Arachidonic acid hydrogen abstraction rate constants

This conclusion is partly true because superoxide is unable to abstract hydrogen atom even from the most active bisallylic positions of unsaturated compounds, while perhydroxyl radical abstracts H atom from linoleic, linolenic, and arachidonic fatty acids with the rate constants of 1-3 x 1031 mol-1 s-1 [24], However, the superoxide damaging activity does not originate from hydrogen atom abstraction reactions but from one-electron reduction processes, leading to the formation of hydroxyl radicals, peroxynitrite, etc, and in these reactions perhydroxyl cannot compete with superoxide. 

The sulfite radical is a sulfur-centered radical and would form a S-H bond if it undergoes a hydrogen abstraction reaction. These bonds are weaker than typical C-H bonds and indeed hydrogen abstraction reactions, for example from 2-propanol, have been found to be very slow k < 10 L mol" s ) [53]. SOf does, however, add to unsaturated bonds, not only carbon-carbon double and triple bonds, but also C=S and C=N bonds [68]. The rate constants for the addition of 803" to unsaturated fatty acids were found to increase from about 3x10 L mol" s" for non-conjugated, straight chain fatty acids (linoleate, linolenate, and arachidonate) to about 1 x lO L mol" s for branched chain, conjugated fatty acids (crocin and crocetin) [69]. 

Rate constants for the above H atom abstraction process are on the order of 10 M" s with the actual values depending both on the nature of the thiyl and the PUFA. For a given thiyl radical the measured rate increases with the number of bisallylic hydrogens. For the reaction of the thiyl radical from glutathione, GS, with PUFAs, for example, the rate constants increase from 0.8 x lO M" s (linoleic acid, 18 2, two double bonds in one bisallylic functionality), over 1.9 x 10 M" s (linolenic acid, 18 3, three double bonds in two bisallylic functionalities), to 3.1 X 10 M s (arachidonic acid, 20 4, four double bonds in three bisallylic functionalities). As expected, the mere length of the aliphatic chain does not play any role, as indicated by identical rate constants for the 18 3 and 22 3 PUFAs. Also, practically no reaction k < 10 M" s" ) is observed for the corresponding reaction of RS with oleic acid (18 1, one double bond) which lacks bisallylic hydrogens. 

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