GLUTAMINE-METABOLISM IN CHICK ENTEROCYTES - ABSENCE OF PYRROLINE-5-CARBOXYLASE SYNTHASE AND CITRULLINE SYNTHESIS

This study was designed to determine whether pyrroline-5-carboxylate ( P-5-C) synthase is deficient in chick enterocytes therefore resulting in the lack of synthesis of ornithine and citrulline from glutamine. P ost-weaning pig enterocytes, which are known to contain P-5-C synthase and to synthesize both ornithine and citrulline from glutamine, were used as positive controls. Enterocytes were incubated at 37 degrees C for 0-30 min in the presence of 2 mM [U-C-14]glutamine or 2 mM ornithi ne plus 2 mM NH4Cl. In chick enterocytes, glutamine was metabolized to NH3, CO2, glutamate, alanine and aspartate, but not to ornithine, cit rulline, arginine or proline. Likewise, there was no formation of citr ulline, arginine, alanine or aspartate from ornithine in chick enteroc ytes. Furthermore, the rate of conversion of ornithine into proline in chick enterocytes was only about 4% of that in cells from pigs. To el ucidate the reason for the inability of chick enterocytes to synthesiz e ornithine and citrulline from glutamine, the activities of the enzym es involved were measured. No activity of P-5-C synthase or ornithine carbamoyltransferase was found in chick enterocytes, in contrast with cells from postweaning pigs. It was also demonstrated that the activit y of ornithine aminotransferase in chick enterocytes was only 3 % of t hat in cells from pigs. Thus the present findings elucidate the bioche mical reason for the lack of endogenous synthesis of ornithine and cit rulline in chicks. Our results also explain previous observations that ornithine cannot replace arginine or proline in the diet of chicks. W e suggest that the absence of P-5-C synthase and ornithine carbamoyltr ansferase in enterocytes is the metabolic basis for the nutritional re quirement of arginine in the chick.