Arginine degradation in pig enterocytes
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Enterocytes were prepared from the jejunum of 0-, 4-, 7-, 14-, 21-day-old suckling pigs, and 29-58-day-old pigs weaned at age of 21 days. Cells were incubated at 37C for 30 min in Krebs bicarbonate buffer (pH 7.4) containing 1 mM [U-14C]arginine and 5 mM glucose, with or without 2 mM NG-nitro-L-arginine (an inhibitor of NO synthase), 2 mM gabaculine (an inhibitor of ornithine aminotransferase), or 10 mM valine (an inhibitor of arginase). Activities of major enzymes involved in arginine degradation were determined in cell extracts. Production of ornithine, proline and CO2 from arginine by enterocytes was negligible or low in newborn and suckling pigs, markedly increased in weaned pigs due to induction of arginase, and decreased 90-95% in the presence of valine or gabaculine. There was no measurable formation of glutamate, aspartate or alanine from arginine. In enterocytes of 0-58-day-old pigs, citrulline was produced from arginine by NO synthase, which was decreased 90-95% by NG-nitro-L-arginine. NO synthesis by enterocytes decreased with increasing age during the suckling period and was markedly enhanced in weaned pigs. In cells from suckling and weaned pigs, rates of conversion of arginine to NO were lower than those to the sum of ornithine, proline plus C02; the activity of NO synthase was lower than that of arginase; the activity of pyrroline-5-carboxylate (P5C) reductase was greater than that of P5C dehydrogenase; and the activity of ornithine decarboxylase was low. Our results indicate that arginine degradation by enterocytes was negligible in newborn and suckling pigs and markedly increased in weaned pigs via the arginase pathway.
