SYNTHESIS OF CITRULLINE FROM GLUTAMINE IN PIG ENTEROCYTES

The synthesis of citrulline from glutamine was quantified in enterocyt es from pre-weaning(14-21 days old) and post-weaning (29-58 days old) pigs. The cells were incubated at 37 degrees C for 30 min in Krebs-Hen seleit bicarbonate buffer (pH 7.4) containing 0, 0.5, 2 and 5 mM gluta mine. Oxygen consumption was linear during the 30 min incubation perio d. The rates of citrulline synthesis were low or negligible in enteroc ytes from 14-21-day-old pigs, but increased 10-20-fold in the cells fr om 29-58-day-old pigs. This marked elevation of citrulline synthesis c oincided with an increase in the activity of pyrroline-5-carboxylate s ynthase with the animal's post-weaning growth. In contrast, decreases in the activities of phosphate-dependent glutaminase, ornithine aminot ransferase, ornithine carbamoyltransferase and carbamoyl-phosphate syn thase were observed as the age of the pigs increased. The concentratio ns of carbamoyl phosphate in enterocytes from pre-weaning pigs were hi gher than. or similar to, those in the cells from post-weaning pigs. I t is possible that the low rate of citrulline synthesis from glutamine in enterocytes from pre-weaning pigs was due to a limited availabilit y of ornithine, rather than that of carbamoyl phosphate. We suggest th at this limited availability of ornithine in pre-weaning-pig enterocyt es results from (i) the low rate of pyrroline-5-carboxylate synthesis from glutamate, due to the low activity of pyrroline-5-carboxylate syn thase, and (ii) the competitive conversion of pyrroline-5-carboxylate into proline. Our present findings on the developmental aspect of citr ulline synthesis in pig enterocytes may offer a biochemical mechanism for the previous observations that arginine is a nutritionally essenti al amino acid for suckling piglets, but not for adult pigs.