Intramitochondrial malate

A typical intramitochondrial concentration of malate is 0.22 mM. If the [NAD ]/[NADH] ratio in mitochondria is 20 and if the malate dehydrogenase reaction is at equilibrium, calculate the intramitochondrial concentration of oxaloacetate at 25°C. 

Mitochondria from body wall muscle and probably the pharynx lack a functional TCA cycle and their novel anaerobic pathways rely on reduced organic acids as terminal electron acceptors, instead of oxygen (Saz, 1971 Ma et al, 1993 Duran et al, 1998). Malate and pyruvate are oxidized intramitochondrially by malic enzyme and the pyruvate dehydrogenase complex, respectively, and excess reducing power in the form of NADH drives Complex II and [3-oxidation in the direction opposite to that observed in aerobic organelles (Kita, 1992 Duran et al, 1993 Ma et al,... 

All parasitic flatworms capable of anaerobic metabolism favour malate as the primary mitochondrial substrate and the oxidative decarboxylations of first malate and then pyruvate generate intramitochondrial reducing power in the form of NADH (Fig. 20.1). In contrast, the pathways used to reoxidize intramitochondrial NADH are quite diverse and depend on the stage or species of parasite under examination, but in all cases, redox balance is maintained and electron-transport associated ATP is generated by the NADH-reduction of fumarate to succinate. In the cestode, hi. diminuta, succinate and acetate are the major end products of anaerobic malate dismutation and are excreted in the predicted 2 1 ratio. In the trematode F. hepatica, succinate is then further decarboxylated to propionate with an additional substrate level phosphorylation coupled to the decarboxylation of methylmalonyl CoA. F. hepatica forms primarily propionate and acetate as end products, again in a ratio of 2 1 to maintain redox balance. 

Fig. 5.9. Proposed scheme for the intramitochondrial metabolism of malate by Hymenolepis diminuta. Abbreviations ME, malic enzyme F, fumarase T, transhydrogenase FR, fumarate reductase ETS, electron transport system. Once within the matrix compartment, malate oxidation, as catalysed by malic enzyme, results in NADPH formation. Via the activity of fumarase, malate also is converted to fumarate in the matrix compartment. NADPH then serves as a substrate for the inner-membrane-associated transhydrogenase and transhydrogenation between NADPH and matrix NAD is a scalar reaction associated with the matrix side of the inner membrane. Matrix NADH so formed reduces the electron transport system via a site on the matrix side of the inner membrane permitting fumarate reductase activity. The reduction of fumarate to succinate results in succinate accumulation within the matrix compartment. (After McKelvey Fioravanti, 1985.)...

Schmidt and Katz [45] suggested an alternative cyclic process involving malate, which would bypass the citrate cleavage enzyme and transfer intramitochondrial reducing equivalents to the cytosol without producing acetyl units. Were this short-circuit of the malate transhydrogenation cycle found to play a major role in adipose tissue, it could supply more than 50% of the NADPH required by the synthetase. Another potential source of extramitochrondial NADPH is isocitrate dehydrogenase however, it does not appear to be of major importance in fatty acid synthesis, as will be discussed later. 

An increased intramitochondrial NADH concentration leads to an increase in cytosolic NADH through shuttles in the mitochondrial inner membrane, i.e. the malate-aspartate shuttle. The raised NADH/NAD" ratio in the cytoplasm shifts the lactate dehydrogenase equilibrium in the direction of lactate. An increased lactate/pyruvate ratio can be found in most but not in all patients with a mitochondrial disorder. In the case of a disturbed respiratory chain in liver mitochondria, the reduced intramitochondrial redox state leads to an increased 3-OH-butyrate/acetoacetate ratio. 

The malate-aspartateshuttle (Figure 5.11) involves reduction of oxaloacetate in the cytosol to malate (with the oxidation of cytosolic NADH to NAD ). Malate enters the mitochondria and is reduced back to oxaloacetate, with the reduction of intramitochondrial NAD to NADH. Oxaloacetate cannot cross the... 

To test the latter possibility mitochondria were further fractionated with digitonin and the enzymes determined in intramitochondrial compartments. As shown in Pig,3 over 80% of the activity of all three sulphur-transferases was recovered in the matrix fraction, similarly to malate and glutamate dehydrogenases. There is no indication that the sulphurtransferases are located either in the intermembrane space or in the membranes, since their activities in these fractions are the same as those of the two marker enzymes for the matrix. 

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