Acetate stimulates flux through the tricarboxylic acid cycle in rabbit renal proximal tubules synthesizing glutamine from alanine: a C-13 NMR study

Although glutamine synthesis has a major role in the control of acid-base b alance and ammonia detoxification in the kidney of herbivorous species, ver y little is known about the regulation of this process. We therefore studie d the influence of acetate, which is readily metabolized by the kidney and whose metabolism is accompanied by the production of bicarbonate, on glutam ine synthesis from variously labelled [C-13]alanine and [C-14]alanine molec ules in isolated rabbit renal proximal tubules. With alanine as sole exogen ous substrate, glutamine and, to a smaller extent, glutamate and CO2, were the only significant products of the metabolism of this amino acid, which w as removed at high rates. Absolute fluxes through the enzymes involved in a lanine conversion into glutamine were assessed by using a novel model descr ibing the corresponding reactions in conjunction with the C-13 NMR, and to a smaller extent, the radioactive and enzymic data. The presence of acetate (5 mM) led to a large stimulation of fluxes through citrate synthase and a lpha-oxoglutarate dehydrogenase. These effects were accompanied by increase s in the removal of alanine, in the accumulation of glutamate and in flux t hrough the anaplerotic enzyme pyruvate carboxylase. Acetate did not alter f luxes through glutamate dehydrogenase and glutamine synthetase; as a result , acetate did not change the accumulation of ammonia, which was negligible under both experimental conditions. We conclude that acetate, which seems t o be an important energy-provider to the rabbit renal proximal tubule, simu ltaneously traps as glutamate the extra nitrogen removed as alanine, thus p reventing the release of additional ammonia by the glutamate dehydrogenase reaction.