Mercaptopropionate inhibits butyrate uptake in isolated apical membrane vesicles of the rat distal colon.

Abstract

Previous observations have shown that mercapto- and bromo- short-chain fatty acids diminish fatty acid use in colonic epithelium. The aim of this study was to investigate whether this effect is attributable to the inhibition of short-chain fatty acid uptake. Apical membrane vesicles of rat colonocytes were prepared by a discontinuous sucrose gradient after isolation of membrane caps. [14C]butyrate uptake was measured by rapid filtration technique. Preloading of isolated apical membrane vesicles with bicarbonate or butyrate stimulated [14C]butyrate uptake and resulted in up to fivefold overshoots. Increasing extravesicular butyrate concentrations saturated the bicarbonate-stimulated butyrate uptake with a binding constant of 44.7 +/- 5.9 mmol/L and a maximum velocity of 33.2 +/- 2.7 nmol.mg protein-1.3 s-1. Intravesicular butyrate uptake was inhibited by addition of 20 mmol/L 3-mercaptopropionate (43.0% +/- 5.6%), whereas 2-bromo-propionate (13.9% +/- 4.1%) and 4-bromobutyrate (22.6% +/- 5.3%) did not significantly alter butyrate uptake. Increasing concentrations of 3-mercaptopropionate had a competitive inhibitory effect on butyrate uptake with a binding constant following inhibition of 6.25 +/- 0.87 mmol/L and a maximum velocity of 5.82 +/- 1.01 nmol.mg protein-1.3 s-1. Butyrate uptake in apical membrane vesicles of rat distal colon is mediated by a low-affinity anion transport system, which can be competitively inhibited by 3-mercaptopropionate but not by 2-bromopropionate and 4-bromobutyrate.