Role of Carrier Proteins in the Movement and Metabolism of Retinoids in the Seminiferous Tubule

Vitamin A is required for maintenance of the proper state of many of the differentiated epithelia of the body and for processes as diverse as morphogenesis and vision. As details about the structure and biochemistry of vitamin A have been revealed, it has become clear that a family of compounds, the retinoids, are involved in fulfilling the functions of vitamin A. Retinol is the only “required” retinoid because it can serve as a precursor to all other retinoids and provides the appropriate forms for storage and transport (reviewed in 1). It is present in the blood, bound to its carrier protein, retinol-binding protein (RBP), at a relatively constant level in the normally nourished animal. This serves to supply most of the organs and cells that then produce the retinoids that act as hormones, principally retinoic acid. It is clear that the hormonal effects of retinoids can be explained in many cases by the presence of nuclear receptor proteins that bind retinoids and then activate and repress the expression of specific genes (2). But for the forms of vitamin A that appear to function as hormones, we still know little of their sites of production, the regulation of that production, and what means are present to modulate their action.