[Effect of cyclic-3',5'-AMP on rat liver polynucleotide phosphorylase activity]

Effect of cAMP on the activity of polynucleotide Phosphorylase (PNPase) was studied in polyribosome fraction of rat liver tissue. Intraperitonel administration of cAMP or of theophilline into rats distinctly decreased the PNPase activity in the polyribosome fraction. The cAMP (1-10(-4) M) inhibited the enzymatic activity only by 8% in polyribosome fraction in vitro, as it was estimated by the reaction of phosphorolysis of endogenous RNA and polyA added. Any attempts were proved to be uncucessful to reveal cAMP, ATP-dependent proteinkinase in rat liver, responsible for the decrease in the PNPase activity in the polyribosome fraction. The cAMP inhibited the increase in the PNPase activity, coupled with protein biosynthesis in polyribosomes. Moreover, cAMP caused a decrease in the PNPase activity in reaction of polyA phosphorolysis and did not affect the rate of endogenous RNA phosphorolysis in polyribosome fraction, isolated from postmitochondrial fraction after incubation for 15 min at 30 degrees. The 3',5'-cyclo AMP (2-10(-6)-2-10(-4) M) stimulated incorporation of 14C-leucine into acid-insoluble material, when postmitochondrial fraction was incubated under the same conditions. The data obtained suggest that cAMP either inhibits specifically the PNPase synthesis or represses the coupled with protein biosynthesis formation of active "heavy" type of PNPase from less active "light" type.