Ammoniagenesis catalyzed by hippurate-activated γ-glutamyltransferase in the lumen of the proximal tubule

Abstract

γ-Glutamyltransferase (γ-GT) is located in the brushborder membrane of the proximal tubule where the catalytic site of the enzyme faces the lumen. The (phosphate-independent) glutaminase activity of γ-GT in vitro is activated by hippurate. In order to investigate glutamine deamidation in the tubule lumen in vivo,¹⁴C-l-glutaminecontaining solutions were continuously microperfused through sections of the proximal convoluted tubule in vivo and in situ.d-aspartate andl-phenylalanine (10 mmol/l, each) were added to the perfusate in order keep the reabsorption ofl-glutamine as such low and to block reabsorption of any glutamate possibly formed, respectively. Intraluminal formation of glutamate from glutamine in the absence of hippurate is small. In presence of 10 mmol/l hippurate, 5%–70% of the recovered¹⁴C-activity was¹⁴C-glutamate at an initial¹⁴C-l-glutamine concentration of 1 mmol/l. The respective absolute rate (±SEM) of glutamate formation, i.e., 36±5 pmol · s⁻¹ · m⁻¹, was increased 1.4-fold at an initiall-glutamine concentration of 3 mmol/l, but dropped to one third at initially 0.3 mmol/l. A rough estimate of the apparent kinetic constants resulted in aK _m of 0.58 (0.19–0.97) mmol/l and aV _max of 56 (40–93) pmol · s⁻¹ · m⁻¹. Deamidation of glutamine occurred also in the absence ofl-phenylalanine. Acivicin (AT 125), a γ-GT inhibitor, completely blocked glutamate formation. Endogenous hippurate concentrations determined by free flow micropuncture and HPLC were 0.16 mmol/l in the late proximal convulation, 0.6 mmol/l in the early distal convolution, and 4.9 mmol/l in the final urine. In conclusion, glutamine deamidation, i.e. ammoniagenesis, can take place in the lumen of the proximal tubule under favourable conditions. It can be estimated that this luminal ammoniagenesis makes up about 5%–10% of the normal renal ammonium excretion in these rats at endogenous proximal hippurate levels.