L'un des avantages majeurs des réactions de bioconversion résulte du fait que le milieu réactionnel des biocatalyseurs est l'eau. Ce dit avantage constitue l'une des principales limitations de ces réactions de bioconversion lorsqu'il s'agit de substances lipophiles non solubles dans l'eau connue les stérols. L'efficacité d'un procédé de bioconversion de substances lipophiles solides dépend essentiellement du contact et de 1'interaction entre le biocatalyseur et ce substrat lipophile. Les solutions proposées dans la littérature font appel à des solvants et des produits chimiques de natures souvent toxiques, inflammables et explosives. Ces solutions décrites font perdre à la bioconversion son caractère de biotechnologie blanche. Dans ce travail, nous avons montré qu'en plus de ses avantages connus, l'immobilisation passive de biocatalyseurs au sein d'un support poreux peut favoriser l'interaction cellule-substrat lipophile et augmenter le taux de bioconversion sans utiliser de solvants et de produits chimiques. La réaction modèle étudiée est le clivage de la chaine latérale des stérols par des mycobactéries en vue de l'obtention des androsténones précurseurs naturels des stéroïdes, molécules à forte valeur biologique ajoutée. Le support d'immobilisation le plus performant a été le fruit sec de Luffa cylindrica. Par rapport aux supports organiques classiques tels que les gels de polyacrylamide, les mousses de polyuréthane, la silicone et les plastiques, le fruit sec de Luffa cylindrica présente les avantages suivants : (i) c'est un produit naturel, (ii) biodégradable, (iii) peu onéreux, (iv) non toxique pour les microorganismes, (v) stable du point de vue mécanique et thermique, (vi) et réutilisable. / One of the major advantages of using biocatalysts in organic synthesis is that water constitutes the reaction medium. However, water becomes a serious problem when bioconversion deals with lipophilic compounds, in particular those poorly soluble in water such as sterols. Bioconversion of lipophilic compounds depends on the close contact between the hydrophobic substrate and the biocatalyst. Increasing this contact requires usually the use of huge amounts of chemical which are often toxic, flammable and explosive. In this work, we showed that passive cell immobilization in porous materials can increase the contact between microorganisms and lipophilic substrates without using chemicals. The side chain cleavage of sterols was studied as a model multistep microbial bioconversion of lipophilic compounds. This reaction allows the production of androstenones which are the natural precursors of steroids. Among the studied immobilization carriers, the dried fruit of Luffa cylindrica was the most efficient Compared to other organic support carrier such as alginate beads, polyurethane foams, silicones and plastics, the dried fruit of Luffa cylindrica is advantageous since it is natural, renewable, biodegradable, cheap, mechanically strong, free of toxicity and it doesn't need a chemical pretreatment.

Sewage pollution from humans, animal and domestic sources (land and agricultural run-off) are recognized as a major cause of deteriorating water quality along Australia's coastline. Management of water quality has primarily relied on the use of bacterial indicator methods. However the validity and source-specificity of these methods have been met with increasing reservations for several years now. A relatively recent methodology uses a different chemical biomarker approach using 'sterols', a group of compounds related to the common bio-membrane lipid cholesterol and its derivatives. Sterols can offer an additional diagnostic tool to distinguish and discriminate between sources of faecal contamination in marine, freshwater and estuarine environments in both sediments and the water column. This study investigates for the first time, the degradation of coprostanol and selected faecal sterols in 'natural' sediments from a highly mixed (marine and estuarine) sub-tropical environment following a simulated pollution event (primary effluent); the use of faecal sterols as an additional indicator for determining non-point source sewage discharges at popular anchorages in the Moreton Bay and Gold Coast Broadwater system; and the use of sterol ratios in the determination of the fate and transportation of nutrients from a Sewage Treatment Plant (STP) point-source outlet pipe during plant malfunction. The microcosm degradation experiment revealed that faecal and selected sterols are continually synthesised and degraded over time by auto- and hetero trophic organisms within the sediment matrix. Coprostanol was the only sterol to degrade continually, with only minor fluctuations over a time period of two months. Results from this degradation experiment further revealed a sharp decline of coprostanol within the first week. From this it could be concluded that, without any further addition, external inputs of coprostanol are reduced to background levels within this time period. Therefore, removal of coprostanol after six days was 94% and 73% in mud and sand, respectively. The removal of coprostanol was much higher in mud than sand, reflecting a higher level of microbial activity in muddy sediments for assimilation of sterols. The field study undertaken at popular anchorages in Moreton Bay and the Gold Coast Broadwater revealed extremely low levels of sterols and bacterial indicators over both a spatial and temporal scale consistent with a shallow, oligotrophic, highly dynamic sand dominated system. Even though sterols analysed were found at extremely low levels (mostly in the nano-gram range), they were found to be highly correlated and were successful in identifying an unexpected once off pollution event from a point source at Moreton Bay Island. Other than this one incident, both sterol and bacterial levels were consistently low even when anchorages were at full capacity. Thus, sewage from recreational vessels was found to have very little, if any, effect on the water quality at anchorages in Moreton Bay and Gold Coast Broadwater. The point-source study conducted during a local sewage treatment plant malfunction revealed that even though absolute concentrations of sterols did not change during this event, the distribution of sterols within the samples changed, hence changing the sterol ratios. Further, nutrients (mainly nitrogen) can be transported several kilometres by currents, flocculate out of the water column and settle out into the sediment in areas with low tidal and hydrological flushing. There, the nutrients can cause in situ production of sterols in sediments changing sterol ratios. Overall, this study revealed that analyses of sterol biomarkers have the potential to indicate nutrient inputs (such as nitrogen) as well as sewage, post-hoc pollution events at extremely low levels/high dilutions in coastal sediments.

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The objective of this research was to examine the effects of sitosterol and sitostanol supplementation on plasma cholesterol levels and cholesterol metabolism in hypercholesterolemic subjects consuming a fixed foods diet in a four-phase crossover design. It was hypothesized that addition of either phytosterols, phytostenols, or a 50:50 mixture of sterols and stanols to butter would reduce circulating cholesterol levels, despite butter's hypercholesterolemic effect, through actions involving cholesterol absorption, synthesis, and turnover rates. The data obtained indicate that in their free, unesterified form, plant sterols and stanols lower plasma LDL cholesterol equivalently in hypercholesterolemic subjects. Results of this study provide new data that phytosterols and stanols function by suppressing cholesterol absorption while increasing cholesterol synthesis, however, the de-suppression in synthesis cannot fully compensate for the decrease in absorption making the treatment effective, thus may assist in the development of a food which offers health-promoting advantages related to the prevention of cardiovascular disease.

Background. A high ratio of total cholesterol to high-density lipoprotein (HDL) cholesterol, in addition to increased levels of small low-density lipoprotein (LDL) particles, are important indicators of cardiovascular disease risk. Therefore, interventions that combine the lowering of total cholesterol and raising of HDL cholesterol concentrations that also increase LDL particle size, may be preventive against cardiovascular disease. Plant sterols decrease total cholesterol and LDL cholesterol levels by 10-15%, while exercise increases HDL cholesterol levels by 4-22%. In view of their complementary effects, combining plant sterols with exercise would appear to be an effective lifestyle therapy to decrease the risk of future cardiovascular disease. / Objective. The aim of this study was to examine the independent and combined effects of plant sterols and exercise on blood lipid levels, and LDL particle size in previously sedentary, hypercholesterolemic adults. An additional objective of this trial was to assess the underlying mechanism by which this combination therapy modulates whole body cholesterol metabolism, to in turn improve lipid profiles. / Methods. In an 8-week, parallel-arm trial, 84 subjects were randomized to 1 of 4 interventions: (1) plant sterols and exercise,(2) plant sterols alone, (3) exercise alone, or (4) control. Blood lipid concentrations were measured using enzymatic kits, and LDL particle size was assessed using polyacrylamide gel electrophoresis. Cholesterol absorption and synthesis were determined using the single isotope single tracer technique and the deuterium incorporation approach, respectively. / Results. Plant sterol supplementation decreased (P < 0.01) total cholesterol concentrations by 8.2% when compared to baseline. Exercise increased (P < 0.01) HDL cholesterol levels by 7.5% while decreasing (P < 0.01) triglyceride concentrations by 13.3% when compared to baseline. Exercise reduced (P < 0.05) post-treatment LDL peak particle size from 255 to 253 A, and decreased (P < 0.05) the proportion of large LDL particles by 13.1%. Plant sterols had no effect on particle size distribution. Plant sterol supplementation decreased (P < 0.01) intestinal cholesterol absorption by 18%, while exercise had no effect on cholesterol absorption. Non-significant increases in cholesterol synthesis rates of 63%, 59%, and 57%, were observed in the combination, exercise, and plant sterol groups, respectively, relative to control. / Conclusion. These findings suggest that this combination therapy yields the most favourable alterations in lipid profiles when compared to each intervention alone. This combined intervention exerts its beneficial effects on lipid profiles by suppressing intestinal cholesterol absorption. Therefore, this lifestyle therapy may be an effective means of decreasing the risk of cardiovascular disease in hypercholesterolemic adults.

Reduced bioavailability in some formulations of phytosterols accounts for the variable results observed in LDL- C- lowering efficacy among trials. This study examined the effects of a water-dispersible formulation of free phytosterols (WD-PS) versus phytosterol esters (PS-esters) on plasma lipid and fat soluble vitamins concentrations in hypercholesterolemic individuals. Using a double-blind, randomized, crossover study, 47 hypercholesterolemics were provided for 4 wk: WD-PS-enriched yogurt (2g/d), PS-esters-enriched yogurt (2g/d), or yogurt alone (placebo), in a random order. Each study phase was separated by 4 wk washout intervals. Supplementation of WD-PS or PS-esters similarly decreased serum TC (7.7% and 6.3%, respectively) and LDL-C levels (11.7% and 11.6%, respectively, p<0.001). The ratio of TC/ HDL-C decreased for WD-PS (10.6%, p<0.05) but not for PS-esters. Moreover, WD-PS reduced serum TG (13.9%, p<0.05) as compared to PS-esters (0.6%). The results of the current study confirm the importance of the formulation of phytosterols in their bioavailability and efficacy.

To evaluate the relative efficacy of plant sterols (PS) esterified with the fatty acids from fish oil (PS-FO), olive oil (PS-OO) and sunflower oil (PS-SO) on blood lipid and PS concentrations, 21 hyperlipidemic subjects were randomly assigned to each of five treatments for 28 days using a cross-over design. The results showed that: (1) in a comparison of olive oil (OO), fish oil (FO), PS-FO and PS-SO subgroup, PS-FO reduced triacylglycerols (TG) relative to PS-SO. Total cholesterol (T-C)/high-density lipoprotein cholesterol (HDL-C) ratio was reduced with PS-FO compared with FO. Plasma PS levels were increased with PS-SO and PS-FO. (2) in a comparison of OO, PS-SO and PS-OO subgroup, PS-OO had a larger decrease in T-C than OO, while PS-SO and OO reduced T-C equally. Both PS-SO and PS-OO elevated plasma PS levels. Overall, PS-FO and PS-OO have a higher potential for decreasing the risk of cardiovascular disease in hyperlipidemic subjects than PS-SO and OO supplementations.

Sterols are lipid components of eukaryotic membranes. Alterations of membrane sterol composition perturb the execution of cell division, which in diverse eukaryotes can have severe consequences for development of the organism. Partitioning of the cytoplasm during cell division occurs at the final stage of cell division named cytokinesis. In somatic plant cells, cytokinesis is initiated by fusion of membrane vesicles in the plane of cell division resulting in a transient compartment termed the cell plate. Cell plate maturation relies on temporal and spatial orchestration of membrane fusion and endocytosis. Impaired vesicle fusion or defects in endocytosis result in cytokinetic defects. In Arabidopsis thaliana, the KNOLLE and DYNAMIN-RELATED PROTEIN 1A (DRP1A) contribute to cytokinesis. KNOLLE mediates fusion of vesicles at the plane of cell division while DRP1A appears to be involved in cell plate maturation through its role in clathrin-mediated endocytosis. This thesis shows that KNOLLE is specifically restricted to the cell division plane through sterol-dependent endocytosis that involves a clathrin- and DRP1A-mediated mechanism. Sterols affect internalization of KNOLLE through their role in lateral membrane organization by keeping diffusion of KNOLLE to lateral membranes in check via its endocytic removal. It is shown that the cell plate represents a high-lipid-order membrane domain that depends on the correct composition and the right concentration of sterols. Accumulation of DRP1A at the cell plate requires correct sterol concentration and composition similar to high-lipid order. Conversely, high-lipid-order at the cell plate relies on DRP1A activity suggesting a feedback between DRP1A function and lipid order establishment. Finally, it is shown that sterols are also present at the tonoplast of dividing and elongated root cells. Taken together, the results reveal that formation of the cell plate in Arabidopsis thaliana depends on an intricate interplay between cytokinetic vesicle fusion, sterol-dependent lateral membrane and high-lipid-order domain organization as well as endocytic machinery function.

Reduced bioavailability in some formulations of phytosterols accounts for the variable results observed in LDL- C- lowering efficacy among trials. This study examined the effects of a water-dispersible formulation of free phytosterols (WD-PS) versus phytosterol esters (PS-esters) on plasma lipid and fat soluble vitamins concentrations in hypercholesterolemic individuals. Using a double-blind, randomized, crossover study, 47 hypercholesterolemics were provided for 4 wk: WD-PS-enriched yogurt (2g/d), PS-esters-enriched yogurt (2g/d), or yogurt alone (placebo), in a random order. Each study phase was separated by 4 wk washout intervals. Supplementation of WD-PS or PS-esters similarly decreased serum TC (7.7% and 6.3%, respectively) and LDL-C levels (11.7% and 11.6%, respectively, p<0.001). The ratio of TC/ HDL-C decreased for WD-PS (10.6%, p<0.05) but not for PS-esters. Moreover, WD-PS reduced serum TG (13.9%, p<0.05) as compared to PS-esters (0.6%). The results of the current study confirm the importance of the formulation of phytosterols in their bioavailability and efficacy.

Plant sterols are effective cholesterol-lowering agents; however, recent evidence suggests that this treatment may not be safe and beneficial in all individuals. This study determined whether high and low baseline circulating plasma campesterol and sitosterol are related to subsequent changes in plasma LDL-C, plant sterol or CRP levels, after accounting for plant sterol supplementation in hypercholesterolemic men (n=82). This trial was a 2-phase randomized cross-over design consisting of a controlled diet with and without a dose of 2.0 g/d of plant sterols over 4 weeks. There was no significant difference in plasma LDL-C, in the elevation of plasma plant sterol or in the changes of CRP levels for high and low groups, respectively. In view of these data, a supplement of 2.0 g/d of plant sterols should be viewed as a safe and beneficial cholesterol-lowering therapy for all individuals, with respect to their baseline plasma plant sterol levels.