Retinoids biological functions

The excited-state properties of retinoids and carotenoids have been reviewed, together with chlorophylls, in relation to their biological functions [1], In the present review, we will briefly summarize the previous results concerning polyenes (Sections 3.2.1 and 3.3.1), and add most recent results along these lines (Sections 3.2.2 and 3.2.3, and 3.3.2-3.3.4), focussing on a retinoid (retinal) and two carotenoids (/1-carotene and spheroidene). 

The retinoids comprise a family of polyisoprenoid lipids that inclndes vitamin A (retinol) and stracturaUy related componnds. The biological activity of retinoids can be mod-ihed, for example, by changes in the molecules state of oxidation and cis/trans isomerization. Their activity is also dependent on the levels of specific types of retinoid-binding proteins that exist in extracellular, cytosolic, and nuclear compartments. The role of retinoids in gene expression represents an important biological function for this family of molecules. Retinoid-dependent modulation of gene expression is critical for normal cell and tissue function in mature as well as developing animals. 

Vitamin A (retinol) and retinoic acid are carotenoid oxidation compounds that are very important for human health. The main functions of retinoids relate to vision and cellular differentiation. With the exception of retinoids, it was only about 10 years ago that other carotenoid oxidation products were first thought to possibly exert biological effects in humans and were implicated in the prevention - or promotion of degenerative diseases. A review on this subject was recently published. ... 

Retinoids They influence a wide variety of biological activities including cellular proliferation, cellular differentiation, immune function, inflammation, e.g. tretinoin, isotretinoin, etretinate. 

Retinoic Acid Receptor. Most of the biological effects of retinoids are mediated through the retinoic acid receptor (RAR) and the retinoid X receptor (RXR). Both all-/ran.s-retinoic acid and 9-d.v-rctinoic acid serve as agonists of RAR, while only 9-d.v-rctinoic acid functions as an agonist of RXR. The functional RAR exists as a heterodimer with RXR, while functional RXR exists as a homodimer. Methoprene is a juvenile hormone III analogue that mimics the activity of this insect hormone. 

Vitamin A is a generic term embracing substances having the biological actions of retinol and related substances (which are called retinoids). The principal functions of retinol are to ... 

The function of certain carotenoids as provitamins A (retinol (13), 3,4-didehydroretinol (14) and retinal (18)), treated in the previous section, is during the last two decades further expanded as precursor role for additional biologically important retinoids, namely the all-trans and 9-cis isomers of retinoic acid (19), Fig. 6. Their function in growth regulation, development and cell differentiation via specific nuclear receptors are being clarified [24,31], The origin of all-trans and 9-cis retinoic acid (19) has been considered [29,32,33],... 

Morriss-Kay GM. Retinoids their physiological function and therapeutic potential. In Advances in Organ Biology, Vol. 3, Ed. Bittar EE. JAI Press, Greenwich, CT, 1997, p. 79. 

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