The Stimulation of Rat Liver Phenylalanine Hydroxylase by Lysolecithin and α-Chymotrypsin

In the presence of the natural cofactor, tetrahydrobiopterin, rat liver phenylalanine hydroxylase has a sigmoidal saturation curve (Hill coefficient 2.0) for phenylalanine concentration versus initial velocity. Lysolecithin or α-chymotrypsin increases the maximum velocity 20-fold, decreases the Km for phenylalanine by 50%, and converts the phenylalanine-saturation curve from the sigmoidal to the hyperbolic form. Lysolecithin and the phenylalanine hydroxylase stimulator increase the hydroxylase activity in a synergistic fashion. Lysolecithin also stimulates the hydroxylation of tryptophan (to 5-hydroxytryptophan) and the hydroxylation of meta-tyrosine (to 3,4-dihydroxyphenylalanine) by highly purified phenylalanine hydroxylase. Lysolecithin stimulates para-tyrosine-dependent TPNH oxidation by the hydroxylase without hydroxylation of the para-tyrosine. Lysolecithin exposes a sulfhydryl group of the hydroxylase and therefore appears to alter the conformation of the enzyme. Chymotrypsin, in contrast, partially hydrolyzes the hydroxylase, reducing its size from a dimer with a molecular weight of 100,000 (composed of two 50,000 molecular weight subunits) to a dimer with a molecular weight of 67,000 (composed of two 35,000 molecular weight subunits).