Transport of l-leucine hydroxy analogue and l-lactate in rabbit small-intestinal brush-border membrane vesicles

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Abstract
Substitution of the α-amino group of amino acids by hydroxyl groups yields hydroxy analogues (HA), which have been ascribed beneficial effects in nitrogensparing diets for uremic patients. In this study, intestinal uptake of l-leucine HA (l-LeuHA) and l-lactate into rabbit jejunal brush-border membrane vesicles was investigated. An inward-directed H+ or Na+ gradient stimulated uptake of both labelled substrates in a voltageclamped assay. The H+ gradient was the major driving force of uptake as compared with the Na+ gradient, and it led to a transient accumulation of both l-LeuHA and l-lactate. The proton ionophore carbonylcyanide p trifluoromethoxyphenylhydrazone (FCCP) reduced the initial H+-gradient-driven uptake rates of both substrates, but was without effect on Na+-gradient-driven uptakes. The H+-gradient-driven l-LeuHA uptake was saturable (apparent Kt = 15.4 mM). α-HA of l-leucine, l-isoleucine, l-valine, d-leucine, d-valine or l-lactate inhibited the H+-gradient-driven l-LeuHA or l-lactate uptakes whereas free branched-chain amino acids had no effect. Preloading the vesicles with one of the l-or d-HA of branched-chain amino acids or with l-lactate stimulated tracer l-LeuHA and also tracer l-lactate uptakes in the presence of a H+ gradient. It is concluded that H+-gradient-driven transport of l- and d-stereoisomeric HA of branched-chain amino acids as well as of l-lactate across rabbit intestinal brush-border membranes is mediated by the same carrier. Furthermore, there exists a Na+gradient-driven l-lactate transport system in the rabbit intestinal brush-border membrane.