ABSTRACT
INTRODUCTION Retinoids represent a class of lipophilic compounds that are comprised of natural derivatives of vitamin A (retinol) as well as synthetic analogues. Naturally-occurring retinoids are sourced via dietary intake of beta-carotene as well as retinyl esters, both of which are found in a broad range of foodstuffs and animal by-products. Structurally, betacarotene exists essentially as a dimer form of retinol and the enzymatic conversion in the intestinal tract via retinal production is the initial sourcing of retinoids for usage by the human body. Most eukaryotic cells have the capability of enzymatically converting retinol and retinyl esters to various metabolites that are critical in maintaining cellular homeostasis, regulating proliferation and differentiation patterns, as well as embryonic development. Of the metabolites generated, the true “business end” of retinoids is retinoic acid, which is present in cells as various cis conformations as well as trans-retinoic acid (tRA). Retinoids play such a critical role in developmental biology and the metabolic pathways are tightly regulated. This includes not only enzymatic processes but transport via cellular retinoid binding proteins as well.
