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Apoptotic endonuclease is a key enzyme that mediates regulated DNA fragmentation and chromatin condensation in response to apoptotic signals such as the Fas ligand, ionizing radiation, and anticancer agents. An endonuclease that is activated specifically by caspase-3 has been identified in humans and mice. The human gene for this protein has been termed DFF40 (DNA fragmentation factor, 40-kd subunit) or caspase-activated nuclease (CPAN), whereas the mouse homologue has been named caspase-activated deoxyribonuclease (CAD). Although CAD/DFF40 is known as a major apoptotic nuclease, mice lacking inhibitor of CAD (ICAD) (also known as DFF45) are viable and still show DNA fragmentation, suggesting that alternative endonucleases play an important role during apoptosis. Endonuclease G has been reported to possibly be responsible for DNA fragmentation in various cells during apoptosis. Furthermore, we also have found that apurinic/apyrimidinic endonuclease 1 (Ape1) and its N-terminal-truncated form (AN34) are involved in DNA fragmentation during apoptosis in leukemia cells. In this review, we describe the features of several endonucleases that are involved in the apoptosis of human leukemia cells. Apoptotic endonuclease may vary among different leukemia cell types. [ABSTRACT FROM AUTHOR]

Apurinic/apyrimidinic (AP) endonuclease (Ape1) is the major cellular enzyme responsible for repairing AP-sites in DNA. It can cleave the DNA phosphodiester backbone immediately 5′ to an AP-site. Ape1 also shows 3′-phosphodiesterase activity, a 3′-phosphatase activity, and an RNaseH activity. However, regarding its exonuclease activity, it remains controversial whether human Ape1 may possess a 3′–5′ exonuclease activity. During the course of study to search for the major nuclease activity to double-stranded DNA in human leukemia cells, we purified a 37 kDa Mg2+-dependent exonuclease from cytosolic fraction of human leukemia U937 cells. Surprisingly, this exonuclease is Ape1. We demonstrated for the first time that Ape1 possesses a significant activity as major 3′–5′ exonuclease in human leukemia cells. In addition, we also observed that translocation of cytoplasmic Ape1 into nucleus occurs during DNA damage. [Copyright &y& Elsevier]

Human hypoxanthine/guanine phosphoribosyltransferase (EC 2.4.2.8) was purified from red blood cells by the following two methods. Method A includes (a) elimination of hemoglobin by DEAE-cellulose, (b) DEAE-Sephadex chromatography, (c) specific elution of the enzyme from CM-Sephadex by pyrophosphate and (d) Sephadex G-100 gel filtration. Method B includes (a) elimination of hemoglobin by DEAE-cellulose, (b) acid treatment at pH 4.5, (c) ammonium sulfate fractionation. (d) DEAE-Sephadex chromatography, (e) heat treatment at 85 °C and (f) Sephadex G-100 gel filtration. Homogeneous enzyme preparation was obtained by the two methods with 8000–9000-fold purification. The sedimentation coefficient (S20, w) was 5.5–5.6 S, and the molecular weight of the enzyme was estimated at about 85000 by the sedimentation equilibrium method. The subunit molecular weight of the untreated protein and S-carboxymethylmaleyl protein was estimated as 41000–45000 by the sedimentation equilibrium method in the presence of guanidine hydrochloride. However, the subunit size estimated by the sodium dodecylsulfate gel electrophoresis was only 26000. Amino acid composition of the enzyme was determined. Glucosamine, sialic acid and hexose were not detected in the enzyme. [ABSTRACT FROM AUTHOR]