The uptake of C14-beta-carotene and its conversion to retinol ester (vitamin A ester) by the isolated perfused rat liver.

The small intestine is probably the major site for the conversion of p-carotene into retinol ester (vitamin A ester)1 in the mammal (2-5), but other tissues are capable of catalyzing this cleavage reaction, e.g. the conversion takes place in viva after removal of the intestine (6) and of various other organs (6-9). Direct evidence for the participation of other tissues, however, is lacking. Although the concentrations of p-carotene and retinol in the lung increase in viva after the injection of @-carotene (lo), practically no p-carotene was converted into retinol ester in the dog heart-lung preparation (11). As early as 1931, Olcott and McCann (12) claimed that ground liver tissues contained the enzyme “carotenase,” which converted carotene into retinol. However, Woolf and Moore (13), with the use of more careful analytical methods, were unable to repeat these experiments. With a variety of techniques, others (14-19) supported the view that carotene could be converted into retinol in liver tissues. On the other hand, conversion did not occur in cat liver perfused with carotene suspended in gum-NaCl (20), and other experiments with liver tissue (21-24) failed to demonstrate the conversion. Moore (25) has critically reviewed these studies. Since CY-p-carotene has been used to demonstrate retinol ester formation in the rat intestine both in viva (5) and in vitro (26), labeled p-carotene was employed in investigating the role of the liver in p-carotene metabolism. In the present communication, the conversion of p-carotene into retinol ester by an isolated perfused rat liver is clearly demonstrated.