Abstract
Summary
The daily rhythm of plasma tyrosine that occurs in rats fed ad libitum was altered by controlling the time of food intake. The experimental data suggested that (1) absorption of tyrosine from dietary protein and (2) metabolism of tyrosine by liver tyrosine transaminase may be important mechanisms in the daily rhythm of tyrosine. A normal daily rhythm in serum tyrosine occurred in human phenylketonuric subjects lacking phenylalanine hydroxylase, so variations in the hydroxylation of phenylalanine to tyrosine probably do not play a part in the tyrosine rhythm. In rats fed ad libitum, there was little or no daily variation in plasma phenylalanine, but some changes were produced by restricting the period of food intake. Control human subjects exhibited a phenylalanine rhythm identical to that for tyrosine in serum.
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