The intestinal transport of zinc studied using brush-border-membrane vesicles from the piglet

Brush-border-membrane vesicles were prepared from piglet small intestines and the uptake of 65Zn was studied using a rapid filtration assay. The mechanism of 65Zn uptake was complex and two processes were identified. In the first process, 65Zn uptake was rapid, reached equilibrium in 5-15 min and had an optimum pH of 7.5. The uptake was saturable and involved both binding to, and transport across, the membrane. The initial phase of 65Zn uptake (1 min incubation) approached saturation at high levels of substrate 65Zn and a Michaelis-Menten constant (Km) of 67.0 microM was calculated. Maximum uptake at equilibrium was approximately 100 nmol/mg protein. Cupric, ferrous and ferric ions had no effect on the uptake, but cadmium ions inhibited it competitively. The results are consistent with a carrier-mediated process, possibly involving a protein receptor in the membrane which is specific for Zn and elements close to it in the periodic table. Lactoferrin stimulated 65Zn uptake by approximately 20% when pre-incubated with 65Zn at a concentration of 0.01 microgram/ml. It had no significant effect on 65Zn uptake at higher concentrations or when pre-incubated with brush-border-membrane vesicles. The second mechanism of 65Zn uptake was linear with respect to time and involved binding to the membrane only. It was inhibited by all divalent and trivalent metal ions tested and is probably a passive binding process. The results are discussed with respect to the use of brush-border-membrane vesicles in examining the role of nutrient interactions and their effect on the biological availability of nutrients.