Partial purification and characterization of rat liver phenylalanine hydroxylase phosphatase

A phosphatase which catalyzes the release of 32Pi from 32P-labeled phenylalanine hydroxylase has been purified about 50-fold from rat liver extracts. The phosphatase is able to catalyze the removal of only 70 to 80% of the 32Pi leaving about the same amount of Pi still bound to the hydroxylase as was originally present in the native enzyme. Dephosphorylation is accompanied by a corresponding loss in phenylalanine hydroxylase activity when the activity is measured with the natural cofactor, tetrahydrobiopterin, but not when measured with the synthetic cofactor, 6-methyltetrahydropterin. The partially purified phosphatase has very low activity toward p-nitrophenylphosphate, histone phosphate, and phosphorylase a. The activity toward these substrates has not been purified to the same extent as the phenylalanine hydroxylase phosphatase activity.