ARGININE-METABOLIZING ENZYMES IN THE DEVELOPING RAT SMALL-INTESTINE

Before weaning, arginine biosynthesis from citrulline most likely take s place in the small intestine rather than in the kidney. We studied t he expression of ornithine cycle enzymes in the rat small intestine du ring perinatal development. The spatiotemporal patterns of expression of ornithine aminotransferase, carbamoylphosphate synthetase, ornithin e transcarbamoylase, argininosuccinate synthetase, argininosuccinate l yase, and arginase mRNAs were studied by Northern blot analysis and in situ hybridization. In addition, the expression of carbamoylphosphate synthetase and argininosuccinate synthetase protein was studied by im munohistochemistry. Before birth, the developmentally more mature prox imal loops of the intestine expressed the mRNAs at higher concentratio ns than the more distal loops. After birth, this difference was no lon ger obvious. The mRNAs of argininosuccinate synthetase and argininosuc cinate lyase, the enzymes that metabolize citrulline to arginine, were detectable only in the upper part of the villi, whereas the other mRN As were concentrated in the crypts. The distribution of argininosuccin ate synthetase protein corresponded with that of the mRNA, whereas car bamoylphosphate synthetase protein was present in all enterocytes of t he crypts and villi. Hepatic arginase mRNA could not be detected in th e enterocytes. The spatial distribution of the respective mRNAs and pr oteins along the villus axis of the suckling small intestine indicates that the basal enterocytes synthesize citrulline, whereas the enteroc ytes in the upper half of the villus synthesize arginine.