Physical and Chemical Properties of Yeast Phosphoglucose Isomerase Isoenzymes

Three isoenzymes of brewers' yeast phosphoglucose isomerase which can be resolved by column chromatography on DEAE-cellulose (Nakagawa, Y., and Noltmann, E. A. (1967) J. Biol. Chem. 242, 4782–4788) have been subjected to physical and chemical characterization. In contrast to the pseudoisoenzymes of rabbit muscle phosphoglucose isomerase (Blackburn, M. N., Chirgwin J., M. James, G. T., Kempe, T. D., Parsons, T. F., Register, A. M., Schnackerz, K. D., and Noltmann, E. A. (1972) J. Biol. Chem. 247, 1170–1179), these isoenzymes do not originate from various extents of sulfhydryl oxidation since the presence of dithiothreitol during both autolysis and chromatography of the crude extract did not affect the distribution of phosphoglucose isomerase activity. Equilibrium sedimentation and gel electrophoresis studies established the molecular weight of all three isoenzymes to be 120,000. Also, identical values of s20,ω (6.7 S) and v (0.735 ml g-1) were obtained for the three proteins. Amino acid analyses yielded small but significant differences in the over-all amino acid composition. As the only notable feature, the isoenzymes do not contain cystine and their cysteine content was found to be maximally 2 residues per molecule. Electrophoresis yielded isoelectric points of pH 5.4, 5.2, and 5.0 for Isoenzymes A, B, and C, respectively, corresponding to the order of their elution positions on anion exchange columns. Polyacrylamide gel electrophoresis indicated, furthermore, that the isoenzymes differ in net charge, but not in molecular weight; hence, they may be classified as “charge isomers.”