Metabolism of D- and L-[(13)C]alanine in rat liver detected by (1)H and (13)C NMR spectroscopy in vivo and in vitro.

Alanine is the major amino acid utilized by the liver for gluconeogenesis under normal conditions. The metabolism of alanine in rat liver was investigated by means of (1)H and (13)C NMR spectroscopic studies in vivo and in vitro after infusion of L- and D-alanine labelled with (13)C at the carboxyl and methyl group into normal, fasted rats. Valuable information about different metabolic pathways of alanine in rat liver and their regulation under the conditions of gluconeogenesis were obtained. The enrichment of the alanine pool by the infusate was estimated to be 11% for L-alanine and 70% for D-alanine. After infusion of labelled D-alanines, the metabolic pathway via D-amino acid oxidase was observed. The labelled alanines entered the tricarboxylic acid cycle mainly via pyruvate carboxylase; the ratio of pyruvate dehydrogenase activity to that of pyruvate carboxylase is about 28%. The ratio of flux from phosphoenolpyruvate (PEP) through phosphoenolpyruvate kinase as compared with the flux from PEP to glucose was approximately 42%. From the labelling pattern of glucose it was concluded that the pentose phosphate cycle was active under the experimental conditions.