SOME OBSERVATIONS ON POSSIBLE NUTRITIONAL SIGNIFICANCE OF VITAMIN-B12-BINDING AND FOLATE-BINDING PROTEINS IN MILK

1. The folate and vitamin B12 in milk are strongly bound to minor whey proteins. These binder proteins are present in excess, and so milk has capacity – which varies between milks of different mammalian species – to sequester added cyanocobalamin and folic acid. 2. A study was made of the influence of different milks on uptake of [3H]cyanocobalamin and [3H]folic acid in selected bacteria, mostly of types that are commonly found in the intestine. 3. None of the test cultures required exogenous vitamin B12. Nevertheless, when free cyanocobalamin was added, in seven of nine cultures it was rapidly taken up into the cells, though there were large differences between the different bacterial species in their absorptive capacity. In presence of sow's milk, little or none of the added cyanocobalamin was taken up, even after incubation of the test mixture for 1 h at 37°. The avidity of sow's milk for cyanocobalamin, as judged from its retention of the vitamin against competition by bacterial cells, was greater than that of a preparation of porcine Intrinsic Factor of similar binding capacity. 4. Of ten cultures of bacteria representing seven species, only the five cultures that required exogenous folate took up added folic acid into the cells. Uptake was severely depressed by the simultaneous addition of goat's colostrum, even after incubation of the test mixture for 1 h at 37°. 5. The physiological role of the binders is discussed. It is postulated that they may act in the mammary gland as trapping mechanisms to accumulate the vitamins from blood plasma into milk and in the gut to facilitate their absorption, both directly, and indirectly by preventing their uptake by intestinal micro-organisms. It is concluded that the binders might well influence the vitamin economy in the neonatal period, and the ecology of the gut microflora. © 1974, The Nutrition Society. All rights reserved.