7] Differentiation between central and excentric cleavage of β-carotene

Abstract

A series of observations is presented in the chapter that culminate in the hypothesis that β -carotene could undergo both central and excentric cleavage. This hypothesis is partially supported by observations that the apocarotenals are effectively converted to retinol in rats as well as are being oxidized to their corresponding carotenoic acids. In addition, these apocarotenals are identified in the intestine of chickens after the administration of β -carotene and could serve as a substrate for a partially purified enzyme preparation from both rabbit and guinea pig intestine. Olson has stated that to remain viable as an alternative pathway of vitamin A formation from carotenoids in mammals, the excentric cleavage hypothesis clearly requires unambiguous direct supporting evidence. Three recent publications offer this direct supporting evidence, and the methods that they used are described in the chapter. Apart from the intrinsic interest in the fact that β -carotene can be enzymatically cleaved at several locations along the polyene chain, there is now a strong possibility that excentric cleavage may play an important role in the formation of retinoic acid. It has been shown that retinoic acid is an important metabolite of β -carotene in mammalian tissues, including the small intestine, as well as liver, kidney, lung, and testes.