Synthesis of citrulline and arginine from proline in enterocytes of postnatal pigs

The synthesis of ornithine, citrulline, and arginine from proline was quantified in enterocytes of newborn (0-day-old) pigs, 2- to 81-day-old suckling pigs, and 29- to 58-day-old pigs weaned at 21 days of age. Mitochondria were prepared from enterocytes for measurement of proline oxidase activity. For metabolic studies, cells were incubated at 37 degrees C for 30 min in Krebs bicarbonate buffer (pH 7.4) containing 5 mM D-glucose and 0.0-5.0 mM L-[U-C-14]proline with or without 2.0 mM L-glutamine. Proline oxidase activity and rates of synthesis of citrulline and arginine from proline were high in enterocytes of newborn pigs and markedly decreased in 7-day-old pigs. With increasing piglet age from 7 to 21 days, enterocyte proline oxidase activity and rate of conversion of proline into citrulline progressively increased, but the values in 21-day-old pigs remained much lower than in newborn pigs. The synthesis of ornithine, citrulline, and arginine from 0.5-5 mM proline was concentration dependent and required the addition of glutamine. About 80-90% of utilized proline carbons were recovered in ornithine plus citrulline plus arginine, with CO2 being a minor product. These results demonstrate a hitherto unrecognized pathway for the synthesis of ornithine, citrulline, and arginine via proline oxidase in enterocytes and challenge the current concept that pyrroline-5-carboxylate (P5C) is synthesized in the small intestine only from glutamine/glutamate via P5C synthase on the basis of previous work with postnatal rats.