The primary producs formed from the autoxidation of lipids can be understood based upon a mechanism that involves five different reaction types. These reactions are: reaction of a carbon radical and molecular oxygen, atom transfer of a hydrogen from substrate to the chain carrying peroxyl, fragmentation of the chain carrying peroxyl to give oxygen and a carbon radical, rearrangement of the peroxyl, and cyclization of the peroxyl. The mechanisms of these primary reaction steps has been the focus of extensive research over the past fifty years, and the current level of understanding of these transformations is the subject of this review.

Mechanisms of free radical oxidation of unsaturated lipids

A recognized feature of psoriasis and other proliferative dermatoses is accumulation in the skin of the unusual arachidonic acid metabolite, 12R-hydroxyeicosatetraenoic acid (12R-HETE) This hydroxy fatty acid is opposite in chirality to the product of the well-known 12S-lipoxygenase and heretofore in mammals is known only as a product of cytochrome P450s Here we provide mechanistic evidence for a lipoxygenase route to 12R-HETE in human psoriatic tissue and describe a 12R-lipoxygenase that can account for the biosynthesis Initially we demonstrated retention of the C-12 deuterium of octadeuterated arachidonic acid in its conversion to 12R-HETE in incubations of psoriatic scales, indicating the end product is not formed by isomerization from 12S-H(P)ETE via the 12-keto derivative Secondly, analysis of product formed from [10R-3H] and [10S-3H]-labeled arachidonic acids revealed that 12R-HETE synthesis is associated with stereospecific removal of the pro-R hydrogen from the 10-carbon of arachidonate This result is compatible with 12R-lipoxygenase-catalyzed formation of 12R-HETE and not with a P450-catalyzed route to 12R-HETE in psoriatic scales We cloned a lipoxygenase from human keratinocytes; the cDNA and deduced amino acid sequences share ≤50% identity to other human lipoxygenases This enzyme, when expressed in Hela cells, oxygenates arachidonic acid to 12-HPETE, >98% 12R in configuration The 12R-lipoxygenase cDNA is detectable by PCR in psoriatic scales and as a 25-kilobase mRNA by Northern analysis of keratinocytes Identification of this enzyme extends the known distribution of R-lipoxygenases to humans and presents an additional target for potential therapeutic interventions in psoriasis

https://www.pnas.org/content/pnas/95/12/6744.full.pdf

A 12R-lipoxygenase in human skin: mechanistic evidence, molecular cloning, and expression.

Stereochemical difference between 12-hydroxy-5,8,10,14-eicosatetraenoic acid in platelets and psoriatic lesions.

Tremendous advances have been made in the study of biomarkers related to carcinogenesis during the past 20 years. This perspective will briefly review improvements in methodology and instrumentation that have increased our abilities to measure the formation, repair, and consequences of DNA adducts. These biomarkers of exposure, along with surrogates such as protein adducts, have greatly improved our understanding of species differences in metabolism and effects of chemical stability and DNA repair on tissue differences in molecular dose. During this same time frame, improvements in assays for biomarkers of effect have provided better data and an improved understanding of the dose responses for both gene and chromosomal mutations. A framework analysis approach was used to examine the mode of action of genotoxic chemicals and the default assumption that cancer can be expected to be linear at very low doses. This analysis showed that biomarkers of exposure are usually linear at low doses, with the exception being when identical adducts are formed endogenously. Whereas biomarkers of exposure extrapolate down to zero, biomarkers of effect can only be interpolated back to the spontaneous or background number of mutations. The likely explanation for this major difference is that at high exposures, the biology that results in mutagenesis is driven by DNA damage resulting from the chemical exposure. In contrast, at very low exposures, the biology that results in mutagenesis is driven by endogenous DNA damage. The shapes of the dose-response curves for biomarkers of exposure and effect can be very different, with biomarkers of effect better informing quantitative estimates of risk for cancer, a disease that results from multiple mutations. It is also clear, however, that low dose data on mutagenesis are needed for many more chemicals.

Biomarkers in Toxicology and Risk Assessment : Informing Critical Dose-Response Relationships

Eggs of the sea urchin, Strongylocentrotus purpuratus, contain a prominent (11R) and (12R) lipoxygenase activity.

Investigation of the selectivity of hydrogen abstraction in the nonenzymatic formation of hydroxyeicosatetraenoic acids and leukotrienes by autoxidation.

Investigation of the chemical conversion of hydroperoxyeicosatetraenoate to leukotriene epoxide using stereospecifically labeled arachidonic acid. Comparison with the enzymatic reaction.

A T Porter

Papers

Investigation of the selectivity of hydrogen abstraction in the nonenzymatic formation of hydroxyeicosatetraenoic acids and leukotrienes by autoxidation.

Investigation of the chemical conversion of hydroperoxyeicosatetraenoate to leukotriene epoxide using stereospecifically labeled arachidonic acid. Comparison with the enzymatic reaction.