The Enzymatic Conversion of All-trans β-Carotene into Retinal

Published on Monday, 11 April 2016

Summary

Studies were conducted dealing with the cleavage in vitro of β-carotene into 2 molecules of retinal. The cleavage reaction was catalyzed by a soluble enzyme, from rat intestinal mucosa, which was partially purified by precipitation with ammonium sulfate between 20 and 45% saturation. The enzyme was stimulated by the addition of thiols, and was strongly inhibited by sulfhydryl inhibitors and by the chelating agents α,α'-dipyridyl and o-phenanthroline. The latter properties are consistent with the postulated mechanism of the reaction (a dioxygenase reaction). The cleavage reaction had a narrow optimal pH range (7.5 to 8.0) and displayed Michaelis kinetics, with a Vmax of 8 x 10-9 mole of retinal formed per hour (for 7 mg of protein of the active ammonium sulfate fraction) and a Km of 3 x 10-6 m.

Addition of an appropriate detergent was required in order to effect the conversion in vitro of β-carotene into retinal. In the absence of bile salt or synthetic detergent no enzyme activity was seen. Maximal activity was obtained by addition of an appropriate combination of detergent plus lipid. The detergent-lipid requirement was found to be nonspecific. Effective detergents included both bile salts and synthetic detergents; effective lipids included phospho- and nonphospholipids, as well as synthetic detergents. Stimulation was not related to the electrical charge of the compound added. In the absence of added lipid, the reaction was stimulated by the addition of cell particles to incubations containing bile salt. The particles apparently functioned to provide membrane-bound lipids, and were able to stimulated the reaction to the same maximal rate as that obtained by the addition of an effective detergent-lipid combination. The role of the detergent-lipid combination, in activating the cleavage reaction, has not been defined.

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