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Protein Synthesis in Liver, Muscle, and Brain of Rats Fed a High Tyrosine-Low Protein Diet¹

Departments of Biochemistry and Nutritional Sciences, University of Wisconsin-Madison, Madison, Wisconsin 53706

Effects of feeding rats a high tyrosine-low protein diet on protein synthesis in liver, muscle, and brain were investigated both in vitro and in vivo. Tissue tyrosine concentrations of rats consuming the high tyrosine diet for 7 days were substantially elevated; this was accompanied by severe growth retardation. In the livers of rats fed the high tyrosine diet for 6 days, polysomal profiles showed a shift toward heavier ribosomal aggregates, while in muscle and brain, extensive disaggregation of polysomes occurred. In an in vitro amino acid incorporating system, the activities of both the microsomal and pH 5 enzyme fractions isolated from the livers of the high tyrosine animals that had been fed the diet for 6 days were elevated. On the other hand, the capacity of muscle or brain ribosomal preparations to incorporate [¹⁴C]leucine was much reduced. Similar results were obtained in a study of [¹⁴C]leucine incorporation in vivo in which rats were force-fed two meals and killed at various times after the last feeding. In rats fed the high tyrosine diet, incorporation of leucine into liver increased progressively; this was accompanied by a gradual decrease in leucine incorporation into muscle. In contrast, leucine incorporation into brain was immediately suppressed. In view of the apparently paradoxical effect of a high tyrosine load on protein synthesis in the liver, rates of the anabolic and catabolic phases of protein turnover in animals fed a high tyrosine diet were determined from radioactivity measurements made after pulse labeling them with [¹⁴C]bicarbonate. Results indicated that the rates of both synthesis and degradation of liver proteins were elevated over control values. Differences in the effects of a toxic load of tyrosine on protein synthesis in the tissues examined could be the consequence of altered metabolic or hormonal balance as a result of nutritional stress.

KEY WORDS: • tyrosine feeding • protein turnover • tyrosine toxicity

¹ This research was supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison, Wisconsin, and by United States Public Health Service Grant AM 10748 from the National Institute of Arthritis, Metabolism and Digestive Diseases.

² Present address: Department of Pharmacology, Upstate Medical Center, SUNY, Syracuse, N.Y. 13210.

³ To whom reprint requests should be addressed.

Manuscript received 14 November 1974.