Stigmasterol, in plants

Lanosterol is converted to stigmasterol (in plants), cholesterol (in animals), and 24-methylenedihydrolanosterol (24-MDL) (in fungi), which is further converted to ergosterol. 

In milk fat, cholesterol is associated with Hpoproteins in the milk fat globule. It is also a component of animal membranes and controls rigidity and permeabihty of the membranes. Cholesterol has interesting surface properties and can occur in Hquid crystalline forms. Plants contain sterols such as P-sitosterol [83-46-5] (4b) or stigmasterol [83-48-7] (4c). Their functions in plant metaboHsm are not yet well understood. Analysis of sterols has proven useful for detection of adulteration of edible fats (9). 

Cholesterol is the sterol characteristic of animal cells plants, fungi, and protists make other, closely related sterols instead. They use the same synthetic pathway as far as squalene 2,3-epoxide, at which point the pathways diverge slightly, yielding other sterols, such as stigmasterol in many plants and ergosterol in fungi (Fig. 21-37). 

Phytosterols are structurally very similar to cholesterol and the major phytosterols (campesterol, sitosterol and stigmasterol) have the same kind of membrane viscosity modulating function in plants that cholesterol (C27 3-OH-C6 C6 G61 C5—C8) has in animals. Campesterol (24-methylcholesterol), sitosterol (24-ethylcholesterol) and stigmasterol (A22, 24-ethylcholesterol) are widespread phytosterols. The animal sterols lanosterol and cholesterol are present in particular plants. Phytosterol esters reduce cholesterol absorption and lower LDL-cholesterol. 

Among the simple lipids are also the cholesterol and cholesterol esters with acids such as palmitic, stearic, or oleic. These compounds are common compounds in most animal cells, although not in plants. Besides cholesterol, other zoosterols are known [la]. Several sterols are also present in plants (phytosterols), the most common being the sitosterol(s) and stigmasterol. 

Fig. 2. Biosynthesis of typical plant sterols. Boxed sterols are probable precursors of various brassinosteroids. Among these sterols, sitosterol, stigmasterol, campesterol, 24-epicampesterol and cholesterol are the most common end-of-pathway sterols in plants. Italic letters refer to the lesions in the biosynthesis mutants of Arabidopsis (dim) and pea (Ikb).

The effects of a mixture of plant sterols (stigmasterol/sitosterol/caropes-terol, 55 15 30, wAich is representative of the mean sterol composition of membranes from maize roots), were found to be similar to those obtained with stigmasterol, the main compound of the mixture (data not shown). Cholesterol, a minor sterol in plants, is shown to be less efficient than sitosterol. In Fig.2 is represented the comparative behavior of sitosterol-and 24-methylpollinastanol-containing LUV. The cyclopropylsterol appears to be much less efficient than sitosterol as a modulator of the fluidity of soybean PC bilayers. It behaves more like (24R)-methylcholesterol. 

Desmethyl sterols, with no methyl group at C-4, are the main class of sterols in plants and thus in the human diet. The proportion of 4-desmethyl sterols among total sterols in common crude vegetable oils is >85% (Piironen et al, 2000a). Sitosterol, the principal sterol in most plants, is usually accompanied by its 22-dehydro analogue, stigmasterol. The molecular stmctures of the most important plant sterols and stands (see Section B.5 below) are shown in... 

The sterols bear a hydroxyl group on C atom 3, which gives them their name. Of the animal sterols, the zoosterols, cholesterol is the most important. Cholesterol also occurs in plants. However, the most important sterols in higher plants are )8-sitosterol and stigmasterol (Fig. 79). They differ from each other with respect to a double bond in the side chain which is present in stigmasterol. The C skeleton of both substances consists of 29 atoms, two of which are attached as a branched chain to the side chain at C 24. This branched chain is absent from cholesterol, which has only 27 C atoms. 

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