FIGURE 3.2 An outline of vitamin A activation into RA and the degradation of RA illustrating the roles of retinoid-specific binding proteins. The abbreviations used are CRABP, cellular retinoic acid-binding protein; CRBP, cellular retinol-binding protein; CYP, cytochrome P450; LRAT, lecithin:retinol acyltransferase; RALDH, retinal dehydrogenase; RalR1 and RRD, retinal reductases; RDH and RoDH, retinol dehydrogenases; RE, retinyl esters; REH, retinyl ester hydrolase.

there are numerous potential combinations of RAR and RXR isoforms. In addition, the numerous genes regulated by RA manifest a variety of receptor het-erodimer response elements. This entire system, illustrated in Figure 3.2, is subject to feedback and other types of control that are only beginning to be understood. Any imposition of exogenous retinoids, especially in high concentrations and for the long term, invites a plethora of compensatory and potentially adverse responses, which almost certainly will have unpredictable effects.

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