An Important Role for Pentose Cycle in the Synthesis of Citrulline and Proline from Glutamine in Porcine Enterocytes

This study was designed to determine a role of pentose cycle in the provision of NADPH for the synthesis of citrulline and proline from glutamine in porcine enterocytes. Enterocytes from 4-day-old pigs were incubated at 37°C for 0 to 30 min in Krebs–Henseleit bicarbonate buffer (pH 7.4) containing 2 mMglutamine and 5 mMglucose in the presence of 0, 0.1, or 0.25 mMdehydroepiandrosterone (DHEA), a potent inhibitor of glucose-6-phosphate dehydrogenase which is the key regulatory enzyme of pentose cycle. The activity of this cycle was estimated with the use of [1-14C]glucose and [6-14C]glucose. In some experiments, the medium included 2 mMornithine and 2 mMNH4Cl (no glutamine). About 14% of glucose taken up by enterocytes was metabolized via pentose cycle. The flux from glucose into this cycle was decreased by 70 and 86%, respectively, in the presence of 0.1 and 0.25 mMDHEA compared with its absence. DHEA inhibited the synthesis of ornithine, citrulline, arginine, and proline from glutamine in a concentration-dependent manner, but had no effect on the formation of citrulline and arginine from ornithine. However, DHEA decreased the synthesis of proline from ornithine by 79%. DHEA had no effect on cellular ATP concentrations. These results provide the first line of evidence suggesting that glucose metabolism via pentose cycle plays an important role in providing NADPH for the conversion of glutamine into pyrroline-5-carboxylate in porcine enterocytes, which may explain the glucose-dependent synthesis of citrulline and proline from glutamine in these cells.