U. Gloor

Absorption and Distribution of Vitamin E in the Tissues

Publisher Summary The mechanism of the absorption of vitamin E is undoubtedly very similar to that of the other fat soluble vitamins, but compared to vitamin A, for example, its efficiency appears to be much lower. The specific site of absorption is not too well established, but it is probably linked with fat and oil absorption and is facilitated by bile. It has also been noted that the difference in the efficiency of absorption of an oil solution and an aqueous emulsion of vitamin E is not a great as the difference encountered with vitamin A. It is apparent from the work of several investigators that a high percentage of a test dose of tocopherol is excreted in the feces. The data from balance studies and the considerable data collected on blood and liver levels of tocopherol, however, strongly indicate that the absorption of vitamin E is incomplete. With the establishment of the quantitative relationship between tocopherol intake and liver storage together with the availability of precise methods of measurement of the tocopherol in the liver, a method for the bioassay of tocopherols based on liver storage was quick to develop. Although there is agreement concerning a linear relationship between tocopherol intake and liver tocopherol storage, there is some controversy over the relationship between tocopherol intake and plasma tocopherol.

Vitamin A and Lipid Metabolism

Publisher Summary The main aspects of the function of vitamin A in metabolism have been reviewed by Lowe and Morton. Since then, the problem of the functional role of vitamin A in steroid metabolism, already indicated in that review, has been developed further. Part of this problem, that concerning vitamin A and steroid hormones is discussed by Johnson and Wolf. This chapter mainly discusses the function of vitamin A in cholesterol and ubiquinone biosynthesis. The structure of ubiquinones (SA) and ubichromenol (SC) previously found to be enriched in livers of vitamin A-deficient rats has been elucidated. The ubiquinones widely distributedin natural material are dimethoxymethylbenzoquinone derivatives with long isoprenoid side chains. The ubichromenol turned out to be the corresponding chromene derivative, obviously formed by ring closure of the ubiquinone. In vitamin A deficiency, the incorporation of C 14 -mevalonic acid in both ubiquinones and ubichromenol is enhanced. A remarkable rise of the incorporation into squalene is also observed, whereas the biosynthesis of cholesterol is reduced. The sum of the total incorporation into these substances is little affected by vitamin A deficiency.