Metabolism of l-Carnitine Esters of β-Substituted Palmitic Acid by Rat Liver Mitochondria

Redox control of beta-oxidation in rat liver mitochondria

Coupled rat liver mitochondria were incubated with [U-14C]hexadecanoate and carnitine which resulted in the formation of acyl-, 2-enoyl- and 3-hydroxyacyl-CoA and carnitine esters. The production of 2-enoyl-CoA and 3-hydroxyacyl-CoA esters was associated with a significant lowering of the NAD+/NADH ratio, in contrast to rat muscle mitochondria [Eaton, S., Bhuiyan, A. K. M. J., Kler, R. S., Turnbull, D. M. & Bartlett, K. (1993) Biochem. J. 289, 161-172], suggesting that control by the respiratory chain is important under normal conditions. When NAD+/NADH ratios were held low by succinate-induced reverse electron flow, 3-enoyl-CoA esters were also detected, probably formed by the action of 3,2-enoyl-CoA isomerase. Measurement of the flux of beta-oxidation at different osmolalities showed that flux was strongly dependent on osmolality changes in the physiological range. Measurement of the CoA and carnitine esters resulting from incubations made at different osmolalities showed that there was an increase in the amounts of the saturated acyl-CoA esters with respect to 2-enoyl-CoA and 3-hydroxyacyl-CoA esters, consistent with control by the electron-transfer flavoprotein-ubiquinone segment [Halestrap, A. P. & Dunlop, J. L. (1986) Biochem. J. 239, 559-565]. This however could not be the only factor operating as indicated by the continued presence of 2-enoyl-CoA and 3-hydroxyacyl-CoA esters at high osmolalities.

Accumulation of carnitine esters of beta-oxidation intermediates during palmitate oxidation by rat-liver mitochondria

Rat-liver mitochondria were incubated with [14C]palmitate in the presence of L-malate, fluorocitrate, and L-carnitine. The specific activities of acetyl groups incorporated into citrate, ketone bodies and acetyl-L-carnitine were measured. During state-4 oxidation of [1--14C]palmitate the specific activity of the acetyl-CoA pool was 1.3-times higher than that of the average acetyl group of palmitate, indicating an incomplete breakdown of the palmitate molecule. Accumulation of carnitine esters was observed in this condition. The acyl moieties of carnitine esters formed during the state-4 oxidation of [U-14C]palmitate or [16(-14)C]palmitate were analysed by radioactive gas-chromatography. Substantial amounts of beta-oxidation intermediates were found. The accumulation of carnitine esters of C6-C14 intermediates can quantitatively explain the high specific activity of the acetyl-CoA pool during the state-4 oxidation of [1(-14)C] palmitate. The localization and control of beta-oxidation are discussed.

Depressed fatty acid and acetate oxidation and other metabolic defects in homogenates from hearts of hamsters with hereditary cardiomyopathy

Metabolic changes in heart homogenate of Syrian hamsters with hereditary cardiomyopathy (BIO 14.6 strain) were examined. Oxidation of labeled fatty acids and acetate by myopathic homogenates was severely depressed; CO 2 production from labeled acetate was only one-fifteenth of the control value, and butyrate oxidation was suppressed to one-fourth of the control value. Although the addition of carnitine enhanced the oxidation of palmitate and octanoate by homogenates from both healthy and cardiomyopathic hamsters, the magnitude of the depression of oxidation (60-80%) in the myopathic homogenates was not influenced by carnitine. No change occurred in the rate of formation of palmitoyl-CoA from palmitoyl- l -carnitine or in the activity of acyl-CoA synthetases; similarly, the one-step oxidation of [1- 14 C]pyruvate, [1- 14 C]α-ketoglutarate, and succinate was unimpaired in cardiomyopathy, and the oxidation of [2- 14 C]pyruvate and [U- 14 C]α-ketoglutarate was maintained at a relatively high level (63-71% of control). In contrast, oxidation of [1, 4- 14 C]- and [2, 3- 14 C]succinate and [1- 14 C]acetyl-CoA by the myopathic homogenates was depressed to only 40% of the control values. The mechanism responsible for these defects in substrate oxidation by tissue homogenates from cardiomyopathic hearts has not been elucidated; however, significant increases in the activities of two of five hydrolytic enzymes indicate the possible involvement of lysosomes. Ventricular RNA and DNA, the ratio of RNA to DNA and the ratio of heart weight to body weight were all significantly increased, but myocardial water and triglyceride content and esterification of [1- 14 C]palmitate into neutral lipids showed no significant change.