Distribution of gamma-glutamyl transpeptidase and glutaminase isoenzymes in the rabbit single nephron.

"Phosphate-independent maleate-stimulated glutaminase" was investigated as a function of gamma-glutamyl transpeptidase (gamma-GTP). The activity of gamma-GTP in brush border membranes was found to be four times higher than that in the microsomal fraction of the renal cortex. This gamma-GTP activity was exclusively located in the proximal tubule of isolated single nephrons. Specific activity of gamma-GTP was 105 U/g protein (19.8 microU/mm length) in the first 2 mm portion of the proximal tubule and 1352 U/g protein (209 microU/mm) in the last 2 mm portion of the proximal straight tubule. Activity of phosphate independent glutaminase (PIG) was distributed in the same patterns as those of gamma-GTP, not only in the subcellular fractions, but also in the isolated nephron segments. On the other hand, phosphate dependent glutaminase (PDG) was distributed highly in the papillary mitochondrial fraction and in the distal tubule. Observations on the effect of pH on the enzyme activities of gamma-GTP and PDG showed that these enzyme activities were decreased significantly when the pH of the assay mixture was lowered. In the case of PIG, however, the effect of pH was just reversed. From these findings, it may be possible to interpret that gamma-GTP may play an important role in ammonia production in the brush border membrane of the proximal tubule as a function of glutaminase.