Gene expression and activity of enzymes in the arginine biosynthetic pathway in porcine fetal small intestine.

We recently reported the presence of ornithine aminotransferase (OAT) enzymatic activity and mRNA expression in the intestine of fetal pigs from 30 to 110 d of gestation. Here we describe the activities and mRNA expression patterns of other key enzymes in the arginine biosynthetic pathway, specifically carbamoyl phosphate synthase I (CPS-I), ornithine carbamoyl transferase (OCT), and pyrroline-5-carboxylate reductase (P5CR), in the fetal porcine small intestine from 30 to 110 d of gestation. The activities of all three enzymes increased from d 30 to d 110 of gestation, and in situ hybridization demonstrates that 1) CPS-I and OCT genes are expressed in distinct patterns and are confined to the mucosal epithelium and 2) P5CR mRNA is present in mucosal epithelium and lamina propria of the fetal porcine small intestine from d 30 to d 110 of gestation. The presence of CPS-I and OCT in conjunction with the presence of OAT suggests that the fetal porcine small intestine is capable of synthesizing citrulline from P5C. In addition, the presence of P5CR suggests that the fetal porcine small intestine is able to synthesize proline from ornithine via OAT. This ability of the fetal small intestine to synthesize amino acids may be important for development and metabolic activity of the intestine during somatic growth of the fetus.