Vitamin B12 absorption in the neonatal piglet. 3. Influence of vitamin B12-binding protein from sows' milk on uptake of vitamin B12 by microvillus membrane vesicles prepared from small intestine of the piglet.

The influence of the vitamin B12-binding protein isolated from sows' milk on the uptake of vitamin B12 was investigated using microvillus membrane vesicles prepared from the small intestine of 7- and 28-d-old piglets. Uptake of radioiodinated purified binder was also examined. The binder strongly promoted uptake of vitamin B12 at both ages; in the absence of the binder there was little uptake. The uptake mechanism was specific and operative in vesicles prepared from all regions of the small intestine. Uptake was a rapid process, exhibiting saturation kinetics, with a pH optimum at about 7.0, and dependent on the presence of magnesium or calcium ions for maximum activity. Affinity constants of the binding sites for the milk binder were determined. Competition experiments using complexes of the binder with vitamin B12 and with non-cobalamin analogues (cobinamide and Co-alpha-[2-methyladenyl]cobamide) showed that the bound analogues competed with bound vitamin B12 for uptake but with lower efficiency. Intrinsic factor also promoted vitamin B12 uptake by the vesicles but it did not compete with the milk binder for the same binding sites. It promoted uptake only in microvilli isolated from the lower third of the small intestine, and was more effective with preparations from 28-d-old piglets, whereas the milk binder was more effective with the 7-d-old piglets. Porcine gastric cobalophilin competed with the milk cobalophilin, but with lower efficiency. It was concluded that a specific transport mechanism for absorption of vitamin B12, mediated by the vitamin B12-binder in milk, exists at the intestinal brush border of neonatal piglets and strongly reinforces the developing intrinsic factor-mediated mechanism during the early days or weeks of life. It is suggested that the binder in the milk has a wider physiological significance and acts also as a 'host protective' factor and as a scavenger of adventitious vitamin B12.