Jari Toivo

Plant sterols : biosynthesis, biological function and their importance to human nutrition

Plant sterols are an essential component of the membranes of all eukaryotic organisms. They are either synthesised de novo or taken up from the environment. Their function appears to be to control membrane fluidity and permeability, although some plant sterols have a specific function in signal transduction. The phytosterols are products of the isoprenoid pathway. The dedicated pathway to sterol synthesis in photosynthetic plants occurs at the squalene stage through the activity of squalene synthetase. Although the activity of 3-hydroxymethyl-3-glutaryl coenzyme A (HGMR) is rate-limiting in the synthesis of cholesterol, this does not appear to be the case with the plant sterols. Up-regulation of HGMR appears to increase the biosynthesis of cycloartenol but not the Δ5-sterols. A decline in sterol synthesis is associated with a suppression of squalene synthetase activity, which is probably a critical point in controlling carbon flow and end-product formation. The major post-squalene biosynthetic pathway is regulated by critical rate-limiting steps such as the methylation of cycloartenol into cycloeucalenol. Little is known about the factors controlling the biosynthesis of the end-point sterol esters or stanols. The commonly consumed plant sterols are sitosterol, stigmasterol and campesterol which are predominantly supplied by vegetable oils. The oils are a rich source of the steryl esters. Less important sources of sterols are cereals, nuts and vegetables. The nutritional interest derives from the fact that the sterols have a similar structure to cholesterol, and have the capacity to lower plasma cholesterol and LDL cholesterol. Since the morbidity and mortality from cardiovascular disease have been dramatically reduced using cholesterol-lowering drugs (statins), the interest in plant sterols lies in their potential to act as a natural preventive dietary product. Stanols (saturated at C-5) occur in low amounts in the diet and are equally effective in lowering plasma cholesterol and do not cause an increase in plasma levels, unlike the sterols which can be detected in plasma. © 2000 Society of Chemical Industry

Plant Sterols in Cereals and Cereal Products

ABSTRACT The total plant sterol contents (free sterols and covalently bound structures) of the main cereals cultivated in Finland were determined. Furthermore, sterol contents were determined for different flour and bran fractions in the milling process of wheat and rye, as well as plant sterol contents in various milling and retail bakery products. The sample preparation procedure included acid and alkaline hydrolysis to liberate sterols from their glycosides and esters, respectively. Free sterols were extracted and, after recovery using solid-phase extraction, derivatized to trimethylsilyl ethers for gas chromatography (GC) analysis. We used GC with a mass spectrometer (MS) for identification. When two cultivars of rye, wheat, barley, and oats grown in the same year were compared, the highest plant sterol content was observed in rye (mean content 95.5 mg/100 g, wb), whereas the total sterol contents (mg/100 g, wb) of wheat, barley, and oats were 69.0, 76.1, and 44.7, respectively. In addition, the 10 ry...

Sterol and vitamin D2 contents in some wild and cultivated mushrooms

Abstract The contents of vitamin D 2 and sterols in some wild and cultivated mushrooms were determined, and the distribution of these compounds in different parts of the wild mushrooms was evaluated. In addition, the variation in vitamin D 2 contents between individual fruiting bodies of wild mushrooms was studied. Vitamin D 2 was determined using an HPLC method, including saponification and semipreparative normal-phase HPLC purification before analytical reversed-phase quantification with an internal standard. Sterol contents were analysed with gas chromatography using an internal standard method, including saponification before derivatizing sterols to trimethylsilyl ethers. Mass-spectral analyses were used to further confirm the identification of sterols. Vitamin D 2 was almost totally absent in cultivated mushrooms, while some wild mushrooms contained high concentrations of this vitamin (4.7–194 μg/100 g dry weight). Ergosterol was the most abundant sterol found in mushrooms, and its contents were higher in cultivated mushrooms (602.1–678.6) than in wild mushrooms (296–489 mg/100 g dry weight). The contents of vitamin D 2 and ergosterol varied greatly and moderately, respectively, between different parts of the mushrooms and were lowest in stipes. In addition, high variation in vitamin D 2 contents between individual fruiting bodies was found.

Determination of thermo-oxidation products of plant sterols

Plant sterols are subjected to oxidation when exposed to air and, especially, when heated at high temperatures. We developed a method to study thermo-oxidation of plant sterols. The method consisted of cold saponification, purification of oxides by solid-phase extraction and gas chromatography analysis. To compensate for losses during the procedure, an internal standard was added before saponification. The method showed good recovery of added cholesterol oxides, separation of plant sterol oxides and reproducibility in detecting thermo-oxidation products of stigmasterol and rapeseed oil. Based on this study, the major products are 7-hydroxy, 5,6-epoxy and 7-keto compounds and oxides are formed faster in bulk stigmasterol than in rapeseed oil.

Factors affecting sample preparation in the gas chromatographic determination of plant sterols in whole wheat flour.

Abstract A fractional factorial experimental design was applied to study factors affecting sample preparation prior to a gas chromatographic determination of total plant sterols in cereal samples. Whole wheat flour was chosen for the representative matrix. Altogether six factors were studied at two levels. The most affecting factors were a type of hydrolysis (combined acid hydrolysis and alkaline hydrolysis over alkaline hydrolysis alone), extraction solvent after saponification (chloroform over hexane–ether), and sample amount (1.0 g over 2.0 g). Also, it was shown that there is no need to perform total lipid extraction prior to hydrolysis. The use of acid hydrolysis prior to alkaline saponification was found to be essential in a plant sterol analysis of flour matrix. In a further method development, that procedure is to be optimized with various foods of plant origin in order to establish a general method to determine plant sterols. Toivo et al., Factors in the determination of plant sterols

Acid-catalyzed isomerization of fucosterol and Δ5-avenasterol

This work shows that fucosterol, Δ5-avenasterol, and similar ethylidene-side chain sterols can undergo acid-catalyzed isomerization to give a mixture of five isomers. Four isomers formed from fucosterol were analyzed, using gas chromatography-mass spectrometry, and were characterized as Δ5-avenasterol two Δ5,23-stigmastadienols, and Δ5,24(250)-stigmastadienol. When the unsaponifiables fraction from oat oil was subjected to acid hydrolysis, the two Δ5,23-stigmastadienol isomers and Δ5,24(25)-stigmastadienol were detected while fucosterol coeluted with sitosterol. Interisomerization of ethylidene-side chain sterols represents a limitation to the use of the acid hydrolysis method in the determination of sterols in food and other plant materials rich in these sterols, e.g., oat lipids.