ARGININE SYNTHESIS IN ENTEROCYTES OF NEONATAL PIGS

Arginine is deficient in porcine colostrum and milk, and yet the piglet has a particularly high requirement for this essential amino acid for 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 glutamine in 0- to 7-day-old pig enterocytes, but the rates of arginine synthesis 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 conversion of citrulline to arginine, and the activities of the enzymes involved were measured. The rates of metabolism of glutamine to citrulline 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 conversion of citrulline to arginine. The activities of all enzymes that synthesize 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 enzymes decreased by similar to 75% in 7-day-old pigs compared with 8-day-old pigs. Arginase activity was negligible in enterocytes from 0- to 7-day-old pigs, thus minimizing intestinal hydrolysis of newly synthesized arginine and maximizing the endogenous provision of arginine. The results of this study demonstrate the presence of arginine-synthesizing enzymes and their developmental changes in postnatal pig enterocytes. It appears that ppyrroline-5-carboxylate synthase, carbamoyl phosphate synthase I, and ASL are the major regulatory enzymes in arginine synthesis from glutamine in these cells. The intestinal synthesis of arginine may be of physiological and nutritional importance for maintaining arginine homeostasis and supporting the pig's rapid growth during the newborn period when sow's colostrum and milk are deficient in arginine and the capacity of renal arginine synthesis may be limited by a low ASL activity.