Endoperoxides lipid hydroperoxides

A sensitive assay for lipid hydroperoxides is afforded by prostaglandin endoperoxide synthase, that in the presence of hydroperoxides is activated to liberate O2, which is measured with an oxygen electrode . Although it may be of advantage in certain cases, the procedure seems too complex to become of widespread application in practical analysis. 

Lipid hydroperoxides are also generated in singlet molecular oxygen mediated oxidations and by the action of enzymes such as lipoxygenases and cyclooxygenases. Chemiluminescence (CL) arising from lipid peroxidation has been used as a sensitive detector of oxidative stress both in vitro and in vivo . Several authors have attributed ultra-weak CL associated with lipid peroxidation to the radiative deactivation of O2 and to triplet-excited carbonyls (63, 72) (equations 35 and 36) " . It has been proposed that the latter emitters arise from the thermolysis of dioxetane intermediates (61, 62) (equation 35), endoperoxide (73) (equation 37) and annihilation of aUtoxyl, as well as peroxyl radicals ... 

A commonly used technique to follow lipid oxidation is to monitor the formation of malondialdehyde (MDA), a water-soluble compound produced from lipid hydroperoxide and endoperoxide decomposition. The formation of the MDA can be followed by measuring it directly via HPLC - , however most often it is reacted with thiobarbituric acid (TBA) with a 1 2 stoichiometry (equation 26), and formation of the product is followed spectrophotometrically at 535 nm or followed by fluorescence . ... 

The initial products derived from lipid peroxidation are subject to decomposition reactions, which may be facilitated by the presence of metals and other one-elec-tron-donating species. In particular, the lipid hydroperoxide and bicyclic endoper-oxide products are susceptible to degradation. Bicyclic endoperoxides undergo acid-catalyzed ring-scission across the endoperoxide to yield the three-carbon dial-dehyde, malondialdehyde (MDA) (Figure 5.1) [2]. MDAis also produced enzymatically from the endoperoxide-metabolizing enzyme thromboxane synthase [3, 4]. MDA is an abundant product of lipid peroxidation and reacts with DNA nucleophiles (see discussion below). 

Formation. Although bicyclo[2.2.l]endoperoxides are important intermediates in prostaglandin chemistry, they have been synthetically elusive. Autoxidation of polyunsaturated fatty. acids does lead to mixtures of compounds containing the endoper-oxide structure (, 21). We (22) were also able to isolate prostaglandin products from the sequence described below. The lipid hydroperoxide (prepared from soybean lipoxygenase and the fatty acid (23) was treated with a free radical initiator and the products, following borohydride reduction, were compared to authentic prostaglandins (24). 

It has been proposed that 2 can be generated in chemical reactions (so-called dark biochemistry) by unstable oxygen adducts, endoperoxides, metal complexes (101, 102), and peroxyl radical recombinations (103-105), and that the low levels of internal hydroperoxides produced thereby initiate lipid autoxidation chains (102, 106, 107, respectively). 

Unsaturated lipids Oleic, linoleic, Hnolenic, arachidonic acids Polymeric derivatives Endoperoxides, hydroperoxides... 

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