Characterization of l-leucine transport systemin 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 wel l-defined. The leucine transport system in human and rabbit small intestine was characterized using a brush border membrane vesicle (BBMV) model. Sodi um 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 tran sport to be mostly a Nac-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 B-0 in the in testine. This system B-0 transporter demonstrates stereoisomeric specificit y. There is also a minor Nai-independent transport component (<6% in rabbit s). Leucine uptake in both rabbits and humans is significantly greater than the uptake of other clinically relevant nutrients such as glutamine. In th e rabbit, ileal leucine transport is significantly greater than jejunal upt ake. While the affinities of the human and rabbit transporters are similar, the rabbit transporter has greater carrier capacity (maximal transport vel ocity [V-max]). These findings suggest that the transport of leucine in the gut mucosa is significantly different from the transport in other tissues. Copyright (C) 1999 by W.B. Saunders Company.