Stimulation and inhibition of the protein synthetic process by NAD+ in lysed rabbit reticulocytes

NAD+ at 0.16 mM stimulates the initiation step of the protein synthetic process in lysed rabbit reticulocytes. This conclusion is based on the stimulation of (i) the transfer of formylmethionine from f[35S]Met-tRNAfMet into polypeptide, (ii) the accumulation of the initial dipeptide, methionylvaline, in the presence of pactamycin, and (iii) the formation of the 40 S initiation complex. The effect of NAD+ changes from a stimulatory role on protein synthesis to an inhibitory role at concentrations greater than 0.16 mM. At 4.0 mM NAD+, protein synthesis is inhibited. This has been demonstrated experimentally by using the same three assays described above. In addition, 4.0 mM NAD+ inhibits MettRNAfMet.initiation factor.GTP ternary complex formation. The elongation and termination steps of polypeptide synthesis are not affected by 0.16 to 4.0 mM NAD+. The data presented clearly show that the stimulatory activity of 0.16 mM NAD+ and the inhibitory activity of 4.0 mM NAD+ affects the initiation step of the protein synthetic process in lysed rabbit reticulocytes.