Enzymatic synthesis and properties of some novel phytosterol derivatives

Tan, Zhuliang (2011) Enzymatic synthesis and properties of some novel phytosterol derivatives. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Phytosterols and their derivatives occur naturally in the cell membranes of plants and are present at low levels in grains, fruits, and vegetables as well as marine algae. In addition to their well-known effect in lowering low-density lipoprotien cholesterol, phytosterols possess other biological activities, such as anti-inflammatory, antiatherogenicity, and anti-carcinogenic potential. Thus, they have recently received much scientific and commercial attention for the production of nutraceuticals and ingredients for development of functional foods. While most of the research and commercial interest has been on phytosteryl esters with vegetable oil fatty acid mixtures of different chain length, the objectives of this work were to explore enzymatic preparation of phytosteryl esters with phenolic acids or specific fatty acids of interest, such as caprylic acid, oleic acid and docosahexaenoic acid. Phenolic acids are potent antioxidants that scavenge free radicals and other reactive oxygen species. Caprylic acid, oleic acid and docosahexaenoic acid are representatives of medium-chain, long-chain monounsaturated and long-chain polyunsaturated fatty acids, respectively. The health benefits of these fatty acids are well documented. Esterification of phytosterols with phenolic acids, caprylic acid, oleic acid or docosahexaenoic acid may render them better physiochemical properties such as lipid solubility, miscibility, oxidative stability and hence bioactivity and bioavailability. Thus, novel phytosteryl esters, prepared enzymatically, may offer the health benefits of phytosterols and those of phenolic acids, caprylic acid, oleic acid and docosahexaenoic acid, possibly in an additive or synergistic manner. This thesis describes methods for successful enzymatic or chemoenzymatic preparation of selected phytosteryl esters. The identity of the phytosteryl esters so produced was confirmed by using different analytical methods. The enzymatic synthesis of phytosteryl caprylates was optimized using response surface methodology. The antioxidant activity of phytosteryl phenolates was determined using different in vitro assays in a variety of model systems. They exhibited antioxidant activity that was system-dependent and followed different antioxidant mechanisms. Phytosteryl phenolates, especially phytosteryl caffeates, provide an opportunity for future use as food antioxidants. The cholesterol-lowering effect of phytosteryl oleates and docosahexaenoates was evaluated using an apo-E deficient mouse model. Preliminary results indicted that they may be used as excellent ingredients in nutraceuticals and functional foods for cholesterol-lowering purposes. However, phytosteryl oleates and docosahexaenoates exhibited no triacylglycerol-lowering effect in this animal model; in fact they increased the total plasma triacylglycerols level, as expected for this model. Therefore, other animal models may be employed to determine their triacylglycerol-lowering effect. Further research on the evaluation of biological activities of phytosteryl esters so prepared is necessary in order to shed further light on the extent of the beneficial health effects of conjugated compounds reported in this thesis.

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