Studies on carotene metabolism

Over twenty years ago, during the course of the early investigations into chemistry and physiology of vitamin A, it was found that the potency of herbage was related to its carotene content and mainly as a result of the work of Moore, it was established that this carotene could be utilized by animals and converted into vitamin A. Since this recognition of the carotenes as pro-vitamins, the problems of the mode and site of conversion in the animal body have aroused the interest of many workers. Until recently it was considered that the liver was the main site of conversion. Apart from the somewhat equivocal results obtained from attempted in vitro conversions using liver preparations, this assumption was based mainly on the fact that the feeding of carotene to vitamin A-deficient animals resulted in the almost immediate appearance of the vitamin in the livers. At the same time little, if any, carotene appeared in the livers whereas the alimentary tracts contained relatively large amounts of carotene and little vitamin. Recent observations however suggest that in certain animals at least, the conversion takes place in the wall of the intestine rather than in the liver. At the time this investigation was commenced it had been established by Deuel and his co-workers that the intestinal was the main site of conversion in the rat. It seemed possible that there might be species differences and it was resolved to determine the site of conversion in other animals, particularly ruminants, and to investigate the enzyme systems involved. During the past two years reports have appeared of work of a similar nature carried out by various teams and it is now established that the wall of the intestine is the main area of conversion in a range of experimental animals and no evidence has so far been obtained of a secondary site. These findings have of necessity modified the course of this investigation. Prior to the work of Deuel and his team, interest in aspects of carotene metabolism had been aroused in this Department through observations suggesting the relative non-availability to ruminants of carotene from certain pastures. This problem, together with other factors affecting the efficiency of conversion of carotene to vitamin A in ruminants, has also been studies. Since this investigation is continuing at the present time the results presented here are not complete in themselves. Vitamin A or carotene metabolism studies has not previously been undertaken in this Department so that it was necessary to devote some time to perfecting procedures such as the surviving tissue technique and to establishing suitable methods of assay for Vitamin A and carotene in the samples handled. During this preliminary work and while awaiting the arrival of necessary apparatus and chemicals, a short investigation into the vitamin content of New Zealand mutton and lamb was undertaken. This was an extension of work previously carried out and is described in an appendix to this thesis.