Vitamin B12 absorption in the neonatal piglet. 2. Resistance of the vitamin B12-binding protein in sows' milk to proteolysis in vivo.

Abstract

It has been postulated that the vitamin B12-binding protein in the sows' milk may facilitate the intestinal absorption of vitamin B12 in the piglet. This implies that the binder is not rapidly and completely degraded by gut proteases, and the present experiments were devised to test this assumption. Piglets aged 7 and 28 d were given a test feed of sows' milk whey both with and without cyano[3H]cobalamin to saturate the binder, and with 14C-labelled polyethylene glycol (PEG) as a marker. Control piglets were given a milk substitute containing no vitamin B12-binder. At 80 min after the test meal the piglets were killed and the contents of the stomach and small intestine were removed for analysis. Recovery of [14C]PEG, mainly from the middle and distal portions of the small intestine, ranged from 49 to 78%. Apparent loss of vitamin B12-binder from the intestine was calculated from the ratio, PEG:binder. In both 7- and 28-d-old piglets, the content of saturated binder in the middle and lower small intestine was about 20% depleted relative to that in the test fed. With the unsaturated binder the loss was more variable and generally greater, at about 50%. At slaughter the stomach was nearly empty and the high unsaturated binding capacity of the contents was attributed to endogenous binders. However, in the middle and lower small intestine, the binding capacity was derived mainly from sows' milk given with the test meal. No free cyano[3H]cobalamin was found in the intestinal contents of piglets given sows' milk to which enough cyano[3H]cobalamin had been added to saturate the binder. There was no change in the molecular weight of the vitamin B12-binder during its passage down the intestine, as judged by its behaviour on filtration in Sephadex gel G-150. The results indicate that a high proportion of saturated binder, and a smaller proportion of unsaturated binder, survived unchanged in the intestine of piglets at both ages.