The characterization of butyrate transport across pig and human colonic luminal membrane
Corresponding author S. P. Shirazi-Beechey: Epithelial Function and Development Group, Department of Veterinary PreClinical Sciences, The University of Liverpool, Brownlow Hill and Crown Street, Liverpool L69 3BX, UK. Email: firstname.lastname@example.org
- 1Luminal membrane vesicles (LMV) were isolated from human and pig colonic tissues. They were characterized in terms of purity and ability to transport [14C]butyrate.
- 2The activity of cysteine-sensitive alkaline phosphatase, and the abundance of villin, NHE2 and NHE3 proteins, markers of the colonic luminal membrane, were significantly enriched in the LMV compared with the original cellular homogenate. The LMV were free from contamination by other cellular organelles and basolateral membranes, as revealed by the negligible presence of either specific marker enzyme activity or characteristic immunogenic protein.
- 3The transport of butyrate into the luminal membrane vesicles was enhanced 5-fold at pH 5.5 compared with pH 8.0. Butyrate transport was temperature dependent, and was stimulated in the presence of an outward-directed anion gradient in the order of butyrate > bicarbonate > propionate > chloride. Kinetic analysis of increasing substrate concentration showed saturation kinetics with an apparent Km value of 14.8 ± 3.6 mM and a Vmax of 54 ± 14 nmol min−1 (mg protein)−1.
- 4Butyrate transport was significantly reduced in the presence of short chain fatty acids (SCFA), acetate, propionate and other monocarboxylates (pyruvate and L-lactate). Butyrate uptake was inhibited by several cysteine group modifying reagents such as p-chloromercuribenzosulphonic acid (pCMBS), p-chloromercuribenzoate (pCMB), mersalyl acid and HgCl2, but not by the stilbene anion exchange inhibitors, 4,4′-diisothiocyanostilbene-2,2′-disulphonate (DIDS) and 4,4′-dinitrostilbene-2,2′-disulphonate (SITS).
- 5The described properties of butyrate transport across the luminal pole of the colon suggest the involvement of a carrier protein, in the form of a pH-activated anion exchange process. The transporter is distinct from the erythrocyte band-3 type anion exchanger and may belong to the monocarboxylate-type transport proteins (MCT1).