[Metabolism of fat-soluble vitamins by intestinal flora and enterocytes]

Three topics on fat-soluble vitamins in the intestines are described. First, it was suggested that novel cellular retinol binding protein type II, intrinsic to enterocytes, facilitated not only absorption of retinol and its esterification by lecithin:retinol acyltransferase in enterocytes, but also reduction of retinal to retinol by microsomal retinal reductase. Second, it was shown that in enterocytes the C-24 oxidation pathway from 1,25-(OH)2-D3 to calcitroic acid operated at physiological concentrations and the C-23 oxidation pathway to 1,25-(OH)2-D3-26, 23-lactone was also present as a minor route. Third, it was discussed whether menaquinones produced by intestinal microflora could be utilized by the host animals. Further, antibiotic-associated hypoprothrombinemia was shown to be caused by inhibition of K-epoxide reductase by N-methyletrazole, derived from the antibiotics, rather than reduced population of microflora.