Phenylalanine 4-monooxygenase from bovine and rat liver: some physical and chemical properties

Phenylalanine 4-monooxygenase was purified from bovine liver using a modification of the procedure developed for the rat liver enzyme (Shiman, R., Gray, D.W., and Pater, A. 1979. J. Biol. Chem. 254: 11300-11306). The enzyme preparation appeared essentially homogeneous on polyacrylamide gel electrophoresis under non-denaturing conditions. Electrophoresis in the presence of dodecyl sulfate revealed that about 95% of the protein had a mobility corresponding to Mr = 51,000. The remaining 5% was recovered in two minor bands corresponding to Mr of about 35,000 and 15,000 and is likely to result from limited proteolysis of the native enzyme with dissociation of the fragments on denaturation by detergent. The enzyme comigrated with the rat liver enzyme on polyacrylamide gel electrophoresis in both systems studied. No significant difference was observed between the amino acid composition of the bovine and rat liver enzyme, in the reactivity of their sulfhydryl groups or in their iron content (i.e. 1.5-3.0 iron atoms per peptide chain of Mr = 50,000). Both enzymes contained less than 0.01 copper atom per peptide chain. The enzymes were inhibited in a similar manner by the chelator bathophenanthroline disulfonate (selective for iron and copper), but not by bathocuproine disulfonate (specific for copper). The results indicate that the bovine and rat liver enzymes are closely similar and that iron, but not copper, is essential for enzyme activity. High performance size-exclusion liquid chromatography revealed that both native enzymes exist in different oligomeric forms, but further studies are required to understand the physicochemical basis for this phenomenon.