Brassicasterol


Brassicasterol is a 28-carbon sterol synthesised by several unicellular algae and some terrestrial plants, like rape. This compound has frequently been used as a biomarker for the presence of algal matter in the environment, and is one of the ingredients in stigmasterol-rich plant sterols. There is some evidence to suggest that it may also be a relevant additional biomarker in Alzheimer's disease.

Chemical properties

Solubility

Brassicasterol has a low water solubility and, as a consequence, a high octanol-water partition coefficient. This means that, in most environmental systems, brassicasterol will be associated with the solid phase.

Degradation

In anaerobic sediments and soils, brassicasterol is stable for many hundreds of years, enabling it to be used as an indicator of past algal production.

Chemical analysis

Since the molecule has a hydroxyl group, it is frequently bound to other lipids including glycerols; most analytical methods, therefore, utilise a strong alkali to saponify the ester linkages. Typical extraction solvents include 6% KOH in methanol. The free sterols are then separated from the polar lipids by partitioning into a less polar solvent such as hexane. Prior to analysis, the hydroxyl group is frequently derivatised with BSTFA to replace the hydrogen with the less exchangeable trimethylsilyl group. Instrumental analysis is frequently conducted on gas chromatograph with either a flame ionisation detector or mass spectrometer. The mass spectrum for the TMS ether of brassicasterol can be seen in the figure.

Mass fragmentation pattern for brassicasterol at 70eV on a Fisons MD800 mass spectrometer

Formation and occurrences

It can be found in Mirabilis jalapa.

Algal sources

Brassicasterol is formed in plants from the isoprenoid squalene through campesterol as an intermediate. A list of the algae in which brassicasterol has been identified is shown below together with approximate composition.
SpeciesABCDEFGHothers
Gonyaulax spp10000000000
Peridinium foliaceum10000000000
Peridinium foliaceum80200000000
Gonyaulax diegensis390000002932
Pyrocystis lunula76602100015
Gonyaulax polygramma36109700047
Gymnodinium wilczeki263903510000
Glenodinium hallii85000042000
Noctiluca milaris01157306014
Gymnodinium simplex00005300047
Prorocentrum cordatum70005063025

Use as a tracer for marine algae

The principal source of brassicasterol in the environment is from marine algae. Its relatively high concentration and stability allows it to be used in the assessment of the origin of organic matter in samples, especially sediments.

Brassicasterol / cholesterol ratio

The concentration of brassicasterol in a core sample from Loch Striven, Scotland. Highest values may be seen in the top sections of the sediment, which decrease with depth. However, the cholesterol behaves in a similar manner, and the ratio brassicasterol/cholesterol is fairly uniform at all depths, indicating either a comparable degradation rate with no change in source or different degradation rates and a change in source.

Multivariate analysis

such as principal component analysis of a range of lipid biomarkers enable identification of compounds that have similar origins or behaviour. An example can be seen in the loadings plot for sediment samples from the Mawddach Estuary, Wales.
Principal Component Analysis of several lipid biomarkers from the Mawddach Esturay - brassicasterol is highlighted in red

The location of brassicasterol in this figure indicates that the distribution of this compound is similar to that of the short-chain fatty acids and alcohols, which are of marine origin. The terrestrially derived biomarkers such as β-sitosterol are on the opposite side of the figure and are mutually exclusive.