Molecular basis of the isozymes of bovine glucose-6-phosphate isomerase

Glucose-6-phosphate isomerase occurs in different bovine tissues as multiple, catalytically active isozymes which can be resolved by polyacrylamide gel electrophoresis (PAGE) and isoelectric focusing (IEF). Most differentiated tissues have five distinct forms with apparent pI values of 7.2, 7.0, 6.8, 6.6, and 6.4. Young, mitotically active, cells of the intestinal mucosa and the epithelium of the eye lens show only the two more basic isozymes, while old cells in the cortex and nucleus of the eye lens accumulate the more acidic isozymes. All of the isozymes exhibit equal separation based on charge-to-mass ratio (PAGE) and charge (IEF), thus indicating only charge changes. The isozyme patterns are unchanged in the presence of reducing agents or protease inhibitors. Each isozyme was purified to homogeneity and shown to exhibit identical subunit molecular weights (59,000) on SDS-gel electrophoresis. Each of the isolated isozymes, when subjected to PAGE or IEF, exhibited a single band, indicating that the isozymes are not generated as a result of electrophoresis. When the most basic isozyme was incubated in vitro under mild alkaline conditions, there was a spontaneous generation of the more acidic isozymes with properties identical to those found in vivo. The isozymes, thus, appear to be the result of spontaneous, postsynthetic modifications involving the addition of equal numbers of negative charges and are consistent with the deamidation of specific asparagine and/or glutamine residues.