Coenzyme‐Dependent Conformational Properties of Rat Liver Ornithine Aminotransferase

Two apoenzymes of crystalline ornithine aminotransferase from rat liver were separated by gel filtration. They could also be separated by ultracentrifugation and sucrose density gradient centrifugation. These enzymes have different molecular weights, specific activities and absorption spectra. Their molecular weights are 177 100 ± 7700 (form I) and 105000 ± 6300 (form II) and their specific activities are 11.7 and 23.3 U/mg respectively. The native ornithine aminotransferase and the reconstituted holo‐form I and holo‐form II have similar K m values for l ‐ornithine (0.9 to 1.1 mM), K m values for 2‐oxo glutarate (1.4 to 1.8 mM) and K i ; values for l ‐valine (4.0 to 6.5 mM). In the absence of coenzyme both enzymes are inert, but form I has an absorption shoulder at 330 nm, like the pyridoxamine 5′‐phosphate form of enzyme. Apo‐form II associates forming a molecule with the same molecular weight as native enzyme of 177000, on addition of the coenzyme, pyridoxal 5′‐phosphate. The molecular weights of each enzyme protomer of the native ornithine aminotransferase and of the two apo‐forms were all found to be 45000 ± 2000 by dodecylsulfate acrylamide gel disc electrophoresis. The amino acids compositions of form I and form II are very similar, and the total number of SH groups/monomer of the native enzyme, apo‐form I and apo‐form II were all 5.