Coenzyme active site occupancy as an indicator of independence of the subunits of mitochondrial aspartate aminotransferase.

The enzyme, aspartate aminotransferase, is a dimer consisting of two identical subunits which contain overlapping subunit regions ( Eichele , G., Ford, G.C., Glor , M., Jansonius , J.N., Mavrides , C., and Christen , P. (1979) J. Mol. Biol. 133, 161-180), suggesting the possibility of subunit interactions. The structurally similar cytosolic isozyme exhibits noncooperative binding of pyridoxal 5'-phosphate ( Boettcher , M., and Martinez -Carrion, M. (1975) Biochemistry 14, 4528-4531; Relimpio , A., Iriarte , A., Chlebowski , J.F., and Martinez -Carrion, M. (1981) J. Biol. Chem. 256, 4478-4488) in which the apoenzyme/holoenzyme hybrid dimer shows a distinctive thermal stability. Using a nonequilibrium isoelectric focusing technique, it can be shown that mitochondrial aspartate aminotransferase also binds cofactor in a noncooperative random fashion. However, differential scanning calorimetry (DSC) thermograms show different characteristics from the cytosolic form. These differences are interpreted in terms of unique subunit interactions in this isozyme. Heating to the various DSC transition temperatures shows that the anomalous DSC thermograms in partially coenzyme-saturated apoenzyme preparations are due to a selective dissociation of apoenzyme subunits into monomers which are irreversibly denatured. The remaining holoenzyme monomers reassociate and form stable holoenzyme dimers. The net result is retention of the initial concentration of holoenzyme subunits present in any given mixture. Random occupancy of active sites and similar electrophoretic and DSC patterns upon heating of partially saturated apoenzyme preparations is observed whether the coenzyme, pyridoxal phosphate or pyridoxamine phosphate alone, or borohydride-reduced Schiff's bases of coenzyme-substrate analogue derivatives are used as active site directed ligands. The latter resemble covalent enzyme-substrate intermediates.