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The circannual behaviour of glycosylation and protein synthesis in frog liver slices was studied following the incorporation of3H-galactose and14C-glucosamine into glycolipids and glycoproteins and3H-leucine into proteins. The activity of two enzymes the galactosyl-transferase and the N-acetyl-glucosaminyl-1-P-transferase was determined. The incorporations of both sugars into the soluble fraction and into the lipid extract present a maximum during the spring-summer period. The incorporation into the protein fraction displays a different pattern:14C-Glucosamine and3H-leucine incorporation increases from winter to a maximum in autumn; the incorporation of3H-Galactose has a sharp peak during spring. The pattern of glycosyltransferase activities is similar to the pattern of incorporation of the two saccharides into proteins, indicating these enzymes as important control points for glycosylation in Anurae.

Amino acid uptake mediated by system A was studied in cultured fetal and adult hepatocytes, subjected to growth stimulation by EGF and insulin, or to growth inhibition by high cell density. The mitogenic stimulation induced a strong transport increase only in fetal cells, while the cell density-dependent growth inhibition, probably mediated by molecules present on adult hepatocyte membranes, provoked the decrease of amino acid uptake only in the adult cells. The results indicate that the different modulation of amino acid transport by cell growth is dependent on the age and the differentiation stage of hepatocytes.

Acyl CoA: cholesterol acyl transferase (ACAT) activity presents marked oscillations and differential sensitivity to the “in vitro” stimulation of the kinase-phosphatase modulatory system in the perinatal rat liver. The regulation of this enzyme activity by some modulators generally active in adulthood, such as cholesterol, lipoproteins and mevalonate, has been studied in hepatocytes isolated at different developmental stages. A lack of effect of mevalonate and a positive effort of lipoprotein cholesterol have been observed at the fetal and neonatal stages. A differential prevalence is suggested of one of the two modulatory mechanisms (phosphorylation-dephosphorylation system, or substrate effect) at each developmental stage.

Acyl coenzyme A:cholesterol acyltransferase (ACAT), the enzyme catalyzing the hepatic cholesterol esterification could be involved in the modified availability of cholesterol detectable in proliferating systems. While no significant variations are detectable in the regenerating liver, the modified ACAT activity during liver development and its differential sensitivity to the in vitro stimulation of modulatory systems suggest an involvement of the enzyme in this proliferating process.

The regenerating liver presents a changed ability to use mevalonate 16 hr after partial hepatectomy. The dolichol content and its synthesis from mevalonate is increased, while no variation of dolichyl-P and ubiquinone parameters are detectable. The greater amount ofmevalonate utilized to form dolichol, but not dolichyl-P, in this proliferating system, raises some questions about the physiological significance of these isoprenoid compounds and about their biosynthetic sequence.

The binding and uptake of cholesterol enriched lipoproteins by isolated hepatocytes was decreased at 16 hours after partial hepatectomy, with a tendency to return to control values as the regeneration proceeds. The number of lipoprotein binding sites of total cellular membranes remained similar to control at 16 and 24 hours. The plasma lipoprotein pattern, determined by electrophoretic analysis, showed a lower per cent of very low density lipoproteins (VLDL) and a higher per cent of low density lipoproteins (LDL) at 16 and 24 hours post-partial hepatectomy. At these times, plasma lecithin: cholesterol acyltransferase (LCAT) activity was decreased. It is intriguing to suggest that the regenerating liver could regulated the blood lipoprotein pattern and the uptake of lipoproteins by modulating the surface expression of the receptors.