MITOCHONDRIAL ASPARTATE TRANSAMINASE .2. ISOLATION AND CHARACTERIZATION OF MULTIPLE FORMS

Mitochondrial pig heart glutamate-aspartate transaminase (L-aspartate:2-oxoglutarate aminotransferase, EC 2.6.1.1) can be separated by ion-exchange chromatography on carboxymethyl-Sephadex into three fractions each possessing a different electrophoretic mobility. The absorption spectral properties of the fractions have been found to be similar. The subforms are equal in pyridoxal phosphate content, but differ in the relative amounts of “catalytically active” (absorbance at 430 mμ at low pH and 360 mμ at high pH) and “catalytically inactive” (absorbance at 340 rqu at high or low pH) pyridoxal phosphate which accounts for differences in their specific activities and molecular mean residue ellipticity at 430 mμ. The subforms display identical reaction rates with the pseudo-substrates, L-alanine, L-methionine, and L-serine. Their true dissociation constants with α-methylaspartate are equivalent. No distinctions among the multiple forms can be derived from the amino acid composition or peptide maps, pointing out their similar primary structure. Optical rotatory dispersion studies in the ultraviolet region of the spectrum and immunological double diffusion on agar gel cannot distinguish the subforms ruling out the possibility of the mitochondrial glutamate-aspartate transaminase fractions being conformers. Although the pig heart mitochondrial isozyme is distinct from the supernatant one, a comparison among the subforms of each isozyme system points out many similarities. No variation was perceived in the structure of the proteins within each system of multiple forms. The possibility of minor localized structural or conformational differences, which could not be detected by the procedures used, is speculated. © 1969, American Chemical Society. All rights reserved.