Sterols represent a class of hydrophobic ringed lipid molecules that are commonly found in biological membranes throughout the eukarya. Sterols can constitute greater than 50% of the membrane lipid content present in the cells and are known to alter membrane fluidity and structure. Though the mechanisms of these effects are still under debate, sterols have been proved to induce altered lipid phases in bilayer systems, basically increasing the order of acyl chains along with maintaining lipid translational fluidity. Their actions are found to be vital for the formation of lipid rafts with various effects on cell biology and serve as precursors to steroid hormones in animals. The formula of sterol is C17H28O.

Sterol Structure

All naturally occurring sterols are made up of a hydrocarbon tetrameric fused ring structure and it also obtains a small degree of polarity from a 3-hydroxyl in the β configuration; α-hydroxyl sterols are not seen in natural membranes. The very common sterols also possess a short aliphatic tail on the opposite end on C17. In lipid bilayers, the hydroxyl functions as the polar head group and it is directed towards the phospholipid head groups of the bilayer, whereas the rings and short hydrophobic tail extend into the hydrophobic core of the membrane, aligning along the acyl chains of the phospholipids. The arrangement of sterols is such that the α side of the relatively rigid ring structure is flat whereas the β side has protruding methyl groups; this structure maintains their interactions with lipids and allows significant contact on the flat surface.

Common Sterols

Many different types of sterols are found in biological membranes and few among them are specific only to certain phyla. Small changes in their structure can change their effect on membrane structure and dynamics as discussed earlier. Cholesterol is the most common form of a sterol found in animal membranes and it accounts for about 20-30% of sterols found therein. It is characterized by a single double bond in ring B (C5-C6) and the lack of a branching carbon at position 24. Ergosterol another very common sterol is found in the membranes of fungi and protists and it mainly differs from cholesterol by having two more carbon-carbon double bonds (one in ring B, another between carbons 22 and 23 in the tail region) and a methyl group at position 24.

Sterols obtained from plants are referred to as phytosterols. Common phytosterols are as follows: campesterol, stigmasterol and sitosterol, however, hundreds of different compounds have been isolated from plants. Hopanoids are usually pentacyclic compounds that share a common structure with sterols and are generally found in prokaryotic membranes, where they are thought to affect membrane stability and structure similar to sterols.

Significance of Sterols

Sterols and their related compounds play significant roles in the physiology of eukaryotic organisms. For example, cholesterol forms a part of the cellular membrane in animals, where it affects the cell membrane’s fluidity and acts as a secondary messenger in developmental signalling. In humans and other animals, corticosteroids like those of cortisol act as signalling compounds in cellular communication and general metabolism. Sterols are common components of human skin oils.

Phytosterols, more commonly referred to as plant sterols, have been used in clinical trials to block cholesterol absorption sites in the human intestine, thereby helping to reduce cholesterol absorption in humans. They are currently approved by the U.S. Food and Drug Administration for use as a food supplement; although, there is some concern that they may block absorption not only of cholesterol but of other important nutrients as well. 

At present, the American Heart Association has suggested that supplemental plant sterols can be taken only by those who are diagnosed with an increased cholesterol level, and is specifically recommended that they must not be taken by pregnant women or nursing mothers. Preliminary research has shown that phytosterols may also possess anticancer effects.

Conclusion

High serum LDL cholesterol concentration is a huge risk factor for cardiovascular disorders. This risk can be easily lowered by diet. Thus, foods containing plant sterol or stanol esters are useful for mildly- and hypercholesteraemic subjects. Plant sterols and stanols, which are structurally related to cholesterol, help decrease the incorporation of dietary and biliary cholesterol into micelles. This further lowers cholesterol absorption. However, these components increase ABC-transporter expression, that might also contribute to decreased cholesterol absorption. Thus, cholesterol synthesis and LDL receptor activity increase, which ultimately results in decreased serum LDL cholesterol concentrations. 

Research on animals has further shown that these dietary components might also lower atherosclerotic lesion development. Plant sterols and stanols also decrease plasma lipid-standardized concentrations of the hydrocarbon carotenoids, but not those of the oxygenated carotenoids and tocopherols. In this vitamin, A and D concentrations are also not affected. However, absorption of plant sterols and stanols (0.02-3.5%) is low as compared to cholesterol (35-70%), small amounts are found in the circulation and may also influence other physiological functions. Although, there is no relevant evidence that plant sterols or stanols could change the risk of colon or prostate cancer or immune status. In conclusion, plant sterols and stanols help in effectively reducing serum LDL cholesterol and atherosclerotic risk. In addition potential effects of plant sterols and stanols on other metabolic processes further remain to be elucidated.