Lysine and methionine transport by bovine jejunal and ileal brush border membrane vesicles

Purified brush border and basolateral membranes were isolated from homogenized intestinal enterocytes of Holstein steers by divalent cation precipitation followed by differential and sucrose density gradient centrifugation. Membrane marker enzymes were used to determine the effectiveness of the fractionation procedure. Alkaline phosphatase and sodium-potassium adenosine triphosphatase served as the marker enzymes for the brush border and basolateral membranes, respectively. The brush border fraction was enriched 5.1-fold over the cellular homogenate. Purification of 10.1-fold over cellular homogenate was obtained for the basolateral membrane fraction. Electronmicrographs and osmotic response data were used to confirm the vesicular nature of the membrane preparations. Brush border membrane vesicles from bovine jejunal and ileal tissue were used to evaluate lysine (LYS) and methionine (MET) transport. Total transport of LYS and MET was divided into mediated and diffusion components. Mediated uptake was further divided into sodium-dependent (Na⁺) and sodium independent (Na⁻) systems. Total LYS and MET uptake by ileal tissue tended to be higher than jejunal tissue at all concentrations evaluated but differences were significant (P<.O5) at 2.5 and 7.5 mM for LYS and 5, 12.5 and 15 mM for MET. The greater capacity of ileal tissue appeared to be due to the Na⁺ component of LYS uptake and the diffusion component of MET uptake. Methionine transporters had lower affinities and higher capacities than the corresponding LYS transporters in both ileal and jejunal tissue. Methionine transport was greater (P<.O5) than LYS transport in both ileal and jejunal tissue when initial amino acid concentration was 7.5 mM. When initial amino acid concentration was 1.25 mM, MET uptake was greater (P<.13) than LYS uptake in jejunal but not ileal tissue. The relative contribution of mediated and diffusion uptake systems to total MET and LYS uptake was found to be dependent of substrate concentration.