Retinal is not formed in vitro by enzymic central cleavage of .beta.-carotene

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Abstract
Rat intestinal mucosa was prepared and incubated with .beta.-carotene by the procedure of Goodman and Olson [Goodman, DeW. S., and Olson, J.A. (1969) Methods Enzymol. 15, 462-475] to determine .beta.-carotene cleavage activity. A new detection system for the reaction products of the described enzyme .beta.-carotene 15,15''-dioxygenase (EC 1.13.11.21) employs solvent extraction of retinoids and carotenoids followed by high-performance liquid chromatography separation and photometric detection of the pigments. It has not detected any newly formed retinal or other retinoids in the intestinal protein preparations from normal or vitamin A deficient rats. The latter were chosen as a possible source of more active enzyme preparations. With corresponding blank samples subjected to identical conditions of incubation but without added protein, small amounts of .beta.-apocarotenals could be detected. They were previously reported as cleavage products of .beta.-carotene [Ganguly, J., and Sastry, P.S. (1985) World Rev. Nutr. Diet. 45, 198-220] but are clearly not formed as a result of an enzymatic reaction. The failure to detect in vitro enzymatic central or random cleavage of the .beta.-carotene molecule in extracts of rat intestinal mucosa emphasizes the need to reevaluate the existing theory of conversion of .beta.-carotene into vitamin A.
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