Cholesterol

Cholesterol differs from the main steroid ring structures by having an aliphatic side chain at C17 of ring D, methyl groups at C10 and C13, a double bond in ring B, and a hydroxyl group on C3. The possession of the OH group makes cholesterol a steroid alcohol or sterol.

The structures opposite show two contrasting views of cholesterol. The top view shows how straight and flat the molecule appears from the side.

The bottom structure shows a view from the top with the OH group on the far right attached to ring A. Ring B is slightly below ring A, and ring C is above ring B. Ring D is a 5C ring below ring C, and carries the 8C alkyl chain.

This is the most abundant steroid in the body occurring either as a free molecule or bound to FA in the form of cholesteryl esters. Most of the cholesterol in the blood is present as esters, while most of the cholesterol in cells and tissues is the free form. Cholesterol functions in the body are many, but can be grouped under two main headings:

1. structural component of cell membranes

2. precursor for other steroids such as steroid hormones and bile salts.

Cholesterol arises in the body from two sources, natural biosynthesis and dietary. Biosynthesis is a long and complicated process starting from a 2C molecule, and involves many metabolic steps.

Cholesterol pathology

About half of all deaths in developed countries are caused by ateriosclerosis (hardening of the arteries). In this disease, fatty deposits containing mostly cholesterol, accumulate on the walls of arteries.  

The deposits harden and enlarge to form plaques (raised circular structures), gradually narrowing the bore of the artery, and restricting the flow of blood. The irregular and rough textured plaques represent a form of tissue damage, and as such cause formation of blood clots (thrombosis).

If a blood clot forms in one of the arteries serving the heart, then starving the heart of oxygen will cause a heart attack, starving the brain of oxygen will cause a stroke.

The link between blood cholesterol and arteriosclerosis is clear. If blood cholesterol is normally below 150 mg/100 ml blood then the risk of thrombosis arising from arteriosclerosis is low. If blood cholesterol is regularly above 250 mg/100 ml blood then the risk of thrombosis is considerably greater. The blood cholesterol level in the U. K. average male is around 220-260 mg/100 ml blood.

The solution would appear to be simple: lower cholesterol in the diet, but there is a complication! What do you think it is?

Cholesterol is also deposited in the gall bladder, and is the main component of gall stones. The human skin secretes up to 300 mg per day as a protective agent for skin.


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