Effects of L-tryptophan and D,L-. beta. -(1-naphthyl)alanine on rat hepatic protein metabolism

Earlier studies have reported that the administration of L-tryptophan (TRP) caused increases in polyribosomal aggregation, protein synthesis and cytoplasmic poly(A)mRNA in rat liver. This study was concerned with the effects of a TRP analog, D,L-{beta}-(1-naphthyl)alanine ({beta}NA), in comparison with those of TRP. Both {beta}NA and TRP bound to the TRP receptor protein and increased poly(A)polymerase and nucleoside triphosphatase activities of hepatic nuclei as reported earlier with TRP. However, only TRP, but not {beta}NA, revealed increases in release of labeled nuclear RNA (in vitro), in protein synthesis, in polyribosomal aggregation, and in glycosylation ({sup 14}C-glucosamine incorporation into proteins) of rat liver. These results indicate that although {beta}NA affects hepatic nuclei (binding and enzyme levels), it does not stimulate nucleocytoplasmic translocation of mRNA and concomitantly protein synthesis as occurs with TRP. Thus, one may speculate that the TRP-induced stimulation of glycosylation may play an important role in the overall enhancement of protein synthesis due to TRP.