Some nutritional effects of folate-binding protein in bovine milk on the bioavailability of folate to rats.

The excretions of folate compounds into both the urine and bile were investigated in rats after the administration of pteroylglutamic acid (PteGlu) with or without the folate-binding protein (FBP) prepared from bovine milk. When the sample solution, containing either free or bound [3H]PteGlu (i.e., bound to the FBP from milk), was delivered to rats intragastrically via oral intubation, the amounts of [3H]PteGlu excreted into the feces did not change. On the other hand, the urinary excretion of 3H-labeled folate compounds, especially [3H]5-methyltetrahydrofolic acid (5-CH3-H4PteGlu), after the administration of bound [3H]PteGlu was significantly lower (P less than 0.01) than that after the administration of free [3H]PteGlu. The urinary excretion of [3H]5-CH3-H4PteGlu was directly proportional to the initial amount of free [3H]PteGlu administered. The similar effect of FBP was also observed when the biliary excretion of 3H-labeled folate compounds was investigated in situ. Furthermore, the incorporation of [3H]PteGlu into folate-requiring intestinal microorganisms was considerably reduced when it was bound to FBP. These results suggest that milk FBP has some nutritional effects on the bioavailability of folate in vivo.