Effect of Dietary Tryptophan on Hepatic Polyribosomes and Protein Synthesis in Fasted Mice

Abstract

Abstract Mice were fasted overnight, and the following morning the the animals were tube-fed (a) a complete amino acid mixture, (b) the same mixture devoid of one essential amino acid (tryptophan, threonine, isoleucine, or methionine), (c) a single essential amino acid (l-tryptophan, l-threonine, l-isoleucine, or l-methionine), or (d) nothing. All animals were killed 1 hour after the feeding. Postmitochondrial supernatants of liver homogenates were prepared and used for determination of protein synthesis in vitro, or, after addition of deoxycholate, for direct sucrose density gradient analyses. The polyribosome patterns of the livers of all the groups fell generally into one of two basic patterns: A, a shift of polyribosomes toward lighter aggregates, particularly monosomes, and a concomitant decrease of heavier aggregates, or B, a shift toward heavier aggregates with fewer monosomes. All groups fed l-tryptophan alone or amino acid mixtures which contained tryptophan, whether complete or incomplete with respect to other amino acids, showed the B polyribosome pattern, while the fasted controls, the tryptophan-devoid group, or the other single amino acid groups had the A pattern. The A pattern was associated with a low level of incorporation of 14C-leucine or 14C-phenylalanine into protein in vitro, while mice with the B pattern had a considerably enhanced level of incorporation. Four milligrams of l-tryptophan, but not equimolar amounts of single compounds structurally related to tryptophan, induced an increase in stimulation of hepatic protein synthesis, which changed little with higher levels of tryptophan. l-Tryptophan rapidly reached the liver and induced enhanced hepatic protein synthesis within 5 to 10 min, reached its peak effect by 1 hour, then began to decrease within 2 hours, and returned to control fasted levels within 4 to 6 hours. Animals fasted for longer periods (40 and 64 hours) rather than overnight (16 hours), adrenalectomized mice, and mice pretreated with actinomycin D, when tubefed l-tryptophan, showed enhanced hepatic protein synthesis as well as the B polyribosome profile response. Incubation in vitro with microsomal and supernatant fractions revealed results similar to those found with postmitochondrial supernatants and indicated that the microsomes of the tryptophan-fed animals accounted for the main increase in hepatic protein synthesis. Administration of 14C-leucine intraperitoneally to tryptophan-fed mice revealed a small but significant increase in hepatic protein synthesis in vivo over that in control fasted mice. The results indicate that dietary tryptophan plays a special role in the regulation of polyribosome aggregation and protein synthesis in the liver.