Phenylalanine and tyrosine metabolism in renal failure: dipeptides as tyrosine source.

Several lines of evidence suggest that tyrosine formation is impaired in renal failure. The concentration of tyrosine is decreased and the phenylalanine/tyrosine ratio is increased in plasma and in skeletal muscle cells. After an oral or intravenous load, the rise of plasma phenylalanine is augmented, the clearance is decreased, oxidation is diminished and the corresponding rise of plasma tyrosine level is blunted. Tyrosine elimination and oxidation are not altered in uremia. The defect in tyrosine formation may be especially important in uremic patients on a low protein diet supplemented with tyrosine-free essential amino acid preparations and in subjects on artificial nutritional support. Thus, tyrosine should be regarded as a conditionally essential amino acid in renal failure and should be supplied exogenously, at least in these patient groups. Oral tyrosine supplementation was shown to replete plasma and intracellular pools and improve nitrogen balance in chronic renal failure patients on a low protein diet. However, because of poor solubility in aqueous solutions, tyrosine cannot be included in the free form in amino acid solutions for parenteral nutrition. To circumvent stability or solubility problems, tyrosine containing dipeptides and/or N-acetyl-tyrosine may serve as tyrosine sources for parenteral supply. Renal failure does not affect alanyl-tyrosine hydrolysis, and there is an immediate increase of plasma tyrosine concentration after peptide infusion. Elimination and hydrolysis of glycine-tyrosine is retarded in renal failure, but the clearance exceeds clinically relevant infusion rates. After infusion of N-acetyl-tyrosine, no increase in plasma tyrosine is seen, and the half-life N-acetyl-tyrosine is grossly prolonged in uremia.(ABSTRACT TRUNCATED AT 250 WORDS)