The purpose of this study was to investigate the effects of temperature, oxidation time, presence of water, pH, type of buffer and form of substrate used on cholesterol oxidation. Microcrystalline cholesterol films, both solid and melted, and aqueous suspensions of film fragments were used as substrates. Use of dispersing agents was avoided. Quantitative analysis of the unaltered substrate and the products of its autoxidation was carried out by gas chromatography over the course of oxidation. Solid cholesterol films were found to be resistant to autoxidation in the dry state. However, when heated at 125 degrees C, a sudden increase in oxidation rate occurred at a point coinciding with the visible melting followed by a plateau of the oxidation rate. All of the autoxidation products formed underwent further decomposition. Film fragments of cholesterol oxidized at a faster rate in aqueous suspensions than when oxidized in the dry state. In aqueous suspensions, the differences in the resistance of cholesterol to oxidation were not significant within the pH range 6.0-7.4, except for the early stages of oxidation. The 7-ketocholesterol/7-hydroxycholesterol ratio dropped significantly with increasing pH. However, at all pH levels tested, this ratio remained relatively constant during the 6 h of heating. While the 7 beta-hydroxycholesterol/7 alpha-hydroxycholesterol ratio was not affected by pH in the range of 6.0-7.4, at pH 7.4 a high preference was observed for the cholesterol beta-epoxide over its alpha-isomer.(ABSTRACT TRUNCATED AT 250 WORDS)

Triacylglycerols (TGs) accelerated the decomposition of cholesterol at 130°C. Addition of stearic and linoleic acids also accelerated cholesterol decomposition and produced characteristic cholesterol oxide profiles, qualitatively different from those produced in the presence of TGs. Milk fat accelerated cholesterol decomposition at 130°C and produced a cholesterol oxide pattern similar to that arising from the addition of pure TG. Not only did TGs affect cholesterol oxidation, but cholesterol influenced the decomposition of TGs. Addition of cholesterol accelerated the destruction of TGs at the beginning of heating while protecting them later. A similar pattern,i.e., acceleration followed by protection, could also be seen when triacylglycerols were heated in the presence of other triacylglycerols. The results of this work demonstrate that the stability of lipid components in complex mixtures is influenced by interactions among these components and/or their decomposition products. Such interactions do not merely shift,i.e., accelerate or delay, the oxidation rate, they may also modify the shape of the oxidation curve itself.

A simple analytical technique, using a silieic acid minicolumn and capillary gas chromatography, was used for measuring the oxidative interactions of cholesterol vr other compounds. When triacylglycerols were added to cholesterol before heating at 1800C, the latter oxidized faster than cholesterol heated alone, and a relatively high amount of epoxides was found.Dipalmitoylphosphatidylethanolamine and all the amino acids tested showed a protective effect, with cysteine and alanine being the most effective. The results of this study indicate that the varius compounds added not only influenced the rate of cholesterol oxidation, but also exerted different influences on its oxidative pathway.

The effects of oxidative interactions between cholesterol and milk fat globule membrane (MFGM) components, i.e., nonlipid fraction, total lipid, nonpolar lipid and polar lipid, on cholesterol oxidation were studied in the presence and absence of water. In the dry state, cholesterol natively present in MFGM appeared to be protected at 135 degrees C. The nonpolar lipid and nonlipid fraction contributed to the protective effect of MFGM. Added cholesterol accelerated the oxidation of membrane lipid fractions. At 75 degrees C, pure cholesterol and membrane lipid fractions did not show significant interaction. However, cholesterol and other lipids in MFGM were less stable than when these were heated separately. When cholesterol and membrane lipids were mixed in an aqueous medium at 75 degrees C, each accelerated the oxidation of the other. The MFGM exhibited a high protective effect on cholesterol oxidation in an aqueous environment. The nonlipid fraction protected cholesterol against oxidation, whereas the lipid fraction was destructive. In the absence of water, the net balance between these two opposing factors was destructive. The presence of water reversed the balance in favor of protection.