Effect of tyrosine intake on the rate of phenylalanine hydroxylation in adult males

This study evaluated the effect of varying levels of tyrosine intake on the estimation of phenylalanine hydroxylation. Healthy men were fed 1 g protei n kg(-1) . d(-1) for a 2-day period. On the third day, subjects consumed a formula diet containing 1 g protein kg(-1) . d(-1) hourly over 10 hours, an d primed hourly oral doses of L-[N-15]phenylalanine and L-[3,3-H-2(2)]tyros ine for the last 6 hours. Each subject was studied at 7 levels of tyrosine intake (3.0, 4.5, 6.0, 7.5, 9.0, 10.5, and 12.0 mg . kg(-1) . d(-1)) at a c onstant intake of phenylalanine (9 mg . kg(-1) . d(-1), 4.55 mu mol . kg(-1 ) . h(-1)). Phenylalanine hydroxylation was estimated from the ratio of pla sma amino acid isotope enrichment of [N-15]phenylalanine and [N-15]tyrosine and the tyrosine flux estimated from [H-2(2)]tyrosine enrichment. Phenylal anine and tyrosine fluxes showed no significant response to alterations In the intake of tyrosine. Linear regression analysis showed a significant res ponse such that the rate of phenylalanine hydroxylation decreased as tyrosi ne intake increased (R-2 = .21; P = .003). The mean rates of phenylalanine hydroxylation were 3.89 to 8.06 mu mol . kg(-1) . h(-1). Given model uncert ainties, the apparent protein breakdown observed at tyrosine intake levels less than 10.5 mg . kg(-1) . d(-1), and the significant differences observe d between the present data and our prior data, we cannot estimate the tyros ine requirement with any degree of certainty with the present hydroxylation results. Copyright (C) 2000 by W.B. Saunders Company.