Pro-Vitamin A in the Food of Whales

Reviewing the possibilities, Drummond and Hilditch considered that the balance of evidence favoured the latter view. Shortly afterwards, a demonstration that accumulation of vitamin A takes place in the liver of the cod as the fish becomes older was provided by the studies of MacPherson (1932, 1933). Probably the same is true of the halibut (Lovern, Edisbury and Morton, 1933).

In considering this important question it must not be forgotten that attention has been focused almost exclusively on the accepted biological relationship between the lipochrome pigment carotene and vitamin A. At the same time the almost universal employment of the rat as a test animal in making biological tests for the presence of “vitamin A” in materials has tended to detract attention from consideration of the possibility that other species may possess more extensive powers of forming the vitamin from related compounds.

Of the naturally occurring lipochromes certain isomeric forms of carotene are converted into vitamin A by the rat. There are, however, already some reasons for believing that formation of vitamin A from other lipochrome pigments does occur in other species. Macwalter and Drummond (1933), studying the changes in the development of young trout, made observations which suggested that a carotenoid pigment present in the eggs of these fish, and not identical with carotene, might be a precursor of vitamin A.

The investigation of the lipochrome pigments in marine species has recently attracted attention. An extensive survey by Lönnbe and Hellström (1932) and Lönnberg (1933) revealed the interesting fact that xanthophyll and certain other lipochrome pigments are very widely distributed in fish and lower marine animals. An important advance was made when Kuhn and Lederer (1933) isolated the interesting acidic pigment astacin from lobsters. Undoubtedly, earlier investigators had failed to detect this pigment because its acidic nature caused it to remain behind with the soaps when the fatty material was saponified and the non-saponifiable fractions extracted. From an examination of our notes it seems probable that this pigment was present in the materials which were extracted by us from trout eggs. If such were the case the abnormal behaviour of some of the fractions is satisfactorily explained. The occurrence of astacin in many Crustacea is now established (Fabre and Lederer, 1933, 1934) and there are good reasons for believing that its distribution in marine species is much more extensive than was at first suspected (Sorensen, 1934; Emmerie, van Eekelen and Wolff, 1934).

The origin of the large amounts of vitamin A which are found in the livers of whales is relevant to this discussion, but in this case the problem is somewhat simpler because, as is well known, most species of whales feed almost exclusively on plankton, which usually consist largely of copepods. The results of feeding tests with rats have suggested that zooplankton is usually a very poor source of vitamin A. The study of plankton oils recently reported by Drummond and Gunther (1934) provided some evidence that the pigment in the oil derived from common organisms such as Calanus and Euphausia might be a hydrocarbon differing from, but probably related to, carotene. Actually, while the examination of this pigment was being carried further the announcement of the discovery of astacin was made and it immediately became apparent that the newly-discovered pigment was in all probability responsible for a large part of the orange red colour of the zooplankton oils then studied.

Recently it has been possible to carry the investigation further by the gift of a specimen of oil extracted from the material known as “krill.” This name is given to the mass of plankton which forms the chief food supply of the whales in Antarctic waters. The oil had been extracted with all necessary precautions from a “krill” of which a very large proportion was represented by Euphausia. Its general character was similar to that of some of the zooplankton oils studied by Drummond and Gunther (1934). On saponification with alcoholic potash in the usual manner the greater part of the colour was retained by the soap solution when attempts were made to extract the non-saponifiable portion with ether. The colour of the material extracted by ether was found on spectroscopic examination to be due to a very small amount of carotene, with possibly a smaller amount of xanthophyll. The soap solution yielded on acidification and further extraction a deep red-coloured solution containing a pigment which gave a slightly blue-green colour with antimony trichloride.

On shaking a light petroleum solution of the pigment with 90 per cent, methyl alcohol the colour was almost entirely contained in the upper layer, but on the addition of a small quantity of alkali the pigment passed completely into the alcohol layer. The pigment was readily adsorbed from light petroleum solution by aluminium oxide (Merck) and could only be removed from this adsorbent after treatment with alkali. When eluted, the solution showed the characteristic absorption band of astacin with a maximum at 500 mμ in pyridine solution, and there seems little doubt that this interesting pigment is responsible for the greater part of the pinkish red colour of copepods such as Calanus and Euphausia.

From its behaviour with adsorbent aluminium oxide and in the “phase test” it is probable that the pigment is present in the original oil in the form of an ester similar to that described by Kuhn and Lederer (1933). Bearing in mind the very small amount of carotene which was detected spectroscopically as a constituent of the “krill” oil and also the results which seem to show that a pigment similar to astacin may give rise to vitamin A in the developing eggs of trout, it seems reasonable to put forward the suggestion that the large reserves of vitamin A found in the liver of whales might to some extent have originated from this pigment.

We also had the opportunity to examine a sample of oil extracted from the faeces of whales. This material was an oil of a deep blood-red colour. It contained a large amount of astacin, mainly in the form of unhydrolysed esters. It is probable, therefore, that a considerable proportion of the pigment ester is not absorbed during digestion in the whale. Examination of the saponified material gave indications that in addition to astacin a small amount of another pigment was present. It was much less readily adsorbed by the aluminium oxide than astacin and could be slowly washed through the adsorbing column, as a dark brick red zone, by benzene. In this property it resembled the pigment of salmon muscle described by Euler, Hellström and Malmberg (1933).