Biosynthesis, isolation, and NMR analysis of leukotriene A epoxides: substrate chirality as a determinant of the cis or trans epoxide configuration

Abstract

The biosynthetic pathway, unraveled three decades ago, is a model for related transformations to anti-infl ammatory mediators including the lipoxins, eoxins, resolvins, protectins, and maresins ( 2-5 ). 5-Lipoxygenase (5-LOX) catalyzes the fi rst two steps in leukotriene biosynthesis, namely the stereospecifi c oxygenation of arachidonic acid to 5 S -hydroperoxy-eicosatetraenoic acid (5 S -HPETE), and dehydration of the hydroperoxide to LTA 4 , the allylic 5,6-trans -epoxide from which the bioactive leukotrienes are derived ( Fig. 1 ) ( 6-9 ). The importance of this pivotal intermediate is refl ected in the extensive efforts directed toward perfecting the total synthesis of LTA 4 and of closely related 5,6-epoxide isomers (as reviewed in Refs. 10-12 ).LTA 4 is highly unstable in physiological buffer at pH 7.4, with a half-life estimated as approximately 3 s at 25°C and 18 s at 4°C ( 13 ). Despite this instability, LTA 4 was detected as an evanescent intermediate in short-term incubations of arachidonic acid with human leukocytes ( 6 ), and the radiolabeled product was isolated and, as the methyl ester derivative, shown to have similar chromatographic mobility and hydrolysis under acidic conditions as a synthetic standard ( 14 ). The stereochemistry has heretofore not been established directly on the biosynthetic product but was deduced from an understanding of the biosynthetic pathway and by study of the reactions of synthetic LTA 4 and related isomers in transformations to the stable, bioactive leukotrienes LTB 4 and LTC 4 ( 15-17 ). Herein we report the isolation and purifi cation of LTA epoxides as products of LOX enzymes in quantities suffi cient for NMR analysis of the stereoconfi gurations. The isolation method Abstract Leukotriene (LT)A 4 and closely related allylic epoxides are pivotal intermediates in lipoxygenase (LOX) pathways to bioactive lipid mediators that include the leukotrienes, lipoxins, eoxins, resolvins, and protectins. Although the structure and stereochemistry of the 5-LOX product LTA 4 is established through comparison to synthetic standards, this is the exception, and none of these highly unstable epoxides has been analyzed in detail from enzymatic synthesis. Understanding of the mechanistic basis of the cis or trans epoxide confi guration is also limited. To address these issues, we developed methods involving biphasic reaction conditions for the LOX-catalyzed synthesis of LTA epoxides in quantities suffi cient for NMR analysis. As proof of concept, human 15-LOX-1 was shown to convert 15 S -hydroperoxy-eicosatetraenoic acid (15 S -HPETE) to the LTA analog 14 S ,15 S -trans -epoxy-eicosa-5 Z ,8 Z ,10 E ,12 E -tetraenoate, confi rming the proposed structure of eoxin A 4 . Using this methodology we then showed that recombinant Arabidopsis AtLOX1, an arachidonate 5-LOX, converts 5 S -HPETE to the trans epoxide LTA 4 and converts 5 R -HPETE to the cis epoxide 5-epi -LTA 4 , establishing substrate chirality as a determinant of the cis or trans epoxide confi guration.The results are reconciled...

Lipoxygenase-catalyzed transformation of epoxy fatty acids to hydroxy-endoperoxides: a potential P450 and lipoxygenase interaction

“…Meta-chloroperoxybenzoic acid (mCPBA), stannous chloride, and soybean ( Glycine max ) lipoxidase type V were from Sigma-Aldrich. Human 15-LOX-1 was prepared exactly as described by expression of the recombinant enzyme in Escherichia coli ( 9 ), and a baculovirus construct of the murine platelet-type 12-LOX was expressed in Sf9 cells as described ( 10 ).…”

CD64 and Group II Secretory Phospholipase A2 (sPLA2-IIA) as Biomarkers for Distinguishing Adult Sepsis and Bacterial Infections in the Emergency Department

“…All of these metabolites are pro-inflammatory mediators; they include 5-hydroperoxyeicosatetraenoic acid (HPETE, generated by activity of lipoxygenase) [ 19 , 20 ] and prostaglandin G2 [ 21 , 22 ] (catalyzed by cyclooxygenase). HPETE can be converted further into leukotrienes [ 23 ] and hydroxyicosatetraenoic acid (HETE) [ 24 ], while prostaglandin G2 can be catalyzed further to generate thromboxanes (thromboxane A2 and B2) [ 25 ], prostacyclin (PGI2) [ 26 ] and various forms of prostaglandins (prostaglandins D2, E2 and F2α) [ 27 ] which will further amplify the inflammation signals [ 28 – 31 ].…”

Roles of 5‐lipoxygenase and cyclooxygenase‐2 in the biosynthesis of hemiketals E2and D2by activated human leukocytes

“…5‐LOX oxygenation of arachidonic acid gives rise to 5 S ‐hydroperoxyeicosatetraenoic acid (5 S ‐HPETE), which can undergo reduction to 5 S ‐hydroxyeicosatetraenoic acid (5 S ‐HETE) or oxidation to 5‐oxoeicosatetraenoic acid, a potent eosinophil chemoattractant and stimulant (7, 8). The major fate of 5 S ‐HPETE, however, is dehydration to the leukotriene (LT) epoxide LTA 4 in a second reaction catalyzed by 5‐LOX (9, 10). Enzymatic hydrolysis of LTA 4 yields LTB 4 , a proinflammatory neutrophil chemoattractant (11, 12).…”