Separation and characterization of multiactivity peaks of 3-hydroxyanthranilic acid oxygenase from bovine liver

3-Hydroxyanthranilic acid oxygenase (3HAO) which is implicated in Huntington's disease, separated into two activity peaks when beef liver enzyme preparation was chromatographed on hydroxyapatite column (Nandi et al. (1994) FASEB J. 8, 1369). Two similar activity peaks were also separated by chromatofocusing the enzyme preparation before hydroxyapatite treatment, using PBE 94 gel exchange and polybuffer 74 (Pharmacia products) as eluent in the pH range of 4.0 and 7.4. These apparently homogeneous active proteins had pi values of 5.60 and 4.98, and also had different electrophoretic mobilities in non-denaturing PAGE. Their absorptivities at 285 nm and 290 nm appeared different. They had, however, similar Km values (16-18 MM) for 3HA and (40-57 uM) for 4-CH3-3HA, and also similar K, values for competitive inhibitors, anthranilic acid and quinonilic acid. The fact that these apparently different enzyme species had their N-terminal group blocked and their molecular mass (determined by MALDI) was very close, seems to suggest that they are probably isoforms rather than products of proteolysis. A definitive answer awaits further investigation.