Specificity of amino acids as activators and substrates for phenylalanine hydroxylase

Rat liver phenylalanine hydroxylase can be markedly activated by a variety of different procedures including those that lead to covalent modification of the enzyme, such as limited proteolysis by alpha-chymotrypsin and alkylation of sulfhydryl groups by N-ethylmaleimide, and those that lead to reversible changes, such as the interaction of the enzyme with lysolecithin and related compounds. These treatments not only lead to increased activity toward the normal substrate for the enzyme, phenylalanine, but they lead to even greater increases in activity toward normally poor substrates such as m-tyrosine. Activations of this type, therefore, in effect, appear to broaden the amino acid specificity of the enzyme. We have now found that the extent of the change in substrate specificity on activation of the enzyme is much greater than had been heretofore realized. Indeed, the lysolecithin-activated enzyme is able to act on nonaromatic amino acids such as methionine and norleucine, catalyzing their conversion to methionine sulfoxide and epsilon-hydroxynorleucine, respectively. These amino acids are also substrates for phenylalanine hydroxylase that has been activated by limited proteolysis with alpha-chymotrypsin and by reaction with N-ethylmaleimide. By contrast, the unactivated hydroxylase as little or no activity toward methionine and norleucine. Rat liver phenylalanine hydroxylase can also be activated by its substrate, phenylalanine. We have now found that the novel substrates, methionine and norleucine, are also able to activate the enzyme. On a molar basis, however, they are not as active as phenylalanine. A model that can account for the broadened substrate specificity of the activated enzyme is discussed.