Presence of Carotenase in the Liver of the Dog

Abstract

That carotene is converted to vitamin A in vivo has been shown by Moore, 1 Capper and his co-workers. 2 , 3 and by Drummond, Ahmad, and Morton. 4 Moore 5 postulated that this conversion takes place in the liver. Olcott and McCann 6 made a toluene water extract of the livers of A-deficient rats, and with this extract were able to convert colloidal carotene to vitamin A in vitro. They believed the activity of the extract to be due to an enzyme which they named carotenase.

We attempted to prepare carotenase from the livers of normal dogs and of dogs on A-deficient diets. We succeeded only once out of 4 trials. In the successful attempt, we noted that in preparing the carotenase, much of the blood had remained in the liver. It occurred to us that the blood buffers present might have been responsible for the preservation of the enzyme. We therefore made a liver extract, using a phosphate buffering solution of pH 7.4 instead of distilled water. The buffer was made by taking 39.5 cc. of 0.2 N (carbonate free) NaOH, 50 cc. of 0.2 M acid potassium phosphate, and this was made up to 200 cc. with distilled water. We found that such a solution always converted colloidal carotene to vitamin A as shown by a positive antimony trichloride reaction, even when watery extracts of the same liver did not. Colloidal carotene itself failed to give a positive antimony trichloride reaction. The liver extract solution also was tested but gave no reaction.

Olcott and McCann showed that the enzyme was inactive after boiling. We have noted that it is also destroyed by exposure to cold.