19] Retinoic acid synthesis from β-carotene in Vitro

Abstract

It is clear that both retinol and β -carotene serve as initial substrates for retinoic acid synthesis and several pathways of retinoic acid synthesis exist, at least in vitro . One pathway, the conversion of β -carotene to retinoic acid, is discussed in this chapter. It describes high-performance liquid chromatography (HPLC) assays to study the conversion of β -carotene to retinoids. Retinoic acid supports vitamin A-dependent differentiation through modulating gene transcription. As far as the mechanism of the production of retinoids from β -carotene is concerned, the difference between the products of β -carotene and retinal metabolism in vitro is inconsistent with an exclusive “central cleavage” mechanism. Two types of experiments illustrate this point and indicate that β -carotene conversion to retinoids in vitro is not a simple case of generating retinal and releasing it into solution to diffuse to enzymes that convert it to retinol and retinoic acid. The first type is based on the observation that the relative amounts of retinol and retinoic acid generated from β -carotene are different from those generated from retinal. With retinal as substrate, the major product in vitro is retinoic acid. In the absence of cofactor, little or no retinol is detected.