Studies in vitamins A1 and A2

Abstract

Today we know a great deal about the chemistry of vitamin A and the ill?effects produced in humans and animals through its deficiency. But apart from the function of a small portion of this vitamin in the retina, with regard to dark adaptation, our ignorance regarding the role of the large amounts stored in the liver and the general mode of action of this vitamin is profound.

Vitamin A?, which occurs predominantly in fresh?water fish along with vitamin A?, differs from the latter in having one more conjugated double bond in the ??ionone ring. This vitamin has remained a bigger challenge than vitamin A? due to its scarcity and extreme instability. The natural form has not yet been crystallised, and even the synthetic form could not be crystallised until very recently. Comparatively little is known about its chemistry, and still less about its biochemistry. One of the obstacles, scarcity of material, was overcome with the discovery of large amounts of this vitamin in some species of Indian fresh?water fish. This provided an opportunity to study the chemistry and biochemistry of vitamin A? and some of its derivatives, though the drawback of instability became even more apparent with increasing purity of the material.

The thesis entitled Studies in Vitamin A? has been divided into five chapters. The first chapter gives the historical development of vitamin research in general, with special emphasis on vitamins A? and A? and their precursors.

The second chapter, Materials and Methods, describes the historical development, methodology, and the applications of two important techniques—spectrophotometry and chromatography—which have been largely used in the present investigation. This chapter also describes a new method for the separation of certain derivatives of vitamin A? in microgram quantities by reverse?phase paper chromatography.

The third chapter describes the isolation, purification, and properties of a strikingly pure form of natural vitamin A? from the liver oil of a fresh?water fish (Wallago attu), and the preparation and properties of its synthetic esters like vitamin A? acetate, crystalline all?trans vitamin A? p?phenylazobenzoate, and cis vitamin A? p?phenylazobenzoate obtained by isomerisation of the all?trans form. The stereoisomeric forms of natural vitamin A? as well as the synthetic esters have been studied with the help of maleic anhydride reactions and maleic anhydride adducts.

The fourth chapter describes the preparation and characterisation of crystalline vitamin A? aldehyde (retinene?) and its derivatives like crystalline retinene? oxime and 3?dehydroretinylidene methylamine. These derivatives of retinene? are believed to be artificial analogues of indicator yellow (formed during bleaching of porphyropsin). The properties of the natural indicator yellow, in view of those of the synthetic analogues, have been discussed.

A report on physico?chemical and biological studies on one interesting derivative of vitamin A?, viz. crystalline anhydro?vitamin A?, forms the subject of the fourth chapter. When fed to rats, anhydro?vitamin A? is converted to a new compound, “rehydro?vitamin A?.” This has been isolated, characterised, and a tentative structure has been proposed. The site of conversion in rats has also been discussed. To get an idea about the manner in which anhydro?vitamin A? takes up the visual function of vitamin A, the visual pigment from the retina of rats fed crystalline anhydro?vitamin A? was isolated and compared with the pigment from normal rats. Very pure preparations of the pigments were obtained from intact retinas by a modification of known methods.

This chapter also includes the results of a detailed qualitative and quantitative analysis of the liver oil of the fresh?water fish Wallago attu, which has led to the discovery of naturally occurring anhydro?vitamin A? and a new compound named “rehydro?vitamin A?.” The structural differences between natural and synthetic anhydro?vitamin A? have been discussed in view of the differences in physico?chemical properties and infrared data for the two compounds.

In contrast to the third and fourth chapters, which were devoted to studies on vitamin A?, the last chapter deals with a metabolic aspect of vitamin A, and its aldehyde retinene?. Retinene? is believed to be an intermediate formed during the conversion of ??carotene to vitamin A in animals. In view of the known effect of thyroid on the metabolism of ??carotene, the effect of this hormone on absorption, utilisation, and storage of vitamin A aldehyde was studied.

The results obtained suggest that the observed differences in the liver vitamin A levels of control, hyper?, and hypothyroid rats fed retinene? are due only to differences in the rate of utilisation of liver vitamin A by these animals. Thus hyperactivity of thyroid reduces the liver reserves of vitamin A due to increased rate of utilisation, and vice versa for hypothyroid rats. While the total amount of vitamin A absorbed from the intestines is the same in all three types of animals, the hyperthyroid animals are able to absorb this amount at a faster rate than the controls. An opposite liver picture is observed when ??carotene is fed under identical conditions. Hyperthyroid animals have superior liver vitamin A levels, and hypothyroid animals inferior levels, compared to controls. It remains that thyroid hormone has a definite effect on the conversion of ??carotene to vitamin A in the intestines, and that the effect is on the first stage of this conversion.

Thus, in general, this thesis deals with the preparation and properties of pure vitamin A? from Indian fresh?water fish (from the liver oil of Wallago attu) and some of its derivatives, prepared chemically as well as occurring naturally. One of these derivatives, anhydro?vitamin A?, has been studied in great detail for its biological activity and metabolic conversion to a new derivative “rehydro?vitamin A?” in rats. Another aspect of the thesis deals with the action of thyroid hormone on the metabolism of vitamin A aldehyde (retinene?) and the implications of the results obtained for the metabolism of ??carotene.