Experimental Metabolic Acidosis: The Enzymatic Basis of Ammonia Production by the Dog Kidney*

Abstract

Studies were designed to investigate quantitatively the enzymatic basis for NH3 production and H+ ion excretion in healthy dogs under normal metabolic conditions and during NH4Cl induced metabolic acidosis. During the administration of 112 mEq of NH4Cl daily for 7 days, the urine pH fell from 7.85 to 5.47. The excretion of titratable acid increased from 3.6 to 17.4 mmoles/day, of NH3 from 20 to 78 mmoles/day, and of net H+ ion excretion from 16 to 93.5 mmoles/day. Enzyme activities were measured in kidney and liver biopsies taken at the end of a 7 day control metabolic study and at the end of a 7 day period of NH4Cl administration. In the kidney there was no change in the activity of glutaminase I and II, glutamate dehydrogenase, alanine amino transferase, D-a]lanine oxidase, citrate synthase, carbonic anhydrase, lactate dehydrogenase, or malate dehydrogenase; aspartate amino transferase activity increased throughout the nephron in metabolic acidosis. In the liver there was no change in the activity of any of these enzymes, save for a slight increase in aspartate amino transferase activity. From the in vitro data the total in vivo glutaminase I and II and glutamate dehydrogenase activities of the 2 dog kidneys were derived. It was calculated that in metabolic acidosis the glutaminases of the two kidneys would have split 46.9 M[mu]moles of glutamine/minute and that the glutamate dehydrogenase would have split 29.4[mu] moles of glutamate/minute, in each case giving rise to equimolar amounts of NH3. The NH3 production of the dogs was 70 [mu]moles/minute. Thus sufficient glutaminase I and II and glutamate dehydrogenase activities appeared to be present to split all the NH3 produced in metabolic acidosis. About 60% of the NH3 would have derived from glutamine, the remainder from the other amino acids through the amino acid amino transferases and desamination of glutamate.