ARGININE SYNTHESIS IN ENTEROCYTES OF NEONATAL PIGS

Arginine is deficient in porcine colostrum and milk, and yet the pigle t has a particularly high requirement for this essential amino acid fo r rapid postnatal growth. To explain this paradox, arginine synthesis was quantified in enterocytes from newborn (0-day-old) and 2- to 7-day -old suckling pigs. Arginine was found to be synthesized from glutamin e in 0- to 7-day-old pig enterocytes, but the rates of arginine synthe sis were three- to fourfold greater in 0- to 8-day-old pigs than in 7- day-old pigs. To elucidate the developmental change of the intestinal arginine synthesis, the metabolism of glutamine to citrulline, the con version of citrulline to arginine, and the activities of the enzymes i nvolved were measured. The rates of metabolism of glutamine to citrull ine were 2.5- to 3.5-fold greater in enterocytes from 0- to 2-day-old pigs than in cells from 7-day-old pigs, as were the rates of conversio n of citrulline to arginine. The activities of all enzymes that synthe size arginine from glutamine, except pyrroline-5-carboxylate synthase and argininosuccinate lyase (ASL), increased in enterocytes from 2-day -old pigs compared with 0-day-old pigs. The activities of all these en zymes decreased by similar to 75% in 7-day-old pigs compared with 8-da y-old pigs. Arginase activity was negligible in enterocytes from 0- to 7-day-old pigs, thus minimizing intestinal hydrolysis of newly synthe sized arginine and maximizing the endogenous provision of arginine. Th e results of this study demonstrate the presence of arginine-synthesiz ing enzymes and their developmental changes in postnatal pig enterocyt es. It appears that ppyrroline-5-carboxylate synthase, carbamoyl phosp hate synthase I, and ASL are the major regulatory enzymes in arginine synthesis from glutamine in these cells. The intestinal synthesis of a rginine may be of physiological and nutritional importance for maintai ning arginine homeostasis and supporting the pig's rapid growth during the newborn period when sow's colostrum and milk are deficient in arg inine and the capacity of renal arginine synthesis may be limited by a low ASL activity.