Characterization of L-leucine transport system in brush border membranes from human and rabbit small intestine

The branched-chain amino acids (BCAAs) leucine, isoleucine, and valine are beneficial to catabolic patients by improving hepatic protein synthesis and nitrogen economy, yet their transport from the intestinal lumen is not well-defined. The leucine transport system in human and rabbit small intestine was characterized using a brush border membrane vesicle (BBMV) model. Sodium and pH dependence and transport activity along the longitudinal axis of the small bowel were determined. Transport kinetics and inhibition profiles were defined. Although previous studies in other tissues show leucine transport to be mostly a Na⁺-independent process, our studies show that leucine transport is a predominantly Na⁺-dependent process occurring mainly via a single saturable pH-independent transporter resembling system B⁰ in the intestine. This system B⁰ transporter demonstrates stereoisomeric specificity. There is also a minor Na⁺-independent transport component (<6% in rabbits). Leucine uptake in both rabbits and humans is significantly greater than the uptake of other clinically relevant nutrients such as glutamine. In the rabbit, ileal leucine transport is significantly greater than jejunal uptake. While the affinities of the human and rabbit transporters are similar, the rabbit transporter has greater carrier capacity (maximal transport velocity [V(max)]). These findings suggest that the transport of leucine in the gut mucosa is significantly different from the transport in other tissues.