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

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