Plant sterols (such as ^-sitosterol, which differs from cholesterol in the structure of the side-chain) and stanols (which differ from sterols in having a saturated B-ring; see Figure 4.13) inhibit the absorption of cholesterol from the small intestine. As discussed in section 18.104.22.168, in addition to about 500 mg of dietary cholesterol, about 2 g of cholesterol is secreted each day in the bile. Almost all of this is normally reabsorbed; any inhibition of cholesterol absorption is therefore likely to have a more marked hypocholesterolaemic effect than might be expected simply by considering the dietary intake. A number of products, such as margarine, yoghurts and cream, that contain plant sterols and/or stanol esters have been marketed.
The rate-limiting step of cholesterol synthesis is the reduction of hydroxy-
HMG CoA reductase CH2
in plants only
Figure 7.1 9 Compounds synthesized from mevalonate that inhibit cholesterol synthesis by inhibition of hydroxymethylglutaryl CoA (HMG CoA) reductase.
methylglutaryl CoA to mevalonic acid, catalysed by hydroxymethylglutaryl (HMG) CoA reductase. Cholesterol acts to repress synthesis of HMG CoA reductase, and so do a number of other compounds derived from mevalonic acid, as shown in Figure 7.19. Such compounds include:
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