Estimation of apparent diffusional uptake of L-threonine into porcine jejunal brush border membrane vesicles

Using L-threonine (THR) as a probe, the effective dosage of HgCl2 for complete blocking the carrier-mediated L-neutral amino acid transport and the diffusional uptake of L-THR in the jejunal brush border membrane vesicles of 26 kg pigs were determined. In a dose-dependent manner, HgCl2 blocked the carrier-mediated initial uptake of THR in the Naf-gradient condition. At the minimal HgCl2 concentration of 1.5 mM, carrier-mediated THR uptake was completely inhibited and the apparent diffusional uptake of THR was estimated to be 8.6 +/- 0.7% of the total uptake at the THR concentration of 5.0 mu M. Therefore, pretreatment of porcine brush border membrane vesicles with a minimum of 1.5 mM HgCl2 or 0.165 mu mol HgCl2/mg protein of vesicles can completely poison carrier-mediated L-neutral amino acid uptake and allows direct estimation of apparent diffusional uptake of these amino acids.