Conjugated hydroperoxides

Lipoxygenase-Catalyzed Oxidations. Lipoxygenase-1 catalyzes the incorporation of dioxygen into polyunsaturated fatty acids possessing a l(Z),4(Z)-pentadienyi moiety to yield ( ),(Z)-conjugated hydroperoxides. A highly active preparation of the enzyme from soybean is commercially available in purified form. From a practical standpoint it is important to mention that the substrate does not need to be in solution to undergo the oxidation. Indeed, the treatment of 28 g/L of linoleic acid [60-33-3] with 2 mg of the enzyme results in (135)-hydroperoxide of linoleic acid in 80% yield... 

Lipoxygenase-Catalyzed Oxidations. Lipoxygenase-1 catalyzes the incorporation of diuxygen into polyunsaturated fatty acids possessing a l(Z).4(Z)-pentadienyl moiety to yield ( ).highly active preparation of the enz.yme from soybean is commercially available in purified form. 

The major products of autooxidation, but hydroperoxides with the /runs,rranj-configuration can also be formed. This mechanism has been studied by Porter et al. [52,53]. Oxygen addition seems to be... 

A basic reaction scheme of the autoxidative drying process is depicted in Scheme 16.18. A catalyst is needed to give this reaction sufficient speed usually a cobalt(II) salt is used. The diene moieties of the linolenic acids, especially, are involved in this process. After formation of a conjugated hydroperoxide with molecular oxygen from the ambient air, the hydroperoxide decomposes into oxy-radicals that can fur-... 

The properties of a lipoxidase from peas have been studied by Siddiqi and Tappel (660). Because their preparation possesses no fatty acid oxidase or fatty acid dehydrogenase activity and oxidizes linoleate but not oleate, it behaves like a true lipoxidase. The products of linoleate oxidation contain carbonyl groups although a correlation of the intenfflty of absorption with peroxide concentration suggests that the initial products of pea lipoxidase action are totally conjugated hydroperoxides. These appear to be very unstable under the conditions of formation, undergoing scission and polymerization to form aldehydes and trienes, detected by the presence of an absorption band at 280 m/t. 

When a fatty acid residue can form a conjugated hydroperoxide, the following reaction may be envisaged ... 

Free radicals such as ROO- (and RO-, OH- etc. which can be formed by extra side-reactions) can react at random by hydrogen abstration and a variety of addition reactions to damage proteins, other lipids and vitamins (particularly vitamin A). Compounds which react rapidly with free radicals can be useful in slowing peroxidation damage. Thus, naturally-occurring compounds such as vitamin E are powerful anti-oxidants and tissues which are deficient in such compounds may be prone to peroxidation damage (section 8.11). Formation of lipid hydroperoxides can be readily detected by a number of methods of which the absorption of conjugated hydroperoxides at 235 nm is particularly useful. 

Themeasurement of extinction coefficients of theproducts of lipoxidases from various sources has yielded a variable range between8000 and27,000. The initial products appear to be totally conjugated hydroperoxides, on the basis of high extinction coefficients. These are apparently unstable and can decay rapidly to stable products with lower absorption intensities. 

The electrophilic character of boron is again evident when we consider the oxida tion of organoboranes In the oxidation phase of the hydroboration-oxidation sequence as presented m Figure 6 11 the conjugate base of hydrogen peroxide attacks boron Hydroperoxide ion is formed m an acid-base reaction m step 1 and attacks boron m step 2 The empty 2p orbital of boron makes it electrophilic and permits nucleophilic reagents such as HOO to add to it... 

Additional evidence indirectly supports the reversibiUty of reaction 2. The addition of oxygen to a highly conjugated radical is readily reversible even at 40°C according to a study of the isomerization of methyl linolenate hydroperoxides in the presence of 02 (95). 

Physical and Chemical Properties. The (F)- and (Z)-isomers of cinnamaldehyde are both known. (F)-Cinnamaldehyde [14371-10-9] is generally produced commercially and its properties are given in Table 2. Cinnamaldehyde undergoes reactions that are typical of an a,P-unsaturated aromatic aldehyde. Slow oxidation to cinnamic acid is observed upon exposure to air. This process can be accelerated in the presence of transition-metal catalysts such as cobalt acetate (28). Under more vigorous conditions with either nitric or chromic acid, cleavage at the double bond occurs to afford benzoic acid. Epoxidation of cinnamaldehyde via a conjugate addition mechanism is observed upon treatment with a salt of /-butyl hydroperoxide (29). 

A number of methods are available for following the oxidative behaviour of food samples. The consumption of oxygen and the ESR detection of radicals, either directly or indirectly by spin trapping, can be used to follow the initial steps during oxidation (Andersen and Skibsted, 2002). The formation of primary oxidation products, such as hydroperoxides and conjugated dienes, and secondary oxidation products (carbohydrides, carbonyl compounds and acids) in the case of lipid oxidation, can be quantified by several standard chemical and physical analytical methods (Armstrong, 1998 Horwitz, 2000). 

An example of an experiment in which LDL has been treated with 15-lipoxygenase and the oxidation monitored by the formation of conjugated diene is shown in Fig. 2.2. In the absence of transition metal, a rapid increase in absorbance occurs, with no lag phase, which ceases after a period of about 90 min under these conditions. If copper is added to promote LDL oxidation at this point, LDL treated with lipoxygenase oxidizes at a faster rate with a short lag phase when compared to the control. During this procedure there is only a minimal loss of a-tocopherol and so we may ascribe the shortened lag phase to the increase in lipid peroxides brought about by lipoxygenase treatment. A similar result was found when LDL was supplemented with preformed fatty acid hydroperoxides (O Leary eta/., 1992). 

In bronchitics, there have been reports of elevated serum-conjugated dienes, hydroperoxides and aldehydes, and a claim of clinical eflicacy as well as normalization of these parameters after vitamin E therapy (Kleiner et al., 1990). However, these patients were given combined therapy including steroids and thus the effect of vitamin E alone cannot be assessed. N-Acetylcysteine administered to chronic bronchitics increased plasma cysteine from a below-normal baseline but it has not been shown that this intervention had any effect on the disease process, the dosing being of short duration, nor were there short-term effects of the release of ROS from blood neutrophils (reviewed by MacNee et al., 1991). A... 

---