A Protein That Stimulates Rat Liver Phenylalanine Hydroxylase

Abstract

A protein has been found in partially purified preparations of sepiapterin reductase that stimulates highly purified rat liver phenylalanine hydroxylase. The stimulation is dependent on the structure of the pterin cofactor used; a large stimulation is observed with the naturally occurring cofactor, tetrahydrobiopterin, a slight one with 6-methyltetrahydropterin, and none with 6,7-dimethyltetrahydropterin. The extent of the stimulation is minimal at low and maximal at high hydroxylase concentrations. Kinetic data suggest that the stimulating protein influences a reversible association-dissociation of the hydroxylase.