Retinal physiological functions

Since alcohol dehydrogenase is required for the conversion of retinol to retinal, excessive and prolonged ethanol ingestion can impair the physiological function of vitamin A. The decreased conversion of retinol to retinal results from competitive use of the enzyme by ethanol. Night blindness may result, since the visual cycle is a retinol-dependent physiological process. 

Another indirect measure of physiological function relating to PUFA metabolism comes from studies of the electroretinogram (ERG). It is known that adequate n-3 PUFA levels are essential for normal retinal function. Our group has shown that schizophrenic patients show ERG changes similar to those that occur in states of experimental dietary deprivation of n-3 in primates (Warner et al., 1999). 

One of the objectives in performing hydrogenation reactions in situ was to modulate the fluidity of biomembranes and to examine the role of membrane lipid fluidity in biochemical and physiological functions. Secondly, because membrane lipids with six or more unsaturated double bonds were found to be significant components of some membranes, such as the retinal rod membranes of the eye, the hydrogenation of these lipids was thought to be a useful tool to identify their role in these membranes. 

Several studies have suggested that anti-ChE.s can modify retinal physiology and function. Gazzard and Thomas (1975) analyzed the threshold luminance of the central visual fields of human volunteer subjects after expo.sure to. sarin vapor. They found that sarin raised the visual threshold, influencing cone more than rod function. Other studies... 

Here we have another simple chemical transformation, the oxidation of the aldehyde group in retinal to the carboxyl group of retinoic acid. Although the chemical change is simple, the physiology is profoundly different. Retinoic acid has nothing to do with vision but a lot to do with development and differentiation. Here we have yet another example of the sensitive interdependence of chemical structure and biological function. 

Because the normal blood-retinal barrier resists various substances, including fluorescein, the presence of fluorescein in the vitreous humor indicates a functional breakdown of this barrier. Although physiologic factors and instrument artifacts can influence vitreous fluorescence, this technique has been used to detect retinal vascular disease, especially in diabetes.The procedure has also been used to study the integrity of the blood-retinal barrier in various other diseases, including retinitis pigmentosa, optic neuritis, and essential hypertension. 

The best documented and established function of some carotenoids is their provitamin A activity, especially of P-carotene. One mole of P-carotene can theoretically be converted, by cleavage of C 15 = C 15 double bond, to yield two moles of retinal (Reaction 9.1). However, the physiological efficiency of this process appears to be only 50%. The observed average efficiency of intestinal P-carotene absorption is only two thirds of the total content. Thus, a factor of 1/6 is used to calculate the retinol equivalent (RE) from P-carotene, but only 1/12 from the other provitamin A carotenoids in food (Combs, 1992). In fruits and vegetables P-carotene content is used as a measure of the provitamin A content. 

It is apparent that the central nervous system, and retina in particular, appears to go to greatlengths to sustain its levels of DHA. The following discussion will consider biological membranes and retinal function and discuss the possible role that DHA has in its physiology. 

---