Abstract
Alanine is synthesised de novo by skeletal muscle and released for glucose formation in the liver. For alanine to make a net contribution to the body glucose pool in times of dietary insufficiency its carbon must be derived from noncarbohydrate sources. A relationship between the. metabolism of branched-chain amino acids (BCAAs) and alanine release observed in the present study using muscle preparations incubated with clofibric acid or hypoglycin, suggested that BCAAs contributed carbon for alanine formation. A pathway for alanine formation from valine was proposed and developed. The phosphoenolpyruvate carboxykinase (PEPCK) inhibitor, 3-mercaptopicolinate, inhibited alanine formation and correlations were observed between dietary stimulation of BCAA oxidation, PEPCK activity and alanine formation. Amino-methoxybutenoate, an inhibitor which blocks mitochondrial aspartate transfer, inhibited BCAA-stimulated alanine release by muscle from 48 h-starved rats. Aspartate aminotransferase was therefore implicated in the pathway. The subcellular locations of enzymes implicated in this pathway were studied. Using fractional extraction techniques muscle branched-chain aminotransferase activity was found to be associated with the mitochondria-containing fractions. Aspartate aminotransferase, alanine aminotransferase and PEPCK were present in both the soluble and mitochondria-containing fractions. When verification of the pathway was sought, incubating muscle preparations in the presence of [U-[14]C]-valine, negligible amounts of 14C were recovered in the alanine released from the muscle. It was concluded that the pathway for formation of the alanine released from starved muscle preparations therefore does not utilise exogenous BCAA carbon for pyruvate formation to any great. extent but still involves PEPCK and aspartate aminotransferase. The source of pyruvate for alanine formation in these studies is therefore probably the amino acids derived from muscle protein breakdown.