JBC, Vol. 250, Issue 17, 6720-6726, Sep, 1975
Purification and properties of avian liver p-hydroxyphenylpyruvate hydroxylase
G. H. Wada, J. H. Fellman, T. S. Fujita and E. S. Roth
Avian liver p-hydroxyphenylpyruvate hydroxylase (EC 1.13.11.27) was
purified to a 1000-fold increase in specific activity over crude
supernatant, utilizing a substrate analogue, o-hydroxyphenylpyruvate, to
stabilize the enzyme. The preparation was homogeneous with respect to
sedimentation with a sedimentation velocity (s20,w) of 5.3 S. The molecular
weight of the enzyme was determined to be 97,000 +/- 5,000 by sedimentation
equilibrium, and the molecular weight of the subunits was determined to be
49,000 +/- 3,000 by sodium dodecyl sulfate polyacrylamide gel
electrophoresis. Polyacrylamide gel electrophoresis revealed heterogeneity
of the purified enzyme. The multiple molecular forms were separable by
isoelectric focusing, and their isoelectric points ranged from pH 6.8 to
6.0. The amino acid compositions and tryptic peptide maps of the three
forms isolated by isoelectric focusing were very similar. The forms of the
enzyme had the same relative activity toward p-hydroxyphenylpyruvate and
phenylpyruvate. Conditions which are known to accelerate nonenzymic
deamidation of proteins caused interconversion of the multiple molecular
forms. Iron was the only transition metal found to be associated with the
purified enzyme at significant levels. The amount of enzyme-bound iron
present in equilibrium-dialyzed samples was equivalent to 1 atom of iron
per enzyme subunit. Purification of the enzyme activity correlated with the
purification of the enzyme-bound iron. An EPR scan of the purified enzyme
gave a signal at g equal 4.33, which is characteristic of ferric iron in a
rhombic ligand field.