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1995). The obvious routes by which this occurs are either transamination with pyruvate (both in the cytosol and in the mitochondria via alanine aminotransferase, to form alanine and α-ketoglutarate) or the glutamate dehydrogenase reaction, which has recently been discovered (Wibom et al., 1990) to have a somewhat greater capacity in human skeletal muscle than hitherto suspected. There is evidence that in the absence of any other mechanism to increase Krebs cycle intermediates (as in patients with McArdle's disease who are unable to generate oxaloacetate from endogenous glycogen stores via the malic enzyme, pyruvic carboxylase, and PEP carboxykinase), muscle glutamate concentration is reduced at rest, and work capacity appears to be limited when glutamate catabolism bottoms out (Sahlin et al., 1995).

The fall in muscle glutamate is puzzling. Branched chain amino acids are transaminated in the cytoplasmic space to form the branched chain ketoacids which are decarboxylated in mitochondria (see below). The carbon from valine and half of that from isoleucine may enter the Krebs cycle as succinyl CoA, processes which are therefore anaplerotic. The puzzling thing is that branched chain amino acids are transaminated at the expense of α-ketoglutarate to produce glutamate, which ought to protect muscle glutamate concentrations and to deplete α-ketoglutarate. The depletion of α-ketoglutarate would be catapleurotic if it extended to the mitochondria. We currently have no information on this.

What other ways can amino acids contribute to the anaplerotic process? Glutamine crosses the inner mitochondrial membrane with much greater ease than glutamic acid and the mitochondrial phosphate-dependent glutaminase would ensure a plentiful supply of glutamate without the necessity of exchanging glutamate for aspartate across the inner mitochondrial membrane. However, one would then expect to see a fall in muscle glutamine concentration, which is, in fact, usually only seen after long term exercise at moderate intensity at least. However, given the very high background of glutamine and the relative imprecision of measurement of glutamine, it is difficult to be sure about the size of the fall which occurs during heavy exercise. The other possible route of glutamine utilisation and production of α-ketoglutarate would be through the action of glutamine transaminase and to produce α-ketoglutaramide which spontaneously deaminates to produce α-ketoglutarate and ammonia. Unfortunately there is no good data to strengthen or weaken this suggestion. More research is needed on this topic.

The branched-chain amino acids are oxidised in muscle during exercise (Figure 11-1) at a rate which appears to be directly proportional to the overall rate of mitochondrial oxidation, and thus the muscle oxygen uptake possibly because oxygen uptake is blood flow dependent; so is catabolism of the



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