Purification and characterization of phenylalanine 4-monooxygenase from rat liver

Phenylalanine 4-monooxygenase (L-phenylalanine, tetrahydropteridine:oxygen oxidoreductase (4-hydroxylating), EC 1.14.16.1) was purified approx. 600-fold to apparent homogeneity with a 48% yield from rat liver. Two distinct active forms were separable by calcium phosphate gel chromatography and numbered based on their order of elution from the gel column. The predominant form, Form I, had an estimated molecular weight of about 240 000. The enzyme gave a single band on sodium dodecyl sulfate polyacrylamide gel electrophoresis, the molecular weight of which was estimated to be approx. 51 000, indicating that the enzyme might be composed of four identical subunits. The molecular properties of Form I were: sedimentation coefficient, 10.1 S; Stokes radius, 55 A degrees; diffusion coefficient, 3.90 x 10(-7) cm2/s; frictional ratio, 1.33 and isoelectric point, pH 5.6. The enzyme contained approx. 0.6 mol of iron and 0.3 mol of phosphate/mol of subunit of the enzyme. No significant differences in kinetic properties of the two forms, Form I and Form II, were observed. Amino acid analysis studies revealed that the amino acid composition of Form I was essentially identical with that of Form II, indicating that both forms might be the products of the same gene. There were, however, minor differences in the phosphate content and the isoelectric point between the two forms.