Metabolism of absorbed aspartate, asparagine, and arginine by rat small intestine in vivo.

b[U-‘4Claspartate, L-[U-‘4C]asparagine, and L-[U-“‘Clarginine were administered luminally into isolated segments of rat jejunum in situ, and the radioactive products appearing in venous blood from the segment were identified and quantified, in a continuation of similar studies with L-glutamate and L-glutamine (Windmueller, H. G., and Spaeth, A. E. (1975) Arch. Biochem. Biophys. 171, 662-672). Aspartate, administered alone (6 rn@ or with 18 other amino acids plus glucose, was absorbed more rapidly than glutamate, but, as with glutamate, less than 1% was recovered intact in intestinal venous blood. More than 50% of aspartate carbon was recovered in CO,, 24% in organic acids, mostly lactate, 12% in other amino acids (alanine, glutamate, proline, ornithine, and citrulline), and 10% in glucose, apparently the first demonstration of gluconeogenesis by intestine in uiuo. In contrast to aspartate and glutamine, nearly all asparagine was absorbed intact, less than 1% being catabolized. About 4% of the absorbed dose was incorporated into the acid-insoluble fraction of intestine, as was the case with all the amino acids studied. In conventional or germ-free rats, only 60% of arginine was absorbed intact, while 33% was hydrolyzed to ornithine and urea. The urea and 38% of the ornithine were released into the blood; the remaining ornithine was metabolized further by intestine to citrulline, proline, glutamate, organic acids, and CO,. Catabolism of several amino acids from the lumen plus glutamine from arterial blood may provide an important energy source in small intestine.