Multiple Forms of Yeast Phosphoglucose Isomerase

Crystalline phosphoglucose isomerases isolated from brewers' and from bakers' yeast have each been resolved into three isoenzyme fractions.The separate identity of the isoenzymes has been demonstrated analytically by electrophoresis on cellulose acetate at pH 4.7, 5.2, and 6.5.Separation on a preparative scale has been achieved by column chromatography on diethylaminoethyl cellulose with a sodium phosphate gradient at pH 6.0.The following points were investigated in order to establish the origin of the multiple forms of the enzyme.Crystalline phosphoglucose isomerases isolated from three yeast species were all found to contain three isoenzyme fractions.The chromatographic procedure itself was shown not to be responsible for the appearance of the different protein fractions.Conditions known to favor possible proteolysis neither altered the qualitative behavior of the isoenzymes nor changed the relative amounts of isoenzymes found after chromatographic separation.Three isoenzymes were also found in a phosphoglucose isomerase preparation crystallized after isolation from a live, genetically homogeneous yeast culture, thus excluding the possibility that in the dried yeast the existence of several isoenzymes might be explained by assuming genetically mixed strains.All results obtained support the conclusion that the isoenzymes exist in vivo and that they are not artifacts of the purification procedures.In the course of comparative studies on phosphohexose isomerases, phosphoglucose isomerase (D-glucose-B-phosphate ketol-