Phenylalanine Hydroxylase Activity in Liver Biopsies from Hyperphenylalaninemia Heterozygotes: Deviation from Proportionality with Gene Dosage

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Abstract
Extract: Liver biopsy samples from the patients with hyperphenylalaninemia have an average of 5% of the normal hydroxylase activity. The parents of the patients have between 7.3% (excluding the value for one parent) and 10% of the normal hepatic hydroxylase activity. An explanation for these findings involves negative interallelic complementation, which involves protein-protein interaction between subunits in a multitneric enzyme. In support of this model is the evidence that rat liver phenylalanine hydroxylase is a multimeric protein composed of two electrophoretically distinguishable subunits. Speculation: The finding that parents of patients with hyperphenylalaninemia have an average of 10% of the normal level of hepatic phenylalanine hydroxylase, a multimeric enzyme, can be explained on the assumption that the liver tissue of heterozygotes has an excess of enzyme molecules that contain at least one mutant subunit.

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