Intramitochondrial transport

Emoto, K., Kuge, O., Nishijima, M., and Umeda, M., 1999, Isolation ofa Chinese hamster ovary ceh mutant defective in intramitochondrial transport of phosphatidylserine. Proc. Natl. Acad. Sci. USA, 96 12400-12405. 

Fig. 1. Factors involved in the intramitochondrial transport of cholesterol. Left, membrane fusion stimulating reversed hexagonal phase formation right, permeation of cholesterol across membranes (from Ref. 25, with permission).

Carbamyl phosphate condenses with ornithine to yield citrulline in the ornithine transcarbamylase (OTC) reaction. OTC is encoded on band p21.1 of the X chromosome, where the gene contains 8 exons and spans 85 kb of DNA. The activity of this enzyme is directly related to dietary protein. There may be tunneling of ornithine transported from the cytosol to OTC, with the availability of intramitochondrial ornithine serving to regulate the reaction. 

In the third and final step of the carnitine shuttle, the fatty acyl group is enzymatically transferred from carnitine to intramitochondrial coenzyme A by carnitine acyltransferase II. This isozyme, located on the inner face of the inner mitochondrial membrane, regenerates fatty acyl-CoA and releases it, along with free carnitine, into the matrix (Fig. 17-6). Carnitine reenters the intermembrane space via the acyl-camitine/car-nitine transporter. 

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. 

Mitochondria from adult H. diminuta exhibit an NADH-coupled fumarate reductase (Table 5.11). This presents a potential dilemma with respect to the utilisation of intramitochondrial reducing equivalents by this worm. As reducing equivalents are generated by the malic enzyme in the form of NADP, a mechanism for the transfer of hydride ions from NADPH to NAD to produce NADH is required so that electron-transport-associated activities can proceed and terminate with the reduction of fumarate to succinate. Such a mechanism does exist in H. diminuta as there is a non-energy-linked, membrane-associated transhydrogenase (214, 217, 221, 476). This transhydrogenase, which also occurs in H. microstoma (216) and Spirometra mansonoides (220) catalyses the reaction ... 

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.)...

Figure 19.11 Relationship between extra- and intramitochondrial acetyl-CoA and the transport of acetyl-CoA in the form of citrate across the mitochondrial membrane.

A specific transport protein, the carnitine-acylcarnitine translocase, moves the fatty acylcarnitine into the mitochondrial matrix while returning carnitine from the matrix to the cytoplasm. Once inside the mitochondria, another enzyme, carnitine palmitoyltransferase II (CPT II), located on the matrix side of the mitochondrial inner membrane, catalyzes the reconversion of fatty acylcarnitine to fatty acyl-CoA. Intramitochondrial fatty acyl-CoA then undergoes (3-oxidation to generate acetyl-CoA.Acetyl-CoA can enter the Kreb s cycle for complete oxidation or, in the liver, be used for the synthesis of acetoacetate and P-hydroxybutyrate (ketone bodies). 

Primary carnitine deficiency is caused by a deficiency in the plasma-membrane carnitine transporter. Intracellular carnitine deficiency impairs the entry of long-chain fatty acids into the mitochondrial matrix. Consequently, long-chain fatty acids are not available for p oxidation and energy production, and the production of ketone bodies (which are used by the brain) is also impaired. Regulation of intramitochondrial free CoA is also affected, with accumulation of acyl-CoA esters in the mitochondria. This in turn affects the pathways of intermediary metabolism that require CoA, for example the TCA cycle, pyruvate oxidation, amino acid metabolism, and mitochondrial and peroxisomal -oxidation. Cardiac muscle is affected by progressive cardiomyopathy (the most common form of presentation), the CNS is affected by encephalopathy caused by hypoketotic hypoglycaemia, and skeletal muscle is affected by myopathy. 

X1B-X14), and NADP-linked isocitrate dehydrogenase SIS, S16). Numerous studies have been carried out with isolated mitochondria to identify the source of NADPH for 11 -hydroxylation by studying the effect of inhibitors and uncouplers of oxidative phosphorylation in the presence of different hydrogen donors (83, 19S, SIO, SIS, SI4-SSS). Part of this work was inconclusive since several complicating features in the metabolism of adrenocortical mitochondria were insufficiently taken into account such as secondary inhibitory effects of the substrates, inhibitors, or uncouplers used 83, SI4, SSO, SSS) the requirement of transport of substrates across the mitochondrial membrane SS3, SS4) and the possibility of intramitochondrial dismutation reactions SS3). 

Studies performed by the authors of this review [142,144], suggest that the discrepancies are due to functional microcompartmentation between the aspartate aminotransferase and the aspartate transporter. The apparent for aspartate efflux can be dramatically decreased by generation of intramitochondrial aspartate by the aminotransferase reaction. Detailed isotopic studies using labelled matrix aspartate in liver mitochondrial [142] and labelled intramitochondrial glutamate in kidney mitochondria [144] confirmed the initial suggestion. 

Interpretation of the validity of the near-equilibrium concept is dependent on the accuracy of intramitochondrial free NAD/NADH measurements and the difference between extra- and intramitochondrial phosphorylation potentials. In a series of studies, Wilson and associates [40,41,225,226] have presented evidence in support of their hypothesis. Utilizing rat liver mitochondria, Forman and Wilson [225] compared the mass action ratios to calculated equilibrium constants under conditions promoting either forward (net ATP synthesis) or reversed (net ATP hydrolysis) electron transport. Since the mass action ratios calculated under various conditions were similar to the calculated K, these findings were said to support a near-equi-... 

By increasing substrate availability through stimulation of mitochondrial respiration. This decreases the intramitochondrial concentration of H and increases the rate of pyruvate transport. The [ATP]/[ADP] ratio also rises. 

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