PMID: 6981644Oct 10, 1982Paper

Regulation of phenylalanine hydroxylase activity by phenylalanine in vivo, in vitro, and in perfused rat liver.

The Journal of Biological Chemistry
R ShimanD W Gray

Abstract

We show that phenylalanine is able to control the extent of activation and, as a result, the catalytic activity of rat liver phenylalanine hydroxylase in vivo, in perfused liver, and in vitro. Both phosphorylated and unphosphorylated enzyme activities are controlled by phenylalanine activation and, overall, this mechanism appears to be a major means of regulating the enzyme's activity in rat liver. At normal phenylalanine levels in vivo, phenylalanine hydroxylase is at most 1-4% activated, and phosphorylated enzyme (glucagon-induced) appears at most 5-7% activated under similar conditions. In both cases, a phenylalanine load increased the percentage of activated enzyme found in vivo to about 40% of maximal. In perfused rat livers, a plasma phenylalanine concentration of only 4 times normal induced a 4-fold increase in the amount of activated enzyme present and a corresponding functional increase in the rate of phenylalanine hydroxylation by the tissue. Under the latter conditions, more than 25% of the amino acid could be hydroxylated in a single pass through the organ. Purified phosphorylated phenylalanine hydroxylase must be activated to be catalytically active. The activation with phenylalanine, at equilibrium, is a cooperati...Continue Reading

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