On the Site of the Formation of Citric Acid in the Animal Body

Abstract

Recent studies 1 , 2 have demonstrated that the intravenous injection into dogs of the sodium salts of certain dicarboxylic acids, including malic acid, produces a marked and prompt increase in the amount of citric acid excreted in the urine. Simultaneous determinations of the citric acid content of urine and blood 2 suggested that the kidney played an important part in the formation or elimination of citric acid after the injection of sodium malate. The following experiments were designed to more definitely elucidate the question of the site of citric acid formation under these experimental conditions.

Adult male rats weighing from 250 to 450 gm. were given a citrate-low synthetic ration (casein, 18%; dextrin, 51%; Crisco, 27%; Wesson salt mixture, 4%) for a 5-day preliminary period. The animals were then lightly anesthetized with ether and sterile solutions of either sodium chloride (2.5%) or sodium malate (5%) were injected into the heart in amounts containing 100 mg. of sodium per kilo body weight. Preliminary studies showed that, under these conditions, the rat, like the dog, excretes relatively large amounts of citric acid in the urine after the parenteral administration of sodium malate. In the present experiments, the animals were again anesthetized 15 minutes after the injection of the chloride or malate, exsanguinated by bleeding from the abdominal aorta, and the tissues to be analyzed were removed and quickly frozen with carbon dioxide ice. The tissues were then pulverized, extracted with dilute sulfuric acid, and the citric acid content of aliquots of the protein-free (trichloroacetic acid) filtrates was determined. The amount of citric acid in trichloroacetic acid filtrates of the oxalated blood was also determined.

Table I shows that there was little difference between the citric acid content of the liver, muscle, and blood of the chloride-injected control rats and that of the malate-injected animals, whereas the citric acid content of the kidneys of the latter group was approximately twice that of the controls.