Interrelation between salvage of purine nucleotides and protein synthesis in rat heart cells.

The effect of transition from a respiring to a respiration-inhibited state on the rate of protein synthesis was investigated in glycolyzing, cultured rat heart cells. The rate was found to be significantly lower after blocking respiration, and it was further decreased by L-lactate. In contrast, pyruvate or phenazine methosulfate prevented the drop in the rate caused by lack of respiration. The changes in the respiratory state also affected the steady-state concentration of ATP, which varied in the same sense as the rate of protein synthesis. Pyruvate or phenazine methosulfate induced an increment in the concentration of ATP of respiration-inhibited cells. This increment could not be accounted for by more extensive phosphorylation of the available purine nucleotides, but required repletion of the pool by synthesis of purine nucleotides through the salvage pathway. Pyruvate and phenazine methosulfate were found to stimulate incorporation of labeled hypoxanthine into the purine nucleotide fraction in general, and into the nucleotide triphosphates in particular. Under similar incubation conditions an increase in the ATP/ADP ratio was also noted. The stimulatory effect of pyruvate on protein synthesis and on the cellular level of ATP was also observed in respiration-inhibited 3T6 cells and in human fibroblasts, but not in human fibroblasts deficient in the salvage enzyme, hypoxanthine-guanine-phosphoribosyltransferase. Based on the demonstrated influence of L-lactate, pyruvate, and phenazine methosulfate on the salvage synthesis of purine nucleotides [K. Ravid, P. Diamant, and Y. Avi-Dor, (1984) Arch. Biochem. Biophys. 229, 632-639] and on the present findings, the connection between protein synthesis and the salvage activity is discussed.

Regulation of the salvage pathway of purine nucleotide synthesis by the oxidation state of NAD+ in rat heart cells.

The rate of salvage of purine nucleotides from hypoxanthine in glycolyzing, cultured rat heart cells was found to be decreased when respiration was suppressed. Pyruvate or phenazine methosulfate, acting as hydrogen acceptors, reversed the effect of the respiratory block. The inhibition and the reversal could not be attributed to the limitation of energy supply or of 5-phosphoribosyl-1-pyrophosphate. A causal connection was, however, shown to exist between this inhibition and the concomitant shift in the redox state of NAD+ in favor of NADH. NADH also inhibited the key enzyme of the salvage pathway, hypoxanthine-guanine-phosphoribosyltransferase, in cell-free extracts. Regulation of purine nucleotide synthesis by the redox state of NAD+ in heart cells might gain significance during transition from respiring to hypoxic state and vice versa.