Coprosterol
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5β-Coprostanol (5β-cholestan-3β-ol) is a 27-
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
stanol Stanol esters is a heterogeneous group of chemical compounds known to reduce the level of low-density lipoprotein (LDL) cholesterol in blood when ingested, though to a much lesser degree than prescription drugs such as statins. The starting materi ...
formed from the biohydrogenation of
cholesterol Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell mem ...
(cholest-5en-3β-ol) in the gut of most higher animals and birds. This compound has frequently been used as a
biomarker In biomedical contexts, a biomarker, or biological marker, is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues to examine normal biological processes, p ...
for the presence of
human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, cultu ...
faecal matter in the
environment Environment most often refers to: __NOTOC__ * Natural environment, all living and non-living things occurring naturally * Biophysical environment, the physical and biological factors along with their chemical interactions that affect an organism or ...
.


Chemical properties


Solubility

5β-coprostanol has a low water
solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solub ...
, and consequently a high
octanol-water partition coefficient The ''n''-octanol-water partition coefficient, ''K''ow is a partition coefficient for the two-phase system consisting of ''n''-octanol and water. ''K''ow is also frequently referred to by the symbol P, especially in the English literature. It is a ...
(log Kow = 8.82). This means that in most environmental systems, 5β-coprostanol will be associated with the solid phase.


Degradation

In
anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...
sediments and soils, 5β-coprostanol is stable for many hundreds of years enabling it to be used as an indicator of past faecal discharges. As such, records of 5β-coprostanol from paleo-environmental archives have been used to further constrain the timing of human settlements in a region, as well as reconstruct relative changes in human populations and agricultural activities over several thousand years.


Chemical analysis

Since the
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
has a
hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydro ...
(-OH) group, it is frequently bound to other
lipids Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include ...
including
fatty acids In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an B ...
; most analytical methods, therefore, utilise a strong alkali (KOH or NaOH) to saponify the
ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides a ...
linkages. Typical extraction solvents include 6% KOH in methanol. The free sterols and stanols (saturated sterols) are then separated from the
polar lipids Polar may refer to: Geography Polar may refer to: * Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates *Polar climate, the cli ...
by partitioning into a less polar solvent such as
hexane Hexane () is an organic compound, a straight-chain alkane with six carbon atoms and has the molecular formula C6H14. It is a colorless liquid, odorless when pure, and with boiling points approximately . It is widely used as a cheap, relatively ...
. Prior to analysis, the hydroxyl group is frequently derivatised with
BSTFA ''N'',''O''-Bis(trimethylsilyl)trifluoroacetamide (BSTFA) is a chemical compound that is used to derivatise labile groups such as hydroxyl on other chemicals, with the more stable trimethylsilyl group, which protects the labile group and allows th ...
(bis-trimethyl silyl trifluoroacetamide) to replace the hydrogen with the less exchangeable trimethylsilyl (TMS) group. Instrumental analysis is frequently conducted on
gas chromatograph Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. Typical uses of GC include testing the purity of a particular substance, ...
(GC) with either a
flame ionisation detector A flame ionization detector (FID) is a scientific instrument that measures analytes in a gas stream. It is frequently used as a detector in gas chromatography. The measurement of ion per unit time make this a mass sensitive instrument. Standalo ...
(FID) or
mass spectrometer Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a '' mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is us ...
(MS). The
mass spectrum A mass spectrum is a histogram plot of intensity vs. ''mass-to-charge ratio'' (''m/z'') in a chemical sample, usually acquired using an instrument called a ''mass spectrometer''. Not all mass spectra of a given substance are the same; for example ...
for 5β-coprostanol - TMS ether can be seen in the figure.


Isomers

As well as the faecally derived stanol, two other isomers can be identified in the environment; 5α-cholestanol


Formation and occurrence


Faecal sources

5β-coprostanol is formed by the conversion of cholesterol to coprostanol in the gut of most higher animals by intestinal bacteria. The general scheme for its production via a ketone intermediate can be seen in the figure proposed by Grimalt et al., 1990.
There are a small number of animals, however, that have been shown not to produce 5β-coprostanol and these can be seen in the table.


Use as a tracer for sewage

The principal source of 5β-coprostanol in the environment is from human wastes. The concentration of 5β-coprostanol in raw, untreated sewage is around 2-6% of the dry solids. This relatively high concentration and its stability allows it to be used in the assessment of the faecal matter in samples, especially sediments.


5β-coprostanol / cholesterol ratio

Since 5β-coprostanol is formed from cholesterol in the
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...
gut, the ratio of the product over reactant can be used to indicate the degree of faecal matter in samples. Raw untreated sewage typically has a 5β-coprostanol / cholesterol ratio of ~10 which decreases through a sewage treatment plant (STP) such that in the discharged liquid wastewaters the ratio is ~2. Undiluted STP wastewaters may be identified by this high ratio. As the faecal matter is dispersed in the environment, the ratio will decrease as more (non-faecal) cholesterol from animals is encountered. Grimalt & Albaiges have suggested that samples with a 5β-coprostanol / cholesterol greater than 0.2 may be considered as contaminated by faecal material.


5β-Coprostanol / (5β-Coprostanol + 5α-Cholestanol) Ratio

Another measure of human faecal contamination is the proportion of the two 3β-ol isomers of the saturated sterol form. 5α-cholestanol is formed naturally in the environment by bacteria and generally does not have a faecal origin. Samples with ratios greater than 0.7 may be contaminated with human faecal matter; samples with values less than 0.3 may be considered uncontaminated. Samples with ratios between these two cut-offs can not readily be categorised on the basis of this ratio alone.
Sediments falling in the red region are classed as “contaminated” by both of the two ratios and those in the green region are classified as “uncontaminated” by the same measures. Those in the blue region are “uncontaminated” according to the 5β-coprostanol / cholesterol ratio and “uncertain” in the 5β-coprostanol / (5β-coprostanol + 5α-cholestanol) ratio. The majority of the samples between the 0.3 and 0.7 cut-offs are considered as “uncontaminated” according to the 5β-coprostanol / cholesterol ratio and so the 0.3 value must be considered as somewhat conservative.


5β-coprostanol / total sterols ratio

Cut-off values etc.


5β-coprostanol / 24-ethyl coprostanol

Herbivores such as cows and sheep consume terrestrial plant matter (grass) which contains β-sitosterol as the principal sterol. β-sitosterol is the 24-ethyl derivative of cholesterol and can be used as a biomarker for terrestrial plant matter (see section). In the gut of these animals, bacteria biohydrogenate the double bond in the 5 position to create 24-ethyl coprostanol and so this compound can be used as a biomarker for faecal matter from herbivores. Typical values in different source materials can be seen in the table after Gilpin.


Epi-coprostanol / 5β-coprostanol

During sewage treatment, 5β-coprostanol may be converted to 5β-cholestan-3α-ol form, epi-coprostanol. There is also a slow conversion of 5β-coprostanol to epi-coprostanol in the environment and so this ratio will indicate either the degree of treatment of sewage or its age in the environment. A
cross-plot A cross-plot is a scatter plot used primarily in Earth science and social science to describe a specialized chart that compares multiple measurements made at a single time or location along two or more axes. The axes of the plot are commonly linear ...
of the 5β-coprostanol / cholesterol ratio with the epi-coprostanol / 5β-coprostanol can indicate both faecal contamination and treatment.


Related markers


5α-cholestanol / cholesterol

In the environment, bacteria preferentially produce 5α-cholestan-3β-ol (5α-cholestanol) from cholesterol rather than the 5β isomer. This reaction occurs principally in anaerobic reducing sediments and the 5α-cholestanol / cholesterol ratio may be used as a secondary (process) biomarker for such conditions. No cut-off values have been suggested for this marker and so it is used in a relative sense; the greater the ratio, the more reducing the environment. Reducing environments are frequently associated with areas experiencing high organic matter input; this may include sewage derived discharges. The relationship between reducing conditions and the potential source can be seen in a cross plot with a sewage indicator.

It may be suggested from this relationship that sewage discharges are in part responsible for the anaerobic reducing conditions in the sediments.


Use in archaeological studies

Coprostanol and its derivative epicoprostanol are used in archaeological and paleoenvironmental studies as indicators of past human activity due to their longevity in soils and strong association with production in the human gut. Researchers have used the presence of coprostanol to identify archaeological features such as
cesspit A cesspit (or cesspool or soak pit in some contexts) is a term with various meanings: it is used to describe either an underground holding tank (sealed at the bottom) or a soak pit (not sealed at the bottom). It can be used for the temporary co ...
s or landscape activities like manuring. Variations in the concentration of coprostanol over time can be used to create human population reconstructions within a specific depositional environment.


See also

* Coprostane


References

{{reflist Cholestanes Bioindicators Biomarkers