Rat liver phenylalanine hydroxylase. Activation by sulfhydryl modification.

Upon reaction with N-ethylmaleimide, a single sulfhydryl residue/Mr = 50,000 subunit of phenylalanine hydroxylase is modified. This modification is accompanied by a 20-30-fold increase in hydroxylase activity when the activity is measured with tetrahydrobiopterin as cofactor. The N-ethylmaleimide-modified enzyme exhibits many of the characteristics of phenylalanine hydroxylase activated by partial proteolysis or by exposure to phospholipids. For example, a change from sigmoid to hyperbolic kinetics with varying phenylalanine concentration is observed, in addition to broadened substrate specificity and a dependence on phenylalanine hydroxylase stimulator protein at pH 6.8. The binding of phenylalanine to phenylalanine hydroxylase in the absence of pterin cofactor has also been studied. The native enzyme exhibits a sigmoidal phenylalanine binding curve. The N-ethylmaleimide-modified enzyme shows a hyperbolic response to phenylalanine binding in addition to an apparent decrease in total phenylalanine binding.