Evidence against the occurrence of tissue-specific variants and isoenzymes of phosphoglucose isomerase

Phosphoglucose isomerase (EC 5.3.1.9) from a variety of rat and human tissues has been studied by several electrophoretic techniques. When heart, liver, kidney, brain, erythrocyte, or skeletal muscle extracts were subjected to electrophoresis on cellulose acetate, a broad electrophoretic band was observed. When tissues were subjected to isoelectric focusing in sucrose density gradients or polyacrylamide gels, multiple peaks were observed unless the extracts were prepared and focused in the presence of reducing agents. In the presence of 2-mercaptoethanol or dithiothreitol phosphoglucose isomerase from different tissues co-focused as a single peak (apparent isoelectric pH = 9.23 for human and 8.40 for rat), suggesting that the enzyme from the various tissues is identical, and that tissue-specific variants do not exist. The electrophoretic multiplicity previously reported by other investigators may be due to artifacts arising from oxidation of the enzyme.