glutaminase and Acidosis--Renal-Tubular

The renal proximal tubule contains a variety of biochemical pathways, which can metabolize glutamine, the major substrate for renal ammoniagenesis. The intramitochondrially located phosphate-dependent glutaminase (PDG) pathway, rather than the various cytosolic pathways, appears to play the predominant role in regulating the rate of renal NH3 production. Acute acidosis stimulates NH3 production by activating alpha-ketoglutarate dehydrogenase and secondarily glutamate dehydrogenase; whereas the adaptation to chronic metabolic acidosis results primarily from enhanced glutamine transport into the mitochondria and possibly increased activity of PDG. There is no adaptation of ammoniagenesis to chronic respiratory acidosis, because the proximal tubular intracellular pH is not decreased. Alkalosis suppresses NH3 formation but the precise mechanism is not clarified. Ammoniagenesis can be modulated independent of acid-base status by a variety of factors including potassium homeostasis, TCA cycle intermediates, hormones which increase cAMP, prostaglandin F2 alpha, insulin, growth hormone, angiotensin II, corticosteroids, aldosterone, and tubular flow rate.

Topics: Acidosis, Renal Tubular; Ammonia; Animals; Glutamate Dehydrogenase; Glutaminase; Hormones; Humans; Ketoglutarate Dehydrogenase Complex; Renal Tubular Transport, Inborn Errors

The metabolism of a typical North American diet yields a net acid load. Hydrogen ions are removed from the body after combining with bicarbonate to form CO2. This leaves the body with a deficit of bicarbonate. The role of the kidney is to add 'new' bicarbonate to the body. It does so primarily by synthesizing NH4+ plus bicarbonate while making NH4+ an end-product of metabolism (excreting it in the urine). Production of NH4+ occurs primarily in proximal convoluted tubule cells. Although several possible pathways can do this, the primary one stimulated by chronic metabolic acidosis is the glutaminase/glutamate dehydrogenase one. The upper limit on this pathway is set by energy turnover considerations. This, in effect, means control by renal work (sodium reabsorption) and fuel competitions (availability of fat-derived fuels).

Topics: Acidosis, Renal Tubular; Ammonia; Binding, Competitive; Carbonic Acid; Energy Metabolism; Glutamate Dehydrogenase; Glutaminase

The amount of glutamate per microgram protein was measured in renal mitochondria and homogenates as a function of age of rats. Glutamate was higher in the youngest (5-day-old) animals examined, it decreased gradually until about 30 days of age and reached mature levels in animals older than 40 days. The decrease in mitochondrial glutamate content was greater than the decrease in cortical homogenates. Chronic acidosis led to lower levels of glutamate in mature animals, particularly in mitochondria, but produced no effect in either mitochondria or homogenates obtained from kidneys of pups. These observations provide a basis for the hypothesis that high glutamate concentrations in the kidneys of neonatal animals can inhibit mitochondrial glutaminase and thereby limit ammoniagenesis in the postnatal period.

Topics: Acidosis, Renal Tubular; Aging; Ammonia; Animals; Glutamates; Glutaminase; Kidney; Mitochondria; Rats; Rats, Inbred Strains

Mitochondria from control and acidotic rat renal cortex were incubated with labeled compounds to determine the distribution of L-glutamine. The mitochondrial preparations lacked detectable phosphate-independent glutaminase activity. In control mitochondria, the distribution of glutamine exceeded that of mannitol by 8% without an added energy source in the medium and by 9.9% when tetramethyl-p-phenylenediamine (TMPD) and ascorbate were present. The distribution of D-ribose (MW 150) was similar to that of L-glutamine (MW 146), indicating that these molecules penetrate into a slightly larger space than D-mannitol (MW 182). Inhibition of phosphate-dependent glutaminase activity with 5 mM glutamate did not alter the volume of glutamine distribution. In mitochondria from chronically acidotic rats, the glutamine/mannitol ratio was 1.03 in the absence of an energy source. With an energy source this ratio fell to 0.88 due to metabolism of glutamine during the separation process. Inhibition of glutamine metabolism with glutamate or alpha-ketoglutarate resulted in distribution ratios of 1.07 and 0.97, respectively. It is concluded that, although glutamine/mannitol ratios greater than 1.00 can be demonstrated in rat kidney mitochondria, such a finding may arise from causes other than the presence of free glutamine in the matrix space. Thus, it has yet to be established whether the inner membrane carrier for glutamine releases glutamine or glutamate into the matrix space.

Topics: Acidosis, Renal Tubular; Animals; Cell Membrane Permeability; Glutamates; Glutamic Acid; Glutaminase; Glutamine; Kidney; Male; Mannitol; Mitochondria; Rats; Rats, Inbred Strains

The following points summarize these findings: (i) there are 2 glutamine utilizing enzyme systems in the rat kidney; (ii) the cytoplasmic glutamyltransferase system hydrolyzes either glutamine isomer while the mitochondrial localized glutaminase 1 is specific for the L-isomer; (iii) the cytoplasmic pathway contributes 70% of the total renal ammonia production in the normal kidney; (iv) chronic metabolic acidosis results in a 20-fold activation of the mitochondrial glutaminase 1 pathway.

Topics: Acidosis, Renal Tubular; Acyltransferases; Ammonia; Animals; Cytoplasm; Glutaminase; Glutamine; In Vitro Techniques; Kidney; Male; Mitochondria; Rats; Stereoisomerism

Topics: Acidosis, Renal Tubular; Aldosterone; Ammonia; Animals; Carbon Dioxide; Glutaminase; Hydrogen-Ion Concentration; Hyperaldosteronism; Kidney; Male; Nephrosis; Potassium; Rats; Sodium