes respond differently to the addition of d-TTP, d-CTP and sturins A and B: thymidine kinase is strongly inhibited by these agents whereas nucleoside phosphotransferase is insensitive to d-TTP and d-CTP and is only slightly inhibited by sturins. On the other hand the activity of nucleoside phosphotransferase is considerably decreased after addition of ATP. Changes in the activities of both enzymes during 50 hrs following partial hepatectomy were studied. Two activity maxima were observed at 20-22 and 40-46 hrs of regeneration. Using polyacrylamide gel electrophoresis, three isoforms of both enzymes were found. The ratio between the isoenzyme content of the two enzymes from the cytoplasmic fraction of regenerating liver varied as compared to normal.

A study was made of the properties of human lymphoid cell line RPMI-6410 derived from peripheral blood of a patient with acute myeloblastic leukaemia. The lymphoblastoid cell line was found to be resistant to 5-brom-deoxyuridine and to have a low thymidine kinase activity. The modal chromosome number for RPMI-6410 is 46-47 XY. The karyotype includes marker chromosomes: two large submetacentrics --M1 and M2, and two small acrocentrics--M3 and M4. Ways of marker chromosome formation are discussed. The properties of RPMI-6410 line make it possible to use it for somatic cell hybridization, in particular, for obtaining hybridoma synthesizing human monoclonal antibodies.

The influence of phenestrol and XTJI-51 (hexestrol alkylating derivatives) on uterine tissues and their ability to bind to estrogen receptors were studied. The both compounds studied exhibited estrogenic properties and imitated all effects of estradiol: they increased the uterine wet weight and the activity of thymidine kinase and glucose-6-phosphate dehydrogenase. Although their affinity for estrogen receptors was much weaker than the affinity of estradiol, their complexes with estrogen receptors were more stable than estrogen-receptor complexes.

Two mitochondrial fractions, dissimilar in their sedimentation characteristics, were isolated from rat liver homogenates using differential centrifugation. Morphological peculiarities of mitochondria of both these fractions, their capacity to protein and DNA synthesis as well as activities of several key enzymes of DNA biosynthesis were studied. The differences observed are discussed in terms of age dissimilarities of the mitochondria in the population.

A method for the determination of relative values (%) of two pathways of thymidine-5'-phosphate (dTMP) formation, e.g. via de novo biosynthesis and through thymidine reutilization (salvage pathway), is proposed. It is shown that the relative values of dTMP formation through the salvage pathway in the mesometrial part of developing decidua in pregnant rats (9-11th day of ppregnancy) are 1.5-3.4 times higher as compared to those in the antimesometrial part. When dTMP biosynthesis is suppressed by aminopterine, up to 80% of total DNA thymind is synthesized at the expense of thymidine reutilization. The incorporation of 3H-thymidine into DNA was thereby increased approximately 8-fold irrespective of the decrease in the DNA synthesis rate (approximately 2.4 times). The dependence of the relative values of the thymidine reutilization pathway on the correlation of the thymidylate synthetase and thymidine kinase activities in the tissue is discussed. The ability of the cells to reutilize thymidine is interpreted in terms of their relative resistance to the effect of folic acid antagonists.

Isoenzyme composition of thymidine kinase was studied in submitochondrial fractions of liver tissue with various proliferative activity (intact, regenerating livers and Zhaidel ascites hepatoma) using polyacrylamide gel disc electrophoresis. Three zones, corresponding to proteins with Rf 0.1-0.2 (I), Rf 0.5-0.55 (II) and Rf 0.85-0.87 (III) and exhibiting thymidine kinase activity, were found in fractions of cytoplasmic and mitochondrial matrix proteins from resting and proliferating rat liver tissues. In fractions of outer and inner mitochondrial membranes three zones of the enzymatic activity were also observed but two of them did not coincide in the Rf value with the thymidine kinase isoenzymes from cytoplasmic fraction and mitochondrial matrix: I Rf 0.1-0.16, II Rf 0.35-0.4 and III Rf 0.62-0.68. Redistribution of the enzymatic activity between thymidine kinase isozymes occurred in conversion of liver tissue from the resting state to increased proliferation. In these cases slowly migrating enzymatic fraction (Rf 0.1-0.2) was activated in mitochondrial matrix and membranes; formation of TMP, catalyzed by isozymes with fast mobility (Rf 0.5-0.55 in matrix and Rf 0.62-0.68 in membrane fractions of mitochondria), which are typical for intact liver tissue, was decreased, respectively.

Potential sources of errors in 3H-thymidine use in cell proliferation studies are reviewed. Many factors affect the uptake of this agent into newly synthesized DNA: predominance of denovo and salvage pathways of thymidilate biosynthesis, activity of certain enzymes, size of thymidilate and TTP endogenous pools, differences in 3H-thymidine utilization rate during S-period, its catabolism in cell and tissue culture systems, degradation by bacteria, its reutilization in vivo and in vitro, effects of the isotope activity, of the exposition time of autoradiographs and of the beta-particles absorbance in the specimen. Practical recommendations are given to avoid these errors.

Distinct increase in activity of uridine kinase and pronounced decrease in thymidine kinase activity were observed in cell-free extracts of rat liver tissue at early stages of hepatocarcinogenesis caused by administration of N-nitrose diethylamine and 2-acetyl aminofluorene. Exogenous noradrepaline did not affect the altered activity of thymidine kinase but stimulated the effect of N-nitrose diethylamine and lowered the effect of 2-acetyl aminofluorene on the urindine kinase activity. Adrenergic neurotransmitters appear to influence the cell proliferation due to modification of the enzymes participating in the synthesis of nucleic acid precursors.

A single injection of triamcinolone-acetonide evoked multiple changes in adrenectomized rat thymuses, e.g. decrease of DNA polymerase and thymidine kinase activities, incorporation of [3H]thymidine into DNA and fragmentation of chromatin. The decrease of proliferating activity of thymus was preceded by inactivation of cytoplasmic DNA polymerase responsible for DNA replication. Fragmentation of DNA caused by double strand ruptures at the linkage sites affected primarily the non-proliferating thymocytes. Incubation of nuclei isolated from the thymuses of control and hormone-treated animals in Ca2+ and Mg2+-containing media increased the level of low molecular weight compounds in them. It was assumed that the inhibiting action of triamcinolone-acetonide on cytoplasmic DNA polymerase prevents the involvement of proliferating thymocyte subpopulation in a new generation cycle and provides for their terminal differentiation and decay, eventually resulting in fragmentation of DNA.

The activity of thymidine kinase (TK) was studied in series of somatic cell hybrids between the mouse cell line 3T3-4E (TK-) and Chinese hamster cells M-15-1 (HGPRT-). Four groups of hybrid lines with different ratio of parental chromosome sets have been investigated: 1) three lines containing one hamster and one mouse chromosome set (1 hs+1 ms); 2) one line with 2 hs+1 ms; 3) one line containing 3 hs+1 ms and 4) one line containing 1 hs+2 ms. Mixtures of extracts from the parental cells were shown to possess the expected TK activity. The calculation of the activity per cell revealed that the 1 hs+1 ms and 2 hs+1 ms hybrid lines possessed about 50% of the initial hamster cell TK activity. The decreased TK activity in these hybrids might be due either to a loss of hamster chromosomes or to some inhibitory effect of mouse genome in cells with the studied ratio of parental sets. The enzyme activity in the 3 hs+1 ms hybrid was as expected, about three times greater than that of hamster cells.

In animals with experimental transplantable tumors, an effective treatment with antitumor compounds brought about interrelated changes in nucleoside phosphate kinase activity. The decrease in thymidine phosphate kinase activity was as a rule matched by an increase in that of uridine phosphate kinase. Consequently, "thymidine kinase-uridine kinase" shunt responsible for thymidine-uridine interconversion was suggested, which is switched on when the homeostasis of tumor cells is disturbed. In treatment of tumor-bearing animals, the effective dosage of antitumor drugs was considerably decreased due to a combination of the said compounds (inhibiting nucleoside kinases) or with azauridine which inhibits uridine monophosphate synthesis from orotidine-5'-phosphate.

DNA synthesis in the decidual tissue of rats on the 9-10th day of pregnancy has been studied in intact rats and under the effect of chloridin by means of radioautography and biochemical methods. The decidual cells of antimesometral and mesometral regions were shown to differ both by morphological features and intensity of 3H-thymidine utilization and activity of thymidylate synthetase and thymidine kinase. Under the conditions of the block of dihydropholate reductase, differences between the antimesometral and mesometral regions of deciduoma manifest themselves still more markedly. The decidual tissue consists of a heterogenous population of cells which differ by the ratio of different ways of thymidylate synthesis. An estimate is given for the ratio of two ways of thymidine monophosphate synthesis in the antimesometral regions of the decidual tissue.

Factor CPW, isolated from Cl. perfringens/welchii was shown to activate latent thymidine kinase (factor N) from liver tissue of rats beginning from 42 days age. CPW factor activated 3.5-fold also thymidine kinase from rat strumous gland; the enzyme was observed during involution of the tissue. The data suggest that increase in thymidine kinase activity in proliferating tissues is related to activation of latent enzymes.

Fractions of heavy and light mitochondria are isolated from homogenates of homologous rat tissues (intact liver, regenerating liver within 24 hours after hepatectomy and 27 hepatoma) by means of differential centrifugation. It is found that tumour mitochondria have higher heterogeneity and lower buyoant density than mitochondria from normal hepatocytes. The activity of two enzymes of DNA precursors synthesis (ribonucleotide reductase and thymidine kinase) in subcellular fractions is demonstrated to correlate with the tissue growth rate. A single injection of cyclic AMP into hepatectomised rats resulted in the retardation of the regeneration process, and the activity of both enzymes reached its normal level in all the fractions studied after 24 hours after the operation. Thymidine kinase and ribonucleotide reductase are located mainly in the mitochondrial matrix, however, pronounced enzyme activity is observed also in membrane fractions. The activity of the enzymes in the fraction of external mitochondria membranes in rapidly growing tissues is 2--3 times as high as in the same fraction from normal rat liver.

The rapidly growing mouse Gaelstein hepatomas 22 and 22a and rat Zajdela hepatoma are characterized both by a high thymidine kinase activity, increased TTP pool and intense 14C-thymidine incorporation into DNA. This is indicative of an intensive thymidylate biosynthesis via a short, "salvage" pathway. The predominance of this pathway for thymidylate is also characteristic for the spleens of normal animals. On the contrast, in rat and mouse thymus, where the TTP pool was the highest of all normal tissues studied, the thymidine kinase activity and thymidine incorporation into DNA were relatively low. The growth of the three hepatomas under study induces involution of tumour carrier thymus, manifested in a decrease of the TTP pool and the rate of labelled thymidine incorporation into DNA, as well as in a 4-fold decrease of the thymidine kinase activity of rat thymus. In the spleen of mice carrying ascite 22a and solid 22 hepatomas an entirely opposite response to the tumour growth was observed, i. e. in the former case the organ weight and all indices of DNA synthesis were sharply reduced, while in the latter case they were substantially enhanced. In the spleen of Zajdela hepatoma carriers the DNA synthesis is suppressed as can be evidenced from the decrease of labelled thymidine incorporation into DNA and of TTP pool; the weight of organ and the thymidine kinase activity, however, exceed the normal level more than 2-fold.

Biochemical impairments in spleen immunocompetent cells (T- and B-lymphocytes) were revealed in host (C3HA mice) of transplantable and ortoaminoazotoluol-induced hepatomas in the course of their growth. As soon as hepatoma emerged (chemical carcinogenesis), the activity of adenosine deaminase and purine nucleoside phosphorylase in T- and B-lymphocytes were found to be reduced 2-6 and 7-10-fold, respectively in parallel with the impairment of their immune system. These alterations were accompanied by the increase in concentrations of dGTP in T-lymphocytes (5.4-fold) and of dATP in B-lymphocytes (4-fold) as well as with the inhibition of DNA synthesis, predominantly in T-lymphocytes. In both T- and B-lymphocytes, the dCTP pool was decreased. In the spleen, T- and B-lymphocytes of mice carrying transplantable 22 hepatoma 22 by the moment of its maximal growth (5th day), the DNA synthesis was inhibited as revealed by the reduction of (a) thymidine kinase activity, (b) rate of the labeled thymidine incorporation into DNA, and (c) intracellular dTTP and dCTP concentrations. In latter periods (from 8th day up to the moment of death), drastic stimulation of DNA synthesis in spleen T- and B-lymphocytes was observed irrespective of the impairments in the immune function and the decrease of the adenosine deaminase activity. In the course of growth of both transplantable and induced solid hepatomas in host spleen T- lymphocytes, the activity of the CTP-dependent thymidine kinase isoenzyme increased, coinciding in time with the activation of antigen-specific T-suppressors in the same organ.

The activities of enzymes catalyzing the formation of nucleic acid precursors, nucleoside kinases, as well as of those involved in the degradation of nucleic acids, were studied in nuclei of the liver of healthy persons, human hepatomas and the liver of patients with cancer of gastrointestinal tract. The activities of thymydine kinase and uridine kinase in the human hepatoma nuclear sap were found to increase 40- to 50-fold and 120- to 150-fold, respectively, as compared to those in normal human liver. The activities of DNase and RNase in the fraction of chromatin protein of human hepatomas, on the contrary, decreased almost to zero. As to the liver of patients with cancer of gastrointestinal tract, drastic alterations in the activities of nucleoside kinases and nucleases in the direction characteristic of tumors themselves were observed. This phenomenon is regarded as a manifestation of the systemic effects of the tumor on the host.

It has been shown that based on the intensity of labeled 5-fluorouracil incorporation in RNA and activity of uridinekinase and thymidinekinase or alkaline phosphatase activity in tumorous tissue of the stomach one can form the judgement about the sensitivity of gastric carcinomas to chemotherapy with 5-fluorouracil.

The heterokaryons of undifferentiated mouse fibroblasts (L and 3T3-4E/TK-) and various cell elements of the rat peritoneal exudate were obtained under the treatment with inactivated Sendai virus. The reactivation of RNA and DNA synthesis in the nuclei of highly differentiated periotoneal exudate cells and the synthesis of thymidine kinase controlled by the nuclei of peritoneal exudate cells were shown to occur in the heterokaryons. During the process of reactivation, the ring-like nuclei of polymorphonuclear leucocytes acquired the form characteristic of the reactivated nuclei of mononuclears. The morphological changes of heparin-containing granules in the cytoplasm of the heterokaryons of mast cells and undifferentiated fibroblasts suggest the degeneration and breakdown of granules.

The activity of DNA synthesis enzymes was studied in bone marrow of rats aging from 1 month to 2.5 years. The activity of enzymes of thymidine and thymidylic acid phosphorylation in the rat bone marrow is determined to be practically constant during the whole period of studies. Activity of gCMP-desaminase and DNA-polymerase has a tendency to a decrease during the life period of rats from one to six-eight months, being constant in the following periods.

3H-thymidine incorporation into DNA of heavy mitochondria from regenerating rat liver and the change of mitochondrial thymidine kinase and ribonucleotide reductase activities are studied in vivo in regenerating rat liver within 6--48 hours after hepatectomy. Synthesis of mitochondrial DNA and changes in the activity of the enzymes studied are found to be undulate. Thymidine kinase activity maxima coincide with those of 3H-thymidine incorporation. Maximal activity of ribonucleotide reductase pre-exists maxima of mitochondrial DNA synthesis.