Dephosphorylation of Pyrimidine Nucleotides in the Soluble Fraction of Homogenates from Normal and Regenerating Rat Liver

Abstract

The dephosphorylation of the pyrimidine nucleotides UMP, CMP, dUMP, dCMP, and dTMP was determined in the soluble fraction of homogenates from normal rat liver and from regenerating liver during an 8‐day period after partial hepatectomy. The dephosphorylations were assayed at optimal pH in the presence of optimal concentrations of Mg++ ions and substrate. β‐Glycerophosphate was included among the substrates for measurement of non‐specific phosphatase activity. The intracellular origin of the enzymes was investigated by determining the rate of their appearance in the soluble fraction during homogenization and by estimating the rate at which the lysosomes were disrupted.The results showed that the dephosphorylation of the deoxyribonucleotides was mainly due to non‐specific acid phosphatase activity. This enzyme and the alkaline phosphatase activity were found to be localized in the soluble space of the liver cell. The dephosphorylation of the ribonucleotides was due largely to a specific 5′‐nucleotidase. The enzyme was adsorbed or loosely bound to some sedimentable cell constituent at the time of cell disruption, but it was quantitatively released into the soluble fraction during 2 minutes homogenization.During liver regeneration the acid and alkaline phosphatase activities showed slight variations which could not be related to the growth rate of the liver. In contrast, the 5′‐nucleotidase activity showed distinct, cyclic variations which were inversely related to the growth rate. The variations in the 5′‐nucleotidase activity were strikingly similar to the activity variations of uracil reductase and N‐carbamoyl‐β‐alanine amidohydrolase observed previously in regenerating liver.