Vitamin A, Carotenoids and Cell Function

Abstract

Summary I. Both deficiency and excess of vitamin A produce many diverse pathological changes. Lipoprotein membranes have recently been found to be concerned in a number of the actions of excess of the vitamin. This article is devoted principally to a discussion of the interactions and possible functions of vitamin A within membranes, both in hypervitaminosis and under physiological conditions. 2. Excess of vitamin A. (a) As a result of its amphipathic molecular structure, retinol is highly surface active. (b) The initial action of excess of retinol on erythrocytes is an expansion of the cell membrane; this is followed by haemolysis unless the cells are kept cold, or inhibitors, such as vitamin E, are present. (c) Addition of retinol to fibroblasts growing in vitro causes degranulation and swelling of the endoplasmic reticulum, swelling of Golgi vacuoles and mitochondria, and the formation of cytolysomes. (d) Isolated mitochondria from certain tissues also swell in the presence of retinol. This swelling is apparently not dependent on respiration, but is inhibited by vitamin E. (e) Lysosomal enzymes are released by excess of retinol, both in vivo and in vitro. A number of the effects of excess vitamin A on connective tissues may be ascribed to this action; these effects are inhibited by hydrocortisone, which stabilizes lysosomes, but are not significantly inhibited by vitamin E. (f) Closely related, but physiologically inactive, derivatives of retinol are relatively inactive towards membranes; membranes might therefore also be concerned in the physiological functions of retinol. (g) Vitamin A has recently been shown to have marked effects on the membranes of certain bacteria and viruses. 3. Deficiency of vitamin A. (a) The diminished synthesis of mucopolysaccharide observed in deficiency may result from an interference with the synthesis of ‘active sulphate’ but, in view of conflicting observations, this cannot be considered as proven. (b) Suggestions have been made that lipoprotein membranes may be concerned in the actions of vitamin A on both mucopolysaccharide and steroid biosynthesis. (c) The action of the vitamin in biological oxidations is far from clear. Recently it has been found that deficiency increases, and excess of vitamin A decreases, the oxidation of succinate by homogenates of rat liver. (d) Organ culture experiments indicate that vitamin A acts directly on mouse prostate glands to prevent the squamous changes that are characteristic of deficiency. 4. A conjugated chain of alternating single and double bonds is a chemical feature common to vitamin A and the carotenoids. (a) Such a chain is characterized by a high electron mobility; this has been studied both theoretically and experimentally in relation to electron transfer in photosynthesis, to ion transport, and to super‐conduction and semi‐conduction in biological systems. (b) The role of vitamin A in vision may be an evolutionary development of the function of the carotenoids in photosynthesis. In the eye, retinal is intimately conce...

Safety of <7500 RE (<25000 IU) vitamin A daily in adults with retinitis pigmentosa

“…Nonetheless, highly elevated serum retinol in excess of 3.49 mol/L (100 g/dL) usually is a diagnostic indicator of excess vitamin A intake and hypervitaminosis A (6,8). Some of this abnormal concentration of retinol circulates nonspecifically in association with plasma lipoproteins, as opposed to normal transport bound to retinol binding protein, and thus has a potentially destructive effect on cellular membranes leading to cell and tissue damage (22,36,37). During the course of the present study, no significant changes were noted in the serum retinol concentrations of unsupplemented patients in the trace group, whereas significant increases of Ϸ8% and 18% were observed at years 5 and 12, respectively, among those supplemented with 4500 RE (15 000 IU) vitamin A/d.…”

Vitamin A Deficiency and Immunity.

“…Alteration of membrane lymphocyte receptor synthesis and redistribution of lymphocyte populations are not mutually exclusive hypotheses: the former has consequences in terms of the latter [117][118][119]. Numerous authors support the alternative hypothesis of a vitamin A effect on lysosomal membranes [77,[121][122][123]: the enzymes would be delivered in the presence of an excess of vitamin A [123][124][125][126][127][128][129][130][131].…”

LT-STM study of self-organization of β-carotene molecular layers on Cu (1 1 1)

“…At the same time the system is a molecular multilayer structure, where we expect to retain some of the properties which are necessary to understand the important biological activity of this and other related molecules like vitamin A [6]. From chemical evidence, C 40 H 56 (alltrans b-carotene), is a planar, all-trans polyene chain characterized by a set of conjugated double bonds with attached methyl groups, terminated with two b-ionone carbon rings also with attached methyl groups and symmetric with respect to its center [7].…”