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Studies on Carotenoid Metabolism

VI. The Relative Provitamin a Activity of Carotene When Introduced Orally and Parenterally in the Rat¹

Two Figures

Department of Biochemistry and Nutrition, University of Southern California Medical School, Los Angeles

1. No relation was found between the rate at which carotene was destroyed after parenteral injection in the rat and the previous level of vitamin A administration. Vitamin A had been given at levels of 0, 30, 450, 900 or 3000 I.U. daily for 28 days previous to the carotene administration.

2. No carotene could be demonstrated in the livers of rats after oral administration of carotene as a plasma or lecithin sol, or in a cottonseed oil solution or after feeding a diet containing 25% of alfalfa for 14 days. Increased levels of vitamin A were observed in the livers under such conditions.

3. Carotene was deposited in the liver after parenteral injection. It was still present in considerable amounts 46 days after the injection and it was shown by spectrophotometric measurements and by chromatographic separation to be all-trans-ß-carotene.

4. No increase in vitamin A could be demonstrated after the parenteral injection of carotene.

5. When an excess of carotene was administered orally or an excess of vitamin A was injected intrasplenically in a single dose to vitamin A-depleted rats which were then continued on a vitamin A-free diet, identical growth responses were obtained, and the times when the maximum weight was attained were similar. The average length of survival without additional vitamin A was also approximately the same. On the other hand, after a similar excess of carotene was injected intrasplenically, only a slight growth was obtained and the animals died with the livers still containing large amounts of ß-carotene. The quantities of carotene still remaining in the liver at the time of death were sufficient to have maintained the animals well over a year if given in divided doses orally.

6. No intrinsic impairment in carotene metabolism was found in rats showing signs of avitaminosis-A after intrasplenic injection of carotene since they were able to utilize this provitamin when given orally.

7. No carotene was destroyed in the excised gastrointestinal tract of rats kept at 37°C. for 24 hours.

8. The possibility is suggested that the conversion of carotene to vitamin A may be an extra-hepatic function in the rat. The wall of the intestine is suggested as a possible site of such transformation.

² Now with The Best Foods, Inc., Buffalo, New York.

Manuscript received 28 August 1945.