Cause of subunit heterogeneity in purified rat renal phosphate-dependent glutaminase

When electrophoresed on polyacrylamide gels in the presence of sodium lauryl sulfate, highly purified rat renal phosphate-dependent glutaminase exhibits subunits which range in molecular weight from 57,000 to 75,000. Peptide mapping of the separated subunits following limited proteolysis in the presence of sodium lauryl sulfate shows that all of the various subunits are related in structure. The glutaminase, immunoprecipitated from Triton X-100-solubilized mitochondria, is composed primarily of subunits which have molecular weights of 83,000. In addition, the series of smaller subunits is generated during storage of the Triton-solubilized glutaminase at 4 degrees C. These results indicate that the heterogeneity of subunit size found in the purified glutaminase results from a noninactivating partial proteolysis of the native form of the enzyme.