Neonatal role of milk folate-binding protein: studies on the course of digestion of goat's milk folate binder in the 6-d-old kid.

Groups of kids were reared from birth to 5 d on goat's milk. On the 6th day five of the kids received by bottle a morning feed of goat's milk with [3H]folic acid added to saturate the folate-binding proteins (FBP) (Expt 1); three kids received raw goat's milk containing only the endogenous folate and hence a large surplus folate-binding capacity (FBC) (Expt 2). The contents of the stomach, duodenum, jejunum and ileum were recovered by washing out 1.5 h after feeding (Expt 1) or at 0.5, 1 and 3.5 h after feeding (Expt 2). Recovery of [3H]folic acid 1.5 h after feeding (Expt 1) in all segments was 58.4%, mainly in a soluble form, most of this being in the stomach (37.0%) and ileum (14.3%). No surplus FBC was found in any gut segment. Sephadex G-75 chromatography of the soluble fractions of the contents of the various gut segments showed that [3H]folic acid remained bound to FBP throughout the stomach and small intestine. The bound [3H]folic acid exhibited a molecular weight of 81 000 in stomach contents, similar to that in the milk feed, presumably representing an aggregated form of the FBP, whereas in the intestinal contents its molecular weight was 39 000 indicating dissociation to monomer due to dilution in the recovery process. In Expt 2, the total recovery of free FBP in all four gut segments was 67, 54 and 23% respectively at 0.5, 1 and 3.5 h after the milk feed, and the distribution of FBP along the gut at 1 h was similar to that of [3H]folic acid-labelled FBP at 1.5 h in Expt 1.(ABSTRACT TRUNCATED AT 250 WORDS)