A DEFECT IN THE METABOLISM OF TYROSINE AND PHENYLALANINE IN PREMATURE INFANTS. III. DEMONSTRATION OF THE IRREVERSIBLE CONVERSION OF PHENYLALANINE TO TYROSINE IN THE HUMAN ORGANISM

Abstract

Ten healthy <$ infants, 8 premature and 2 full-term, receiving a high protein, vit. C-free diet, were given pure l-tyrosine or d,l-phenylalanine in dosages ranging from 0.5 to 2.0 g./kg./24 hrs. Quantitative uri-nalyses for these amino acids and their derivatives revealed a striking difference, depending on which amino acid was fed. When d,l-phenylalanine was given, the infants excreted large amts. of tyrosine derivatives (p-hydroxy-phenylpyruvic acid and p-hydroxyphenyllactic acid) and tyrosine itself, sometimes in crystalline form, as well as derivatives of phenylalanine. However, when 1-tyrosine was ingested, this compound and its keto and hydroxy derivatives appeared in abundant amts. but there was no significant elevation in phenylalanine or phenylpyruvic acid excretion. In either case, about 40-80% of ingested amino acid was accounted for. These results were interpreted as showing conversion of phenylalanine to tyrosine in the human organism but no reverse conversion. No appreciable amt. of ingested phenylalanine was lost in the feces whereas 18-22% of ingested tyrosine was excreted by this route. No homogentisic acid was excreted in the urine even after large repeated doses of amino acids. Vit. C (l-ascorbic acid), previously shown to diminish or abolish the excretion of aromatic metabolites after single dosage of either amino acid, was, except in 1 infant, ineffective when jointly given with repeated and large dosage of either tyrosine or phenylalanine. The usual vit. effect reappeared promptly on cessation of amino acid ingestion, as evidenced by rapid disappearance of the derivatives from the urine.