Plant Steroids Phytosterols

Some T. have major physiological significance. Thus, lanosterol is converted biosynthetically to cholesterol, the precursor of all steroid hormones, bile acids, and vitamin D3. In fungi, lanosterol is converted to er-gosterol (see sterols), an essential component of the fungal cell membrane. Plant cell membranes also incorporate steroids (phytosterols). In prokaryotes, the hopanoids take over the functions of steroids in the cell membranes. As a component of animal and plant waxes T. strengthen the structures. They protect the plant surface from desiccation and attack by microorganisms (e.g., betulin, lupeol, oleanolic acid, and ursolic acid). 

It is a plant sterol (phytosterol) present in canola oils and rape seed but is also found in some marine algae, diatoms and shellfish (oysters). It inhibits the enzyme A24-sterol reductase that is involved in mammalian cholesterol biosynthesis pemandez et al. Biochem J 336 109 2002], Brassino steroids have plant growth regulating activities [Anastasia et al. J C S Perkin 1379 7982.]... 

V is a - steroid and is contained in cod liver. Its precursors (provitamin Di 3) are found in plant oils (phytosterols). Upon solar irradiation these compounds are able to cure rickets. The active species are mono- and dihydroxylated calciferols. Today, it is possible to synthesize similar antirachitic compounds. 

Insect steroid metabolism has two biochemically distinctive components dealkylation of phytosterols to cholesterol and polyhydroxylation of cholesterol to ecdysone. We will focus on the first of these. Lacking the ability to synthesize sterols de novo, insects instead have evolved a dealkylation pathway to convert plant sterols to cholesterol(7-10). The dealkylation pathways are apparently absent in most other higher and lower organisms, which can convert mevalonate to squalene and thence into sterols( ). Specific insecticides are possible based on these biochemical differences. 

A typical Western diet contains approximately 100-300 mg and 20-50 mg of plant sterol and plant stanol, respectively. The relationship between total dietary phytosterol content and the fatty acid composition of the diet decreases with increasing saturated fatty acids, whereas the total dietary phytosterol content increases with increasing PUFA (89). Fortification of lipid foods, such as margarine, with plant sterols will dramatically increase the daily intake of phytosterols and significantly lower serum cholesterol (90). The dietary consumption of large amounts of plant sterols will interfere with cholesterol absorption, thereby leading to an increased daily neutral steroid excretion. 

Phytosterols are a group of plant chemicals called steroid alcohols (sterols), indicating that they have chemical structures like steroids that dissolve in lipid layers as easily as alcohol does. In the plant, sterols serve as a vehicle for communication between cells and for supporting anatomical structure. 

The biosynthesis of cholesterol, related steroids, and phytosterols is dealt with in this section whereas the further metabolism of these classes and the remaining nonsteroidal triterpenoids are covered in the following two sections. Reviews have appeared on the biosynthesis of sterols and higher terpenoids, the in vivo metabolism of steroids in primates" and in plant tissue culture," and dietary feedback control of cholesterol synthesis." The latter contains a reasoned defence of the hypothesis that HMG-CoA reductase is controlled by alterations to its supporting microsomal membrane. Abstracts of a symposium on all aspects of steroid biosynthesis have appeared." ... 

The structural relationship between phytosterols and BRs has been proposed from the biosynthetic points of view. All naturally occurring BRs possess carbon skeletons identical to those of common phytosterols (e.g., campesterol, 24-methylene-cholesterol, isofiicosterol, sitosterol, and cholesterol). Thus, BRs may be speculatively regarded as the enzymatic oxidation products of phytosterols with the corresponding carbon skeletons, as is the case of the biosynthesis of other steroid hormones (e.g., ecdysteroids (25) and 1,25-dihydroxyvitamin D3 (26)). Although BL has recently been proved to be biosynthesized from CS in crown gall cells of Catharanthus roseus (27), a major part of the biosynthesis of BRs is remained to be investigated. Experiments using radio-labeled precursors are required to clarify the biosynthesis of BRs in a suitable plant system. 

Biological membranes contain, in addition to phosphoglycerides, glycolipids as part of the lipid component. Steroids are present in eukaryotes—cholesterol in animal membranes and similar compounds, called phytosterols, in plants. In the lipid bilayer part of the membrane (Figure 8.10), the polar head groups are in contact with water, and the nonpolar tails lie in the interior of the membrane. The whole bilayer arrangement is held together by noncovalent interactions, such as van der Waals and hydrophobic interactions (Section 2.1). The surface... 

Products of the isoprenoid pathway These include the carotenoids, steroids, terpenes, and phytosterols [5], Phytosterols are present in a broad range of plant foods, are not absorbed effectively from the intestine, but can bind cholesterol and prevent it from being absorbed [3],... 

Squalene is a 30-carbon, straight-chain terpenoid hydrocarbon that serves as a precursor of steroids, both in plant cells and animals. In plant cells, squalene can be converted to phytosterols. It has been reported that squalene significantly decreases total cholesterol, LDL cholesterol, and TAG levels in hypercholesterolemic patients [161,162]. There has been growing interest in squalene as a potential chemopreventative agent [163-165]. In addition, the possible protective effect of squalene maybe attributed to its antioxidant function [166-168]. 

Limonene as a by-product of orange juice production can be used for polymer production together with CO2 to form a polystyrol-like foam. Menthon can be used as a precursor for branched dicarbonic acids and alpha-pinen (a by-product of the forest industry) as precursor for cymol production or for enzymatic verbenone production. Other important terpenoids are phytosterols. They are used in the production of steroid hormones such as cortison or progesteron by Bayer Sobering Pharma. Probably the most important example where plant secondary metabolism and microbial fermentation are synergistically combined, steroid hormones, are produced from phytosterols. A fermentation process by Bayer Schering Pharma uses a plant metaboUte and converts it by a single-step microbial fermentation into the desired end product. 

Cholesterol (7), the major sterol in most animals, also is found in small amounts in many plants, where it serves as a precursor for other steroid derivatives, such as the progestagens and cardiac glycosides (Heftman, 1984). In plants, algae, and fiingi, a variety of other phytosterols may serve... 

Certain compounds related to steroids and derived from plants are now known to lower total blood cholesterol when used in dietary forms approved by the FDA. Called phytostanols and phytosterols, these patented compounds act by inhibiting intestinal absorption of dietary cholesterol. They are marketed as food in the form of edible spreads. An example of a phytostanol is shown here. 

Hundreds of distinct steroids have been found in plants, animals, and microorganisms, respectively, for example, phytosterols and brassinosteroids in plant, ecdysteroids in insect, cholesterol, sex steroids, corticosteroids and anabolic steroids in vertebrate, and ergosterols in microorganism. Up to now, more than 40 steroid drugs have been marketed since the research and development to steroid drugs was triggered in 1950s [6, 7]. 

Lanosterol and cycloartenol are two tetracyclic triterpenoids found in animals and plants, respectively, from which all steroids are derived. Tiiterpenoid squalene is the first precursor in the biosynthesis of phytostanols and phytosterols in plants and... 

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