The Distribution of Glutaminase Isoenzymes in the Various Structures of the Nephron in Normal, Acidotic, and Alkalotic Rat Kidney

Abstract

Rat kidney contains two distinct glutaminase isoenzymes. One requires phosphate for activity; the other is phosphate-independent but is activated 15-fold by maleate. By taking advantage of differences in activators and pH optima, specific assays were devised for each isoenzyme. Their distribution was then examined in individual structures of nephrons dissected from lyophilized 20-µ sections of kidney. Enzymatic cycling, oil well, and fluorometric techniques were combined to provide sufficient sensitivity to measure the enzymes in the 5- to 50-ng samples. The distribution of the two isoenzymes is complementary. Ratios of the two activities in the different structures vary from 0.05 to 50. Phosphate-independent glutaminase activity is high only in proximal straight tubules. All other structures assayed had less than one-tenth as much activity. Phosphate-dependent glutaminase activity is high in distal straight and convoluted tubules, intermediate in proximal convoluted tubules, and low in proximal straight tubules and glomeruli. Only the phosphate-dependent glutaminase activity responds to metabolic acidosis or alkalosis and this response appears to be limited to the proximal convoluted tubules. After administration of NH4Cl for 7 days to induce acidosis the activity increased 20-fold in these tubules, whereas 7 days of NaHCO3 administration to induce alkalosis caused a 40% decrease. Neither acidosis nor alkalosis affected phosphate-independent activity in any renal structure.