METABOLIC INTERRELATIONSHIPS OF PHENYLALANINE AND TYROSINE IN CHICKS AND RATS FED CRYSTALLINE AMINO ACID DIETS
Abstract
Studies were conducted to investigate the effects of a phenylalanine-tyrosine deficiency on serum thyroid hormone concentrations, thyroid gland morphology and growth in chicks from 0 to 14 days of age. Birds fed diets severely deficient in aromatic amino acids (AAA), lysine or valine were found in most trials to have reduced serum concentrations of either 3,5,3'-triiodothyronine (T(,3)) or 3,5,3',5'-tetraiodothyronine (T(,4)). Approximately one-half of the samples assayed did not show detectable levels of 3,3',5'-triiodothyronine (reverse T(,3)). The reduction in serum thyroid hormone levels was not observed until chick growth was markedly affected and was not specific for AAA deficiency. Both mean follicle diameter and epithelial cell height were significantly reduced in the thyroids of birds fed either a lysine-deficient or a phenylalanine-deficient diet as compared with controls. Several possible mechanisms of how amino acid deficiency alters thyroid function are discussed. Phenylalanine alone could not efficiently supply tyrosine when dietary AAA were suboptimal as evidenced by significantly greater weight gains in chicks fed 50:50 or 55:45 (mole:mole) mixtures of phenylalanine and tyrosine as compared with birds fed the same total amount (moles) of AAA supplied solely by phenylalanine. However, at or above requirement levels, phenylalanine could completely meet the chick's demand for tyrosine. Serum tyrosine concentrations generally reflected the dietary levels of AAA. Three subsequent experiments were conducted to investigate the effects of various levels of dietary phenylalanine and tyrosine on weight gain, feed conversion, serum phenylalanine and tyrosine, and hepatic phenylalanine hydroxylase and phenylalanine-pyruvate transaminase activities in Sprague-Dawley rats. In general, the results indicated that these animals were not able to supply tyrosine from dietary phenylalanine with 100% efficiency when fed in diets devoid of tyrosine. At higher levels of phenylalanine (0.8% and above), this phenomenon might be explained by substrate inhibition of phenylalanine hydroxylase. Serum tyrosine appeared to be a better indicator of the amount of dietary phenylalanine and tyrosine consumed than was serum phenylalanine. Additional studies were conducted to examine the physiological effects of feeding chicks diets containing toxic levels of phenylalanine. Marked reductions in weight gain and feed conversion concomitant with elevated levels of circulating phenylalanine and tyrosine were observed in birds fed a diet containing 2.52% L-phenylalanine and 0.47% L-tyrosine. Supraoptimal amounts of glycine, arginine and tryptophan were fed in an attempt to alleviate the toxic symptoms observed in birds fed high phenylalanine diets. Only tryptophan supplementation proved successful in partially alleviating the growth-depressive effects of phenylalanine toxicity. Excessive dietary phenylalanine caused elevations in serum and brain levels of phenylalanine and tyrosine, while cerebral concentrations of free isoleucine, leucine and valine were generally decreased. Tryptophan supplementation increased both serum and brain tryptophan levels. Results of two radiotracer studies with ('14)C-tryptophan suggested that brain uptake of tryptophan was inhibited by hyperphenylalaninemia.
Degree
Ph.D.
Subject Area
Nutrition
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.