Seethala Ramakrishna

The relationship between levels and rates of synthesis of polyamines during mammalian cell cycle

Abstract The objective of this study was to examine the rate of synthesis and the intracellular levels of polyamines as a function of the HeLa cell cycle. The intracellular levels of ornithine, which were high during mitosis and early G1 phase, decreased rapidly during late G1 phase when the ornithine decarboxylase activity was at its peak. The activities of ornithine decarboxylase and S-adenosyl methionine decarboxylase reached a peak during G1 and decreased rapidly during the S phase. The levels of polyamines were maximum in mitosis and S phase. In constrast, the rate of polyamine synthesis during S phase was 5–10 fold lower than that in mitosis or G1 phase. We have also observed fluctuations in diamine-oxidase activity during the cell cycle. The enzyme activity was high during mitosis and late G1 and low during S phase. Thus, the results of this study suggest an important role for the catabolic enzymes in the regulation of polyamine levels during the mammalian cell cycle.

Phosphorylation of ATP‐citrate lyase by a cyclic AMP‐independent protein kinase from a rat liver

Insulin action in isolated adipocytes or hepatocytes incubated with 32Pi rapidly increases the amount of 32P radioactivity associated with a cytosolic phosphoprotein [l-4]. We isolated and purified this phosphoprotein from adipose and liver tissues and identified it as ATP-citrate lyase [5]. Concurrently, Alexander et al. [6] also found that in liver this phosphoprotein is ATP-citrate lyase. When ATP-citrate lyase is incubated with ATP, 2 mol phosphate/m01 enzyme are incorporated into histidine residues at the catalytic site (referred to as catalytic phosphate) which are acid labile [S ,7]. In addition, at least 2 mol acid-stable phosphate (referred to as structural phosphate), principally phosphoserine, have been found with each 1 mol enzyme [8]. Because structural site phosphorylation was affected by hormone action [ 1,2,5], we looked for a cyclic AMP-independent protein kinase in an insulin-sensitive tissue with specificity for phosphorylating the ATP-citrate lyase structural phosphate sites.

Insulin stimulates phosphorylation of a heat-stable protein in rat adipose tissue.

Insulin in rat adipose tissue acts to increase the phosphorylation about 2.5-fold of a low molecular weight protein in the cytosol designated phosphoprotein m. Isoproterenol had no effect on the phosphorylation of phosphoprotein m. Some of the properties of phosphoprotein m are: soluble in 1% trichloro acetic acid, heat-stable and has a molecular weight of 23,000 on polyacrylamide gels in the presence of sodium dodecyl sulfate. Phosphoserine and phosphothreonine are the phosphorylated amino acid residues of phosphoprotein m. The physical and chemical properties of phosphoprotein m are similar to those of previously described inhibitor and modulator proteins.