The implications of multiple forms of phenylalanine hydroxylase in phenylketonuria and related diseases of phenylalanine metabolism

Abstract

Phenylalanine hydroxylase prepared from rat and human liver occurs in three unique isozymal forms. The enzyme was separated into three fractions on neutral calcium phosphate gel columns. Authenticity of the enzyme activities was confirmed by substrate specificity, Michaelis constants for substrate and cofactor, and pH optima. Differentiation of three forms as suggested by column chromatography was confirmed by rechromatography, gel filtration, density gradient centrifugation, and dissimilar responses to temperature, para-chlorophenylalanine, and pretreatment with lecithin. We have demonstrated that the isozymes mature at different times during intrauterine and extrauterine development and that the enzyme is not mature at birth in the rat. We offer various proposals of the significance of isozymes of this enzyme, particularly with regard to the various states of hyperphenylalaninemia and particularly phenylketonuria.