Retinoids and Bone

Abstract

Vitamin A (retinol) can be produced in the body by hydrolysis of retinyl esters or reduction of retinal. Liver and eggs, which are good animal sources of vitamin A, contain retinyl esters. Plant sources such as carrots and spinach contain pro-vitamin A carotenoids, which can be cleaved to retinal. Retinal, also called retinaldehyde, is interconvertible with retinol (Fig. 1). Retinal also serves as an intermediate in the irreversible production of all-transretinoic acid (ATRA), which is considered the major biologically active derivative of vitamin A [Moise et al. 2007, Chambon 1996]. Another important derivative of vitamin A is the visual chromophore 11-cis-retinal [Wald 1968]. Binding of 11-cis-retinal to proteins called opsins is the chemical basis of vision. Vitamin A formed from retinyl esters or carotinoids in the normal diet, or ingested in fortified foods or dietary supplements, is stored in the liver and transported to tissues as a complex bound to retinol binding protein [Moise et al. 2007].