The catabolism of medium- and long-chain dicarboxylic acids.

The w-oxidation of fatty acids results in the formation of mediumand long-chain dicarboxylates. By this accessory metabolic pathway monocarboxylic acids are first converted into w-hydroxymonocarboxylic acids, a reaction catalysed by the microsomal mixed-oxidase function system (Pettersen, 1972). These intermediates are subsequently converted into w-oxomonocarboxylic acids and dicarboxylic acids by the action of cytosolic alcoholand aldehyde-dehydrogenases (Mitz & Heinrikson, 196 1 ). Medium-chain dicarboxylic acids can be formed from long-chain dicarboxylic acids and be excreted in urine. This organic aciduria is increased in a variety of conditions with excessive fatty acid degradation such as ketosis and diabetes mellitus or secondarily to defective mitochondrial fatty acid oxidation, including inherited metabolic defects, hypoglycin intoxication and riboflavin deficiency, as well as in subjects receiving medium-chain triacylglycerols. Dicarboxylic acids can be activated to their CoA esters by a rat liver microsomal synthetase (Vamecq et ul., 1985). In clofibrate-treated rats, esters of longand medium-chain dicarboxylic acids can be /3-oxidized by both liver mitochondrial and peroxisomal fractions (Kolvraa & Grcgersen, 1986), whereas in control animals this catabolism is mainly peroxisomal (Vamecq, 1987). Our studies were thus mainly dedicated to the catabolism of dicarboxylic acids in control, riboflavin-deficient and clofibrate-treated rats in vivo. The shortening of dicarboxylates by isolated mitochondria from the livers of control rats was also investigated. Thc rate of oxidation of dodecanedioic acid was studied in the three groups of animals by the following procedure. Anaesthetized rats were cannulated to give direct access to blood, and urine was continuously collected from the bladder. Dodecanedioic acid was infused in the form of its Na salt in doses of 11, 22 and 44 pmol per 100 g body weight during a 30 min period. Urine was collected over 30 min periods from 1 h before to 3 h after infusion. Organic acids were extracted, and trimethylsilylated derivatives were separated and detected by gas chromatography using an Intersmat IGC12 1 DC gas chromatograph (Suresnes, France) coupled to an Intersmat ICR1B integrator and equipped with a 50 m capillary column (0.32 mm in diameter, Chrompack CP Sil 5 fused silica) as previously described (Draye et ul., 1987). In control and riboflavin-deficient rats, dodecanedioic acid (22 pmo1/100 g body weight) was rapidly transformed into adipic, suberic and sebacic acids, which appeared in urine (Fig. I ) . Excretion of succinic acid remained low and was not modified by the infusion. In control animals, this process lasted less than 2 h, and urinary dicarboxylic acids amounted to 28.6 f 1.0% (mean f S.E.M; 11 = 4) of the dose administered. Over 60"% of this excretion occurred within 30 min after the infusion period. In riboflavin-deficient rats, the process was slower and not complete 3 h after infusion; dicarboxylic acids in urine amounted to 75.6 k 15.5% ( n = 3 ) of the infused dose. In clofibrate-treated rats, only 4.4 f 1.2% ( n = 3 ) of dodecanedioic acid was recovered as shorter dicarboxylic acids in urine. The maximal apparent velocity of dicarboxylate oxidation as calculated from the initial rate of appearance of mediumchain dicarboxylic acids in urine is similar in control and riboflavin-deficient animals. This rate of P-oxidation corresponds to the formation of 10.0 pmol of acetyl-CoA/30 min per 100 g body weight in controls, and 14.8 pmol/30 min per 100 g body weight in riboflavin-deficient rodents. The livers of the latter animals were 1.4 times larger than those of controls, when expressed as a percentage of body weight. In control rats the experiment was repeated with 11 and 44 pmol of dodecanedioic acid and, surprisingly, the same proportions of shorter dicarboxylic acids were recovered in urine (22.1 and 24.9%, respectively compared to 28.6%). Over 70%) of the infused dodecanedioic acid was never recovered in urine, irrespective of dose, the rate of which is presently unknown. Using the ferricyanidc method of Osmundsen &L Bremer (1977) we showed in preliminary experiments that there was no detectable oxidation by intact mitochondria from control rat liver of sebacyl-CoA in the presence of L-carnitine. The same acyl-CoA was a convenient substrate for the peroxi-