CARNITINE AND FATTY ACID OXIDATION IN MITOCHONDRIA OF VARIOUS ORGANS

Response of hepatic mitochondrial redox state to oral glucose load. Redox tolerance test as a new predictor of surgical risk in hepatectomy

The redox tolerance test introduced in this article attempts to quantify the deterioration of hepatic mitochondrial energy metabolism by measuring the changes in arterial ketone body ratio in response to 75-g oral glucose loading, and is discussed in relation to its predictive value for assessing surgical risk in hepatectomy. The indicator, called redox tolerance index (RTI), represents a 100-fold cumulative enhancement of ketone body ratio relative to glucose level (100 x delta KBR/delta glucose). The redox tolerance index was significantly different between 31 liver cirrhotics and 10 normal volunteers (p less than 0.001). Subjects were divided into three classes (I: RTI greater than or equal to 1.0, II: 0.5 less than or equal to RTI less than 1.0, III: RTI less than 0.5). Postoperative mortality was significantly different among the three classes in 127 hepatic resections (chi 2 = 9.843, p less than 0.01). Of 97 hepatocellular carcinoma cases, major hepatic resections in class III showed significantly higher postoperative morbidity and mortality rates (p less than 0.05 and p less than 0.05, respectively). The present findings indicate that RTI based on redox theory is of potential value in predicting posthepatectomy outcome.

Aspects of long-chain acyl-COA metabolism

1. Long-chain acid: CoA ligase (AMP-forming) (trivial name acyl-CoA synthetase; EC 6.2.1.3) is located at the membranes of the endoplasmic reticulum and the outer membrane of the mitochondria. The latter membrane has by far the highest specific activity. 2. GTP-dependent synthesis of acyl-CoA has a very low activity in liver mitochondria (about 5% of the activity measured with ATP). CTP, ITP, UTP and GTP may all provide energy for fatty acid activation in sonicated mitochondria by formation of ATP from endogenous ADP and AMP. 3. In rat liver palmitoyl-CoA: L-carnitine O-palmitoyltransferase (trivial name carnitine palmitoyltransferase; EC 2.3.1.21) is located at the microsomal membranes and in the inner membrane of the mitochondria. Its activity is increased, in both membranes, during fasting and in thyroxine-treated rats. The extramitochondrial carnitine palmitoyltransferase may capture part of the acyl CoA formed at the endoplasmic reticulum as acyl-carnitine, especially during fasting and other metabolic conditions of high fatty acid turnover. This transport form of activated fatty acid can penetrate the inner mitochondrial membrane (the acyl-CoA barrier) where it can be reconverted to acyl-CoA, providing the substrate for beta-oxidation in the inner membrane-matrix compartment. The small part of the mitochondrial carnitine palmitoyltransferase, described to be present at the external surface of the mitochondrial inner membrane, may have the same function in the transport of acyl-CoA formed at the mitochondrial outer membrane. 4. Isolated rat liver mitochondria can oxidize high concentrations of palmitate or oleate in the absence of carnitine. In this case the fatty acids are activated in the inner membrane-matrix compartment of the mitochondria, probably by a medium-chain acyl-CoA synthetase with wide substrate specificity. Because this enzyme is less active in heart and absent in skeletal muscle, these tissues oxidize long-chain fatty acids in an obligatory carnitine-dependent fashion. Also the liver oxidizes long-chain fatty acids in a carnitine-dependent way if lower fatty acid concentrations are used. In this tissue carnitine stimulates specifically the partial oxidation of fatty acids to beta-hydroxybutyrate and acetoacetate. 5. The activities of acyl-CoA: sn-glycerol-3-phosphate O-acyltransferase (trivial name glycerophosphate acyltransferase; EC 2.3.1.15) and carnitine palmitoyltransferase change in opposite directions during fasting. These activity changes, together with the measured kinetic properties of the enzymes in mitochondria and microsomes, allow a switch (relatively) from lipid synthesis to ketogenesis during fasting. This switch may occur at the level of long-chain acyl-CoA both in the endoplasmic reticulum and in the mitochondria.

Adaptation of muscle to exercise. Increase in levels of palmityl Coa synthetase, carnitine palmityltransferase, and palmityl Coa dehydrogenase, and in the capacity to oxidize fatty acids

The capacity of gastrocnemius and quadriceps muscles to oxidize palmitate, oleate, linoleate, palmityl CoA, and palmityl carnitine doubled in rats subjected to a program of treadmill running. The rate of palmitate oxdation by whole homogenates of, or the mitochondrial fraction from, leg muscles was twice as great per gram wet weight of muscle in the trained as in the sedentary animals over a wide range (0.125-1.5 mM) of palmitate concentrations. The levels of activity of carnitine palmityltransferase, palmityl CoA dehydrogenase, and mitochondrial ATP-dependent palmityl CoA synthetase expressed per gram of muscle doubled in gastrocnemius and quadriceps muscles in response to the running program. The protein content of the mitochondrial fraction from these muscles was increased approximately 60%.