Glutamate is known to inhibit the activity of isolated glutaminase I; however, its actual physiologic importance in regulating renal ammoniagenesis has not been established. To determine the regulatory role of glutamate on the metabolism of glutamine by rat kidney slices, we followed the effects on glutamine (2 mM) deamidation of increased removal of glutamate via augmented deamination. Three agents (malonate, 2,4-dinitrophenol, and methylene blue) were known to and shown here to hasten exogenous glutamate deamination. In slices from 10 control rats, 21.5±1.7 (SEM) μmol/g of ammonia were formed from amide nitrogen and 9.3±0.5 (SEM) μmol/g from the amino nitrogen of glutamine in vitro. Over 90% of the glutamine deamidated formed glutamate at one point in its catabolism. After addition of malonate (10 mM), 2,4-dinitrophenol (0.1 mM), or methylene blue (0.5 mM), the production of ammonia from the amino group rose to 29.3±6.0 (SEM) μmol/g, 20.0±1.8 (SEM) μmol/g, and 15.5±4.2 (SEM) μmol/g, respectively; ammonia production from the amide nitrogen rose also, 45.1±7.3 (SEM) μmol/g, 39.7±2.6 (SEM) μmol/g, and 41.9±3.7 (SEM) μmol/g. In the case of the former two, a minimum of 99% and 75% of the glutamine catabolized formed glutamate. Despite increased glutamine catabolism, there was no build up of glutamate in the media. A correlation between the formation of ammonia from the amino and amide nitrogen was apparent. Since none of the three agents selected affected phosphate activated glutaminase I activity directly or appeared to affect glutamine transport, we interpret the increase in deamidation as an expression of deinhibition of glutaminase I activity secondary to lowered glutamate concentrations at the deamidating sites through more rapid removal of glutamate via hastened deamination. Interestingly, slices removed from acidotic rats produced more ammonia from both the amino 29.1±3.8 (SEM) and amide nitrogens 45.9±4.3 (SEM) of glutamine, without a buildup of glutamate in the medium. At least 90% of the glutamine deamidated formed glutamate. A common mechanism is proposed to explain these results and the previous ones.
Harry G. Preuss, Olympia Vivatsi-Manos, Leonard L. Vertuno