Quaternary structure of ornithine aminotransferase in solution and preliminary crystallographic data

Ornithine aminotransferase was purified from rat liver and crystallized in the presence of ammonium sulphate and poly(ethylene glycol) (PEG 4000). The crystallographic threefold symmetry observed for the resulting two crystal forms stimulated a re-examination of the enzyme's quaternary structure in solution by analytical ultracentrifugation and chemical cross-linking. The results indicate that the oligomeric state or ornithine aminotransferase, under conditions similar to those used in crystallization experiments, is a hexamer (Mr = 256,000) rather than a tetramer or higher oligomers as reported previously. The subunits of the enzyme are identical (Mr = 45,000). Only the hexagonal prismatic crystals obtained with PEG 4000 were suitable for crystallographic studies and diffracted X-rays to a resolution of at least 0.16 nm. However, these crystals contained an unusual element of disorder which was persistent under a variety of conditions and was only noticeably diminished in the presence of the non-ionic detergent octyl beta-glucoside. The crystals apparently belong to the trigonal space group P3(1)12 (or enantiomorph) with axial lengths of a = 19.5 nm, c = 5.9 nm and contain three monomers per asymmetric unit.