The origin of alanine produced in skeletal muscle.

These studies were undertaken to clarify the origin of alanine released in large amounts by skeletal muscle. Leutine, a ketogenic amino acid, as well as isoleucine and valine, which are glucogenic, increased the intracellular concentration of glutamate and stimulated the production of alanine and glutamine by isolated rat diaphragms. The extent of transamination of these amino acids (i.e. the sum of cy-keto acid decarboxylation and accumulation) equalled the total increase in alanine, glutamine, and glutamate. Thus the branched-chain amino acids may stimulate alanine and glutamine production simply by supplying amino groups for transamination with cY-ketoglutarate. Providing 0.3 mM NH,+ increased glutamine production because of greater conversion of glutamate to glutamine. The fall in cellular glutamate concentration can explain the associated decrease in alanine production. The production of alanine and glutamine is limited by the supply of carbon chains as well as amino groups. Glucose stimulated alanine production by rat diaphragms. The stimulation was larger when the muscle was also provided with branched-chain amino acids. In addition, Z-deoxyglucose, which inhibited glycolysis, reduced alanine production. Iodoacetate and fluoride also decreased alanine and lactate production. However, observations with the latter inhibitors (unlike those with 2-deoxyglucose) are complicated because they also decreased transamination by inhibiting the oxidative decarboxylation of branched-chain amino acids and simultaneously promoted net protein breakdown. Most of the amino acids in muscle protein have been suggested as sources of the carbons in alanine and glutamine. Experiments with diaphragm, soleus, and extensor digitorum longus, however, showed that only certain I%labeled amino acids could be converted to “C02. Of these, only five amino acids, aspartate, asparagine, glutamine, isoleucine, and valine are converted to tricarboxylic acid cycle intermediates, and only these amino acids could possibly supply the carbon skeleton for alanine or glutamine. The rate of generation of these five amino acids by the net