The purine nucleotide cycle and ammoniagenesis in rat kidney tubules.

The contribution of the purine nucleotide cycle to renal ammoniagenesis was examined in cortical tubule suspensions prepared from acidotic rats and incubated with [alpha-15N]glutamine, [15N]glutamate, or [15N]aspartate. Labeling of ammonia and adenine nucleotides was determined after enzymatic transformations designed to circumvent the technical problem that 15NH3 and H2O have the same nominal mass. Labeling of the adenine nucleotide was undetectable (less than 10%) even after 1 h of incubation. From the measured concentrations of adenine nucleotides and ammonia and the labeling of the ammonia, the flux through the purine nucleotide cycle was calculated to account for less than 1% of the deamination of alpha-amino groups from all three substrates. The glutamate dehydrogenase reaction is therefore the likely pathway for deamination. The rate of 15NH3 production from [alpha-15N]glutamine was two or three times greater than from added [15N]glutamate, indicating a preference for intracellularly generated glutamate. 15NH3 production from added [15N]aspartate was similar to and perhaps slightly greater than that from added [15N]glutamate.