Subhash P. Khanapure

Nomenclature of isoprostanes: a proposal.

We have proposed a nomenclature system for the isoprostanes, a new class of natural products formed in vivo by the free-radical peroxidation of polyunsaturated fatty acids. Our proposed nomenclature is based on the assignment of four isoprostanes, 1, 9, 17, and 25, as representatives of the four classes of isoprostanes derived from arachidonic acid (AA). We have attempted as much as possible to retain elements from the familiar prostaglandin nomenclature. In this proposal, we have used the abbreviation i.p. for isoprostane. We have classified isoprostane classes or types based on omega-carbon as being the starting reference. Roman numerals I-VI refer the six types of isoprostanes derived from eicosapentaenoic acid (EPA) and III-VI refer to the four types derived from AA. This nomenclature can be applied to isoprostanes derived from other PUFAs also.

The Isoprostanes: A Perspective

The isoprostanes are a new class of natural products produced in vivo by a non-enzymatic free-radical-induced peroxidation of polyunsaturated fatty acid. In the case of arachidonic acid, for example, four classes of isoprostanes can be produced. Because of the specific structural features distinguishing them from other free-radical-generated products, e.g., HETEs, etc., the isoprostanes can provide an exclusive and selective index for the oxidant component of several inflammatory and degenerative diseases. The possible mechanisms of formation of the individual isoprostanes is discussed in detail. Class III products, such as 8-iso-PGF2 alpha and 8-iso-PGE2 have been shown to be vasoconstrictors and modulate platelet function. Several synthetic representatives from the four classes of arachidonic-acid-derived isoprostanes have already been prepared by total synthesis. These synthetic standards have been used for the identification and quantitation of these isoprostanes in biological fluids using gas chromatography/mass spectrometry methodology.

IDENTIFICATION OF TWO MAJOR F2 ISOPROSTANES, 8,12-ISO- AND 5-EPI-8,12-ISO-ISOPROSTANE F2ALPHA -VI, IN HUMAN URINE

Isoprostanes (iPs) are nonenzymatic, free radical-derived compounds isomeric with enzymatically formed eicosanoids such as prostaglandins, leukotrienes, and thromboxanes. One group formed by the auto-oxidation of arachidonic acid, the F2-iPs, consists of four classes of isomers of prostaglandin F2α (PGF2α). They are relatively abundant in human urine. This fact, along with their chemical stability and excellent characteristics for quantitation by gas chromatography/mass spectrometry, has made them attractive indices of oxidative stress in humans. We developed a specific assay using gas chromatography/mass spectrometry for the first identified F2-iP, iPF2α-III (previously called 8-iso-PGF2α or 8-epi-PGF2α), which demonstrated the utility of monitoring a specific isomer. Recently, we described an assay for another isomer, iPF2α-VI, which is present in urine in greater concentration than iPF2α-III and which is particularly amenable to quantitation. We now describe the identification in human urine of two more isomers, 8,12-iso-iPF2α-VI and 5-epi-8,12-iso-iPF2α-VI, using high performance liquid chromatography/tandem mass spectrometry and gas chromatography/mass spectrometry. These compounds are each present in ∼5-fold greater concentrations than iPF2α-VI (∼20-fold greater than iPF2α-III). They share the unique chemical characteristics of class VI compounds, which make them attractive targets for quantitation by gas chromatography/mass spectrometry and immunoassay development.

Total synthesis of a novel isoprostane IPF2α-I and its identification in biological fluids

The first total synthesis of IPF2α-I 25 is described using D-glucose as starting material. This novel isoprostane has been used to establish its presence in human urine.

A free radical route to syn lactones and other prostanoid intermediates in isoprostaglandin synthesis.

Abstract The hex-5-enyl radical cyclization methodology was applied to the formation of optical active prostanoid intermediates 7,8,9 with readily available diacetone-D-glucose as starting material. This result should lead to isoprostaglandins, a novel class of arachidonic acid metabolites obtained by a non-cyclooxygenase mechanism involving free radical-catalysed peroxidation of arachidonic acid.

Total synthesis of 12-epi-PGF2α

Abstract A novel synthesis of 12- epi -PGF 2α 5 is described. The key synthon 11 , which has been used as a starting point for the synthesis, is produced by a radical cyclization process using thionocarbonate 9a . The radical cyclization of 9a to 11 has been studied in some detail.

Deblocking of dithioacetals and oxathioacetals using periodic acid under mild nonaqueous conditions

Abstract A novel method for the deblocking of the dithioacetals and oxathioacetals is described. Periodic acid under nonaqueous conditions has been used for the deprotection of the dithio- and oxathio-derivatives to the corresponding carbonyl compounds. This simple high-yield transformation is conveniently carried out in nonaqueous medium and works well with complex sensitive aldehydes and in the presence of other protective groups.

Regioncontrolled formation of iodohy dnns and expoxides from Vic-diols

A novel method for the stereoselective preparation of iodohydrins and epoxides from stereogenic vicinal diols is described. This new high-yield methodology consists of the conversion of thionocarbonates such as 12 to the primary iodo derivative 14 with complete regiocontrol. Deprotection of the secondary alcohol derivative in 14 gives the corresponding synthetically versatile iodohydrin 16, which is converted to epoxide 17. This methodology has been applied to complex tetraols 29 and 34. In the case of diols the transformation is conveniently carried out in high yield as a one-pot reaction.

First total synthesis of isoprostane IPF2α-III

Abstract The first total synthesis of IPF 2α -III, a type III isoprostane is described using a syn-anti-syn lactone 34 prepared from D-glucose and a novel iodohydrin synthon 30 . This is first synthesis of a representative of type III isoprostane which is anticipated to be formed in vivo from arachidonic acid by free-radical mechanism. This synthetic material will help elucidate the formation in vivo of type III isoprostanes.

Calcium/Calmodulin-dependent Conversion of 5-Oxoeicosanoids to 6,7-Dihydro Metabolites by a Cytosolic Olefin Reductase in Human Neutrophils

We previously showed that 6-trans isomers of leukotriene B4 but not leukotriene B4itself are converted to dihydro metabolites by human neutrophils. The first step in the formation of these metabolites is oxidation of the 5-hydroxyl group by 5-hydroxyeicosanoid dehydrogenase. The objective of the present investigation was to characterize the second step in the formation of the dihydro metabolites, reduction of an olefinic double bond. We found that the olefin reductase reduces the 6,7-double bond of 5-oxoeicosanoids, is localized in the cytosolic fraction of neutrophils, and requires NADPH as a cofactor. Neutrophil cytosol converts a variety of both 5-oxo- and 15-oxoeicosanoids to dihydro products. However, conversion of 5-oxoeicosanoids to their 6,7-dihydro metabolites is inhibited by EGTA and a calmodulin antagonist and stimulated by the addition of calcium and calmodulin, whereas the reduction of 15-oxoeicosanoids to their 13,14-dihydro metabolites is slightly inhibited by calcium. Furthermore, eicosanoid Δ6- and Δ13-reductases could be separated by chromatography on DEAE-Sepharose. 5-Oxo-6,8,11,14-eicosatetraenoic acid (5-oxo-ETE) is converted by the Δ6-reductase to 6,7-dihydro-5-oxo-ETE, which is 1000 times less potent than 5-oxo-ETE in mobilizing calcium in neutrophils. We conclude that neutrophils contain both 5-oxoeicosanoid Δ6-reductase and prostaglandin Δ13-reductase. Metabolism of 5-oxo-ETE by the Δ6-reductase results in loss of its biological activity.