Retinoids

The retinoids comprise a class of chemical compounds that are vitamers of vitamin A or are chemically related to it. Retinoids have found use in medicine where they regulate epithelial cell growth.

Retinoids have many important functions throughout the body including roles in vision, regulation of cell proliferation and differentiation, growth of bone tissue, immune function, and activation of tumor suppressor genes.

Research is also being done into their ability to treat skin cancers. Currently, alitretinoin (9-cis-retinoic acid) may be used topically to help treat skin lesions from Kaposi's sarcoma.

There are three generations of retinoids:

Some authors consider retinoids derived from pyranones as the fourth-generation. One such compound is seletinoid G.

The basic structure of the hydrophobic retinoid molecule consists of a cyclic end group, a polyene side chain and a polar end group. The conjugated system formed by alternating C=C double bonds in the polyene side chain are responsible for the color of retinoids (typically yellow, orange, or red). Hence, many retinoids are chromophores. Alternation of side chains and end groups creates the various classes of retinoids.

First- and second-generation retinoids are able to bind with several retinoid receptors due to the flexibility imparted by their alternating single and double bonds.

Third-generation retinoids are less flexible than first- and second-generation retinoids and therefore, interact with fewer retinoid receptors.

The major source of retinoids from the diet are plant pigments such as carotenes and retinyl esters derived from animal sources. Retinyl esters are hydrolyzed in the intestinal lumen to yield free retinol and the corresponding fatty acid (i.e. palmitate or stearate). After hydrolysis, retinol is taken up by the enterocytes. Retinyl ester hydrolysis requires the presence of bile salts that serve to solubilize the retinyl esters in mixed micelles and to activate the hydrolyzing enzymes

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