[Interaction of aspartate transaminase from chicken heart cytosol with pyridoxal phosphate analogs]

The interactions were studied of the apoenzyme of aspartate aminotransferase from chicken heart cytosol with a variety of pyridoxal-P analogues. 2-Norpyridoxal-P, 2'-n-propylpyridoxal-P, 2'-isopropylpyridoxal-P, 6-methylpyridoxal-P, and 5'-methylpyridoxal-P were shown to display coenzyme activity. Estimated relative Vmax values of the complexes of apoenzyme with the above--mentioned analogues amounted respectively to 0.8; 0,2; 0,1; 0.1 and 0.1 (taking the Vmax value of the native holoenzyme as equal 1.0). The pH-dependence of reactivation rates of the apoenzyme with pyridoxal-P and pyridoxamine-P was evaluated. 3-Deoxypyridoxal-P, 3-0-methylpyridoxal-P, 2'-phenylpyridoxal-P, 5-nor-5-beta-carboxyvinylpyridoxal and 5-nor-5-beta-carboxyethylpyridoxal fail to activate the apoenzyme, but inhibit competitively the binding of pyridoxal-P to the protein; the estimated Ki values for these analoges were 2.4-10- minus 6; 3.1-10- minus 6; 3.5-10- minus 6; 7.2-10- minus 6 and 8.3-10- minus 6 M, respectively. It is of interest to compare reactivation effects of pyridoxal-P analogues for the apoenzymes of aspartate aminotransferases from chicken and from pig heart cytosol. Although the observed effects were fairly similar, it should be noted that the relative catalytic efficiencies of complexes of the chicken apoenzyme with pyridoxal-P analogues were much lower than those of complexes formed with the pig heart apoenzyme. It thus appears that of the two enzymes tested, the chicken heart aminotransferase makes more stringent demands with respect to structure of the coenzyme.