Sterols

7 Key excerpts on "Sterols"

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  eBook - ePub

  The Biology of Cholesterol and Related Steroids

  • N. B. Myant(Author)
  • 2014(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Chapter 7

  Sterols in Biological Membranes

  Publisher Summary

  This chapter describes Sterols that play an essential role in determining the structure and physiological properties of biological membranes. In the membranes of animal cells, cholesterol is the major sterol, but other Sterols, including lathosterol, 7-dehydrocholesterol, and 5α-cholesterol are found in small amounts in the membranes of human red cells and of other animal cells. In most plant-cell plasma membranes, stigmasterol and β-sitosterol are the predominant Sterols. Many Sterols other than cholesterol can bring about condensation of phospholipid monolayers composed of a single species of phospholipid, or of mixtures of the lipids extracted from biological membranes, such as red-cell ghosts. The structural features of the sterol and hydrocarbon chains needed for condensation suggest that the effect is because of hydrophobic interaction among the hydrocarbon chains and a planar sterol ring system and side-chain. Sterols influence the physiological properties of natural membranes. The ability of Sterols to keep the hydrocarbon chains of phospholipid bilayers in a state of intermediate fluidity over a wide range of temperature is clearly advantageous to plants, and to animals that do not maintain a constant body temperature as it must permit their cell membranes to function optimally in the face of changes in environmental temperature.

  1 LIPID COMPOSITION OF MEMBRANES 1

  2 PHOSPHOLIPIDS AND MEMBRANE STRUCTURE 2

  3 STEROL-PHOSPHOLIPID INTERACTIONS

  3.1 Condensation in monolayers 3

  3.2 Effect of Sterols on phase changes 4

  4 EFFECTS OF Sterols ON MEMBRANE FUNCTION 5

  4.1 Liposomes 6

  4.2 Natural membranes 7

  5 DISTRIBUTION AND MOBILITY OF MEMBRANE CHOLESTEROL 8

  6 EVOLUTIONARY SIGNIFICANCE OF MEMBRANE Sterols 9

  1 LIPID COMPOSITION OF MEMBRANES

  Sterols are thought to play an essential role in determining the structure and physiological properties of biological membranes. Indeed, this may well be the most primitive, in an evolutionary sense, of all the functions of Sterols, their other functions as precursors of essential metabolites probably arising only at a much later stage of the evolution of living systems. That Sterols have a very long biological history is suggested by their presence in some modern pro-karyotic organisms and by the identification of cyclic hydrocarbons with a cholestane skeleton, undoubtedly of organic origin, in shales known to be at least 3000 million years old (Calvin, 1969

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  eBook - PDF

  Lipid Chemistry

  • Khetarpaul, Vipul(Authors)
  • 2021(Publication Date)
  • Daya Publishing House

    (Publisher)

  Chapter 5 Sterols and Waxes 5.1Introduction Sterols constitute a large group of compounds with a broad range of biological activities and physical properties. They are common in eukaryotic cells but more in prokaryotes. The natural occurring Sterols usually possess the 1,2-cyclopentanophenthrene skeleton with a stereochemistry similar to the trans-syn-trans-anti-trans-anti configuration at their ring junctions, and have 27 to 30 carbon atoms with an hydroxy group at C3 and a side chain of at least seven carbons at C17. Sterols can exhibit both nuclear variations (differences within the ring system) and side chain variations. Sterols have been defined as hydroxylated steroids that retain some or all of the carbon atoms of squalene in the side chain and partition almost completely into an ether layer when shaken with equal volumes of water and ether. Sterols are common in eukaryotic cells but rare in prokaryotes. Without exception, vertebrates confine their sterol biosynthetic activity to producing cholesterol. Most invertebrates do not have the enzymatic machinery for sterol biosynthesis and must rely on an outside supply. Sterols of invertebrates have been found to comprise most complex mixtures arising through food chains. In plants, cholesterol exists only as a minor component. Sitosterol and stigmasterol are the most abundant and widely distributed plant Sterols, while ergosterol is the major occurring sterol in fungus and yeast. The plant Sterols are characterized by an additional alkyl group at C-24 on the This ebook is exclusively for this university only. Cannot be resold/distributed. cholesterol nucleus with either α- or β-chirality. Sterols with methylene and ethylidene substitutes are also found in plants ( e.g. 24-methylene cholesterol, fucosterol). The other major characteristics of plant Sterols are the presence of additional double bonds in the side chain, as in porifeasterol, cyclosadol, and closterol.

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  eBook - PDF

  Chemical, Biological, and Functional Aspects of Food Lipids

  • Zdzislaw Z. E. Sikorski, Anna Kolakowska(Authors)
  • 2010(Publication Date)
  • CRC Press

    (Publisher)

  .The. various.Sterols.differ.only.in.their.side.chain . .Surprisingly,.these.minor.differences. result.in.major.changes.in.biological.functions . .They.are.widely.distributed.in.nature,. and.occur.both.in.the.free.form.or,.most.frequently,.as.esters.of.higher.aliphatic.acids. and.glycosides . .Sterols,.sterol.esters,.and.sterol.glycosides.are,.to.different.degrees,. soluble.in.fats.but.completely.insoluble.in.water . Since. the. discovery. of. cholesterol,. Sterols. have. continued. to. be. the. focus. of. research.activities.of.many.chemists,.biochemists,.and.clinicians,.as.attested.to.by. the.fact.that.three.Nobel.Prizes.awarded.between.1910.and.1985.were.associated. with.work.on.Sterols . .The.continued.interest.of.food.scientists.in.dietary.oxidized. cholesterol.is.related.to.atherosclerosis . .In.this.chapter,.the.recent.developments.in. the.area.of.occurrence.and.biological.effect.of.phytoSterols,.especially.in.the.lower-ing.of.blood.cholesterol,.and.their.presence.in.diet.is.extended.in.relation.to.the.first. edition.of.this.title.[1] . 7.2 CHOLESTEROL 7.2.1 S TRUCTURE AND O CCURRENCE Cholesterol,.with.a.C27.carbon.skeleton.(Figure.7 .1), .is.synthesized.and.utilized.by. animals. .Today,.it.is.most.widely.known.for.its.association.with.atherosclerotic.heart. disease. .However,.despite.its.negative.reputation,.cholesterol.is.a.necessary.constitu-ent.of.all.human.cells.and.fulfills.important.functions,.as.an.essential.component.of. mammalian.cell.membranes,.a.precursor.for.steroid.hormones.and.bile.acids,.as.well. as.being.involved.in.various.cell.signaling.pathways.[2] . .Cholesterol.can.be.present. in.the.free.form.or.esterified.at.the.hydroxyl.group.with.fatty.acids.of.various.chain. length.and.saturation . .It.also.occurs.in.plants,.usually.in.very.small.quantities,.and. marine.algae . .The.content.of.cholesterol.in.some.foods.is.presented.in.Table.7 .1. There. exists. a. strong. positive. correlation. between.

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  eBook - PDF

  Nutraceutical and Specialty Lipids and their Co-Products

  • Fereidoon Shahidi(Author)
  • 2006(Publication Date)
  • CRC Press

    (Publisher)

  500 27.1 INTRODUCTION PhytoSterols are analogs of cholesterol which occur in plant materials and, similar to Sterols in mammalian cells, they are structural and functional constituents of cell membranes 1,2 . Sterols have the tetracyclic ring system specific to steroids, a hydroxyl group at C-3, and a side chain of differ-ent length at C-17. The IUPAC-IUB rules for numbering Sterols and for ring lettering are based on the cholestane (cyclopenta[ α ]phenanthrene) skeleton shown for sitosterol in Figure 27.1. PhytoSterols can be divided, on both structural and biosynthetic bases, into 4-demethylSterols, 4-monomethylSterols, and 4,4-dimethylSterols based on the number of methyl groups at C-4 in the tetracyclic ring system (Figure 27.2). Plant Sterols are not endogenously synthesized in the human body and are derived exclusively from dietary sources.c 4-DemethylSterols which do not carry methyl groups at C-4 are the major class of Sterols in (desmethylSterols) plants. The 4-demethylSterols constitute > 85% of total Sterols in common crude vegetable oils 3 . Sitosterol ( β -sitosterol), the major sterol in most plants, is usually accompanied by its 22-dehydro analog, stigmasterol. The molecular structures of the most important plant Sterols and stanols are shown in Figure 27.3. Another abundant plant sterol is 24-methylcholesterol, Plant Sterols and Steryl Esters in Functional Foods and Nutraceuticals Nikolaus Weber and Kumar D. Mukherjee Institute for Lipid Research, Federal Research Centre for Nutrition and Food, Münster, Germany 27 483 frequently referred to as campesterol. 24-Methylcholesterol is usually a mixture of two epimers, i.e., 24 α -methylcholesterol (campesterol) and 24 β -methylcholesterol (22,23-dihydrobrassicasterol).

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  eBook - ePub

  The Membranes of Cells

  • Philip L. Yeagle(Author)
  • 2016(Publication Date)
  • Academic Press

    (Publisher)

  Chapter 9

  Cholesterol and Related Sterols

  Roles in Membrane Structure and Function

  Abstract

  Consideration of a long record of research, including recent results, on the behavior and influences of cholesterol in the membranes of mammalian cells focuses attention on a new model for the required roles of cholesterol in mammalian cell biology. Most cellular cholesterol is found in the plasma membrane of mammalian cells. Cholesterol decreases membrane permeability through an ordering of the lipid hydrocarbon chains. Membrane cholesterol modulates the activity of integral membrane enzymes. Stimulation of membrane protein activity likely occurs through structurally specific sterol–protein binding, stabilizing the active conformation of the protein. Inhibition of membrane enzyme activity can occur through an analogous mechanism. Inhibition can also be promoted by a cholesterol-driven reduction in free volume within the bilayer, inhibiting protein conformational changes necessary for protein function. The same model can be applied to understand the essential roles of ergosterol in yeast and of plant Sterols in plants. While mammalian cells specifically require cholesterol, yeast specifically requires ergosterol, and plants specifically require plant Sterols for growth.

  Keywords

  Cholesterol; ergosterol; β-sitosterol; lanosterol; free volume; cholesterol binding sites

  Cholesterol poses a fascinating paradox in biology. The bright side of the cholesterol story is the finding that mammalian cell growth is absolutely dependent upon having minimal levels of (specifically) cholesterol in mammalian cell membranes. Without that cholesterol, mammalian cell growth does not occur. Cholesterol is consequently an essential lipid in membranes of mammalian cells. Because of its importance, most cells that require cholesterol for cell growth synthesize cholesterol to satisfy that specific molecular requirement. Fig. 9.1

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  Chemistry Of Marine Natural Products

  • Poul Schever(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  Because of these circumstances well-authenticated, largely recent work will receive major emphasis in the discussion that follows. A recent general review that includes marine Sterols was published in 1970 (Brooks, 1970), but none of the more current work of great interest is covered, because of the normal time lag between manuscript and published book. In addition, Sterols of invertebrates have been reviewed by Austin (1970) molluscan Sterols by Idler and Wiseman (1972), echinoderm Sterols by Goad and co-workers (1972), and crustacean Sterols by Idler and Wiseman (1971). Much new research on marine Sterols is now in progress. Arrangement of the material according to carbon content appears to be simple and straightforward, although carbon content per se is no longer considered to be a significant property. For many years the carbon range of Sterols extended from C 2 7 to C 2 9 , but recent discoveries of a norcholesterol and of gorgosterol and its analogs have increased the carbon range of marine Sterols from C 2 6 to C 3 0 ; yet even now C 2 9 Sterols predominate. It is worth noting that the carbon variation occurs exclusively in the side chain,* and not unexpectedly most of it at C-24. What structural diversity other than carbon content exists among marine Sterols is almost entirely found in the side chain also, with the notable exception of the highly oxygenated C 2 7 moulting hormones (Section 2, B,6) and of some bile alcohols (Section 2, B,9). * Gibbons et al. (1968) in their study of the green algae Enteromorpha intestinalis and Ulva lactuca report indications of the presence of lophenol (a 4-methylsterol) derivatives. None of these has as yet been isolated. 60 2. Sterols There are a number of early reports (e.g., Bergmann et al, 1945) on the isolation of marine Sterols that are saturated in the nucleus, such as cholest-anol and 24a-methylcholest-22-en-3ß-ol (22,23-dehydrocampestanol).

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  eBook - PDF

  Cholesterol

  Chemistry, Biochemistry, and Pathology

  • Robert P. Cook(Author)
  • 2015(Publication Date)
  • Academic Press

    (Publisher)

  Also, besides its intrinsic biological importance, cholesterol has played the part of an easily accessible substance, on which organic chemists have in the past tried and tested many theories of stereochemistry and re-action mechanisms. It will undoubtedly remain their Happy Hunting Ground for several years to come. The chemistry of the whole field of steroids is covered by several monographs: Fieser and Fieser (1949), Lettré and Tschesche (1954), Shoppee and Shoppee (1953), Shoppee (1958), Klyne (1957), and Nazarov and Bergel'son (1955); and by the appropriate volumes of Elsevier's Encyclopaedia of Organic Chemistry (Josephy and Radt, 1940; Georg, 1954, 1956). CHEMISTRY 17 II. Nomenclature The general recommendations of the Ciba Conference on Steroid Nomenclature are followed (Ciba Conference, 1951). Some of the more important aspects of the subject which are encountered in this book are discussed below. 30 31 (I) (Π) The formula (I) shows the skeleton of cholesterol numbered in the generally accepted manner. The formula (II) shows the additional numbering adopted when one, or two extra carbon atoms are present at C-24 in the side chain, and when extra methyl groups are present at C-4 and C-14. The usual conventions are adopted in naming stereo-isomers. Substituents below the plane of the ring are a-substituents; and are joined to the ring by broken lines. Substituents above the plane of the ring are ß-substituents and are joined to the ring by solid lines. Substituents of uncertain configuration are denoted by the Greek letter ξ (Xi) in names, and by wavy lines in formulas. In the formulas above, each angle of the structures represents a carbon atom and a sufficient number of hydrogen atoms to fill all the unused valencies of the carbon atom. Also, the terminations of the side chains and the strokes in the angular positions (C-10, C-13, and C-14) represent methyl groups.

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