Biosynthesis of Cholesterol and Steroids


Cholesterol is one molecule with many functions. It is a lipid that is an essential component of mammalian cell membranes. It isalso the most abundant sterol. Cholesterol is the precursor of three important classes of biologically active compounds: the bile acids, the steroid hormones, and vitamin D. Cholesterol metabolism isimportant in the etiology of cardiovascular disease, and it is a major component of gall stones. |


The typical daily Western diet contains approximately 500 mg (1.2 mmol) of cholesteroldaily, mainly in meat, eggs, and dairy products (see Chapter 21). Under normal circumstances, 30-60% of this is absorbed from the gut. |


After absorption, cholesterol is transported to theliver and to peripheral tissues in the form of chylomicrons. The liver repackages it into another lipoprotein, VLDL (see Chapter 17 and Table 17.3). VLDL subsequently transforms into VLDL remnants (orIDL) and then into LDL. VLDL remnants and LDL can deliver cholesterol to tissues by binding to the apoB/E receptor (LDL receptor). |


The sterol ring of cholesterol cannot be degraded in thehuman body. Therefore it is excreted either in the free form as the biliary cholesterol, or in the form of bile acids. Most of the bile acids are however returned to the liver after reabsorption inthe terminal ileum. This is known as the enterohepatic circulation. |


Human beings synthesize 1g cholesterol each day, mainly in the liver. The rate of its endogenous synthesis isdetermined by dietary intake. For this reason both dietary intake and biosynthesis are important in determining its plasma concentration. |


The structure of cholesterolis shown in Figure 16.1. It has a molecular weight of 386 Da and contains 27 carbon atoms, of which 17 are incorporated into four fused rings (the perhydrocyclopentano-phenanthrene nucleus), two are... [continua]

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