Vitamin A and carotenoids. The enzymic conversion of β-carotene into retinal in hog intestinal mucosa

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Abstract
The conversion of β-carotene into retinal was studied in vitro with enzyme preparations from homogenates of hog intestinal mucosa. The hog mucosal enzyme was purified about 27-fold by precipitation with ammonium sulphate, chromatography on DEAE-Sephadex and gel filtration on Sephadex G-200. The reaction displayed a narrow optimum pH range (approx. 7·8–8·2). The enzyme was stimulated strongly by the addition of thiols, and was inhibited by thiol inhibitors and by the chelating agents αα′-bipyridyl and o-phenanthroline. The reaction required the addition of an appropriate detergent (or bile salt); maximal activity was obtained by addition of an appropriate combination of detergents and lipid (specifically Tween 40, sodium glycocholate and sphingomyelin). The reaction displayed Michaelis kinetics with Km1·3×10−6m and Vmax.1·1nmole of retinal formed/hr. (for 0·7mg. of enzyme protein). The properties of the hog enzyme are similar to those previously reported for a less purified rat enzyme preparation.