Metabolic control mitochondrial metabolism

The well-known fact that in irreversibly damaged cells, respiratory control is lost and is accompanied by oxidation of cytochromes a and as, as well as NADH (Taegtmeyer et al., 1985), was originally thoug it to be due to substrate deficiency (Chance and Williams, 1955) but may be due to an enzymatic defect resulting in an inability to metabolize NADH-linked substrates (Pelican etal., 1987). It seems likely therefore that return of function is dependent on preservation of mitochondrial membrane integrity, and the structure and activities of respiratory chain (R.C) complexes I-IV (Chance and Williams, 1955). 

Long-chain fatty acids can slowly cross the mitochondrial membrane by themselves, but this is too slow to keep up with their metabolism. The carnitine shuttle provides a transport mechanism and allows control of (3 oxidation. Malonyl-CoA, a precursor for fatty acid synthesis, inhibits the carnitine shuttle and slows down (3 oxidation (Fig. 13-5). 

Defects of nuclear DNA also cause mitochondrial diseases. As mentioned above, the vast majority of mitochondrial proteins are encoded by nDNA, synthesized in the cytoplasm and imported into the mitochondria through a complex series of steps. Diseases can be due to mutations in genes encoding respiratory chain subunits, ancillary proteins controlling the proper assembly of the respiratory chain complexes, proteins controlling the importation machinery, or proteins controlling the lipid composition of the inner membrane. All these disorders will be transmitted by mendelian inheritance. From a biochemical point of view, all areas of mitochondrial metabolism can be affected (see below). 

There are many examples of phosphorylation/dephosphorylation control of enzymes found in carbohydrate, fat and amino acid metabolism and most are ultimately under the control of a hormone induced second messenger usually, cytosolic cyclic AMP (cAMP). PDH is one of the relatively few mitochondrial enzymes to show covalent modification control, but PDH kinase and PDH phosphatase are controlled primarily by allosteric effects of NADH, acetyl-CoA and calcium ions rather than cAMP (see Table 6.6). 

Where two enzymes compete for the same substrate, we expect to see some form of metabolic control and in this case the concentrations of NADH and acetyl-CoA are the key controlling factors (Figure 6.44). When glucose is not available as a fuel, metabolism switches to 3- oxidation of fatty acids, which generates more than sufficient quantities of both NADH and acetyl-CoA to drive the TCA cycle and to maintain oxidative phosphorylation. Pyruvate dehydrogenase activity is suppressed and pyruvate carboxylase is stimulated by ATP, NADH and acetyl-CoA (strictly speaking by low mitochondrial ratios of ADP/ATP, NAD+/NADH and coenzyme A/acetyl-CoA), so... 

The cells are permitted to "plant" to the ECM and adjust to the incubator temperature (37°C) and C02 concentration. Then test compounds or controls (both in 0.1% dimethyl sulfoxide, DMSO) are added to the test wells. The cells are then incubated overnight, and the indicator dye Alamar blue10 is added. This noncytotoxic dye reacts to mitochondrial redox reactions and is measured fluorometrically. Cell metabolic activity is determined starting at 3 h after the dye is added and daily thereafter. 

Today s mitochondria lack most of the genes involved in phosphohpid metabolism. Therefore, mitochondria have to import most of their hpids. Phospholipids such as phosphatidylcholine, phosphatidylserine, phosphatidylglycerol, and phosphatidylinositol must be synthesized in the endoplasmatic reticulum under the control of nuclear genes and then transferred to mitochondria (Voelker, 2000) (Figure 1). Mitochondria use both nuclear and mitochondrial encoded proteins to further diversify phospholipids (Dowhan, 1997 Kent, 1995 Daum, 1985). Thus, a nuclear phosphatidylserine decarboxylase converts phosphatidylserine into phosphatidylethanolamine, or mitochondrial encoded cardiolipin synthase converts phosphatidylglycerol into cardiolipin which is incorporated exclusively into the inner mitochondrial membrane. 

Gerhart-Hines, Z., Rodgers, J.T, Bare, 0 Lerin, C., Kim, S.H., Mostoslavsky, R., Alt, F.W., Wu, Z. and Puigserver, P. (2007) Metabolic control of muscle mitochondrial function and fatty acid oxidation through SIRTl/PGC-lalpha. The EMBO Journal, 26, 1913-1923. 

Poole1305 has reviewed the bacterial cytochrome oxidases, and has drawn attention to features which are not present in the mitochondrial enzyme, and which reflect the metabolic diversity and adaptability of bacteria. These are (1) the synthesis of the oxidases is controlled dramatically by the prevailing environmental conditions (2) some oxidases are multifunctional, and may use electron acceptors other than dioxygen (3) more than one type of oxidase may be present, each terminating a branched electron-transfer pathway. 

Halestrap, A.P. (1989). The regulation of the matrix volume of mammalian mitochondria in vivo and in vitro and its role in the control of mitochondrial metabolism. Biochim. Biophys. Acta 973, 355-382. 

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