Lactate inhibits citrulline and arginine synthesis from proline in pig enterocytes

Hypocitrullinemia and hypoargininemia but hyperprolinemia are associated wi th elevated plasma concentration of lactate in infants. Because the small i ntestine may be a major organ for initiating proline catabolism via proline oxidase in the body and is the major source of circulating citrulline and arginine in neonates, we hypothesized that lactate is an inhibitor of intes tinal synthesis of citrulline and arginine from proline. To test this hypot hesis, jejunum was obtained from 14-day-old suckling pigs for preparation o f enterocyte mitochondria and metabolic studies. Mitochondria were used for measuring proline oxidase activity in the presence of 0-10 mM L-lactate. F or metabolic studies, enterocytes were incubated at 37 degrees C for 30 min in Krebs bicarbonate buffer (pH 7.4) containing 5 mM D-glucose, 2 mM L-glu tamine, 2 mM L-[U-C-14]proline, and 0, 1, 5, or 10 mM L-lactate. Kinetics a nalysis revealed noncompetitive inhibition of intestinal proline oxidase by lactate (decreased maximal velocity and unaltered Michaelis constant). Lac tate had no effect on either activities of other enzymes for arginine synth esis from proline or proline uptake by enterocytes but decreased the synthe sis of ornithine, citrulline, and arginine from proline in a concentration- dependent manner. These results demonstrate that lactate decreased intestin al synthesis of citrulline and arginine from proline via an inhibition of p roline oxidase and provide a biochemical basis for explaining hyperprolinem ia, hypocitrullinemia, and hypoargininemia in infants with hyperlactacidemi a.