Introduction

Retinoids include both the natural forms of vitamin A and a large number of synthetic analogues which may or may not display the biological activity of vitamin A. The major natural forms of vitamin A include retinol, retinal and retinoic acid (see Figure 1). All of these forms of vitamin A have a conjugated double bond system which renders them extremely hydrophobic and thus quite water insoluble.

Vitamin A is an essential nutrient in the diet for normal growth (Wolback & Howe, 1925, 1933), differentiation (Wolback & Howe, 1925, 1933), reproduction (Thompson et al., 1964) and vision (Wald, 1968). Vitamin A from the diet is absorbed and transported to the liver. Once in the liver, vitamin A can either be stored or transported to individual cells of target tissues where vitamin A elicits its biological functions. The target tissues include not just the eye but virtually all tissues of the body. In addition, once within the target cell, vitamin A must be transported to its appropriate site(s) of action. Finally, vitamin A may be transported from the target tissue back to the liver or to another target tissue. In each case, an extremely hydrophobic molecule must be transported through an aqueous environment in order to perform its physiological roles.

Since the isolation of the first retinoid binding protein, retinol-binding protein (RBP) in 1968 (Kanai et al., 1968), much information has been obtained related to the identification, characterization and most recently the molecular biology of a number of proteins which bind retinoids.