Influence of goat's-milk folate-binding protein on transport of 5-methyltetrahydrofolate in neonatal-goat small intestinal brush-border-membrane vesicles.

1. The influence of goat's-milk folate-binding protein (FBP) on the uptake of 5-methyltetrahydrofolate (MTHF) by brush-border-membrane vesicles prepared from the small intestine of the 6-d-old goat was investigated using a rapid-filtration assay. 2. Uptake of MTHF by the membrane vesicles was strongly enhanced by FBP within the pH range 4.5-6.5, with an optimum at pH 5-5.5. 3. Both the initial rate of MTHF uptake and uptake of MTHF at equilibrium were markedly increased in the presence of FBP. 4. Uptake of MTHF by brush-border-membrane vesicles was maximal when the molar ratio FBP:MTHF was 1.0-2.5. 5. The relation between pH and 125I-labelled FBP binding to the membranes was similar to that for uptake of MTHF, with an optimum at pH 5. 6. In experiments in which the osmotic pressure of the incubation medium was progressively increased with cellobiose, 125I-labelled FBP was found to be taken up primarily by binding to the brush-border-membrane surface. 7. Uptake of 125I-labelled FBP was time-dependent and saturable, with a Km of 0.39 (SE 0.07) microM and Vmax of 6.73 (SE 0.92) micrograms/mg protein. 8. Experiments in which various milk proteins (cow FBP, goat FBP, alpha-lactalbumin, beta-lactoglobulin, bovine serum albumin and lactoferrin) were allowed to compete in turn with 125I-labelled FBP for uptake by brush-border-membrane vesicles indicated that high-affinity binding was probably specific to FBP, although lactoferrin reduced uptake possibly by non-specific coating of the mucosal surface. 9. It was concluded that a folate transport mechanism mediated by the FBP in milk exists at the intestinal brush border of neonatal goats. It is suggested that this may reinforce the developing endogenous transport system.