Heterogeneity of presumably homogeneous protein preparations.

Some highly purified glycolytic enzymes have been subjected to isoelectric focusing and found to contain a number of enzymatically active species. Crystalline aldolase A and glyceraldehyde-3-phosphate dehydrogenase from rabbit muscle were resolved into five components, crystalline aldolase from yeast was resolved into three components, pyruvate kinase from rabbit muscle yielded four components, and yeast enolase was resolved into two components. Rabbit muscle lactate dehydrogenase (M(4)) gave one major peak of protein and enzymatic activity. The profiles of aldolase, glyceraldehyde-3-phosphate dehydrogenase, and yeast aldolases suggest random combinations of two closely related subunits into tetramers and dimers, respectively. The molecular heterogeneity of the other enzymes is not so easily related to subunit structure.