In a study of the mechanism of regulation of protein synthesis in the liver cell during starvation, the hepatic concentrations of eighteen amino acids and charging levels of tRNA of four essential amino acids, viz., phenylalanine, isoleucine, tryptophan and leucine, were measured in rats fed or starved for 48 h. The charging levels, measured by the periodate oxidation method, ranged from 79-91% and 77-101% in the fed and starved rats, respectively, but neither charging levels nor amino acid concentrations were lowered in starvation. Hepatic levels of essential amino acids in the starved rats were well above the known Km values of aminoacyl synthetases, indicating thereby that the liver maintained sufficiently high amino acid concentrations during starvation to maintain the capacity to charge tRNA near V. However, the total acceptor capacity as measured for tRNAPhe was reduced during starvation. Since the acceptor capacity per mg tRNA was not changed, the drop in the total acceptor capacity indicated a drop in the total amount of biologically active tRNAPhe. These results suggest that (i) the mechanism of regulation of protein synthesis in the liver cell during starvation is not mediated through a decrease in amino acid concentrations or in charging levels (periodate-protected capacity) of tRNA, and that (ii) the reduced protein synthesis observed during starvation is attributed, at least in part, to a reduced total acceptor capacity of the liver cell sap.
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