Carbocations in the synthesis of prostaglandins by the cyclooxygenase of PGH synthase? A radical departure!

Lipidomics Reveals Dramatic Physiological Kinetic Isotope Effects during the Enzymatic Oxygenation of Polyunsaturated Fatty Acids Ex Vivo

Arachidonic acid (AA, 20:4) is an omega-6 polyunsaturated fatty acid (PUFA) and the main precursor to the class of lipid mediators known as eicosanoids. The enzymes that catalyze the oxygenation of AA begin by abstracting hydrogen from one of three bis-allylic carbons within 1,4-cis,cis-diene units. Substitution of deuterium for hydrogen has been shown to lead to massive kinetic isotope effects (KIE) for soybean lipoxygenase (sLOX) oxygenation of linoleic acid (LA, 18:2). Yet, experimental determination of the KIE during oxygenation of AA and LA by mammalian enzymes including cyclooxygenase (COX) and lipoxygenase (LOX) has revealed far lower values. All prior studies investigating the KIE of PUFA oxygenation have relied on in vitro systems using purified enzymes and were limited by availability of deuterated substrates. Here we demonstrate the use of macrophages as an ex vivo model system to study the physiological KIE (PKIE) during enzymatic AA oxygenation by living cells using a newly synthesized library of deuterated AA isotopologues. By extending lipidomic UPLC-MS/MS approaches to simultaneously quantify native and deuterated lipid products, we were able to demonstrate that the magnitude of the PKIE measured in macrophages for COX and LOX oxygenation of AA is similar to KIEs determined in previous reports using the AA isotopologue deuterated at carbon 13 (C13). However, for the first time we show that increasing the number of deuterated bis-allylic carbons to include both C10 and C13 leads to a massive increase in the PKIE for COX oxygenation of AA. We provide evidence that hydrogen(s) present at C10 of AA play a critical role in the catalysis of prostaglandin and thromboxane synthesis. Furthermore, we discovered that deuteration of C10 promotes the formation of the resolving lipid mediator lipoxin B4, likely by interfering with AA cyclization and shunting AA to the LOX pathway under physiological conditions.

COX inhibitors for airway inflammation

INTRODUCTION

The cyclooxygenase (COX) enzyme, which is responsible for the production of prostaglandins (PGs), key mediators of inflammation, may have the potential to become an attractive target for anti-inflammatory therapy. COX catalyzes the conversion of arachidonic acid (AA) into PGs, which play a significant role in disease. PGs are lipid mediators of central importance in the regulation of inflammation and smooth muscle tone. Airway-resident inflammatory cells release PGs: PGD2 and PDF2a amplify smooth muscle contraction and airway inflammation. Following its conversion from membrane phospholipids by phospholipase, AA enters the prostanoid pathway via COX, which catalyzes the conversion of AA to PGH2. PGH2 is then converted to biologically active PGs by cell-specific PG synthases. As COX is the rate limiting step in the PG pathway, the regulation of this enzyme is of critical importance in PG production.

AREAS COVERED

This review addresses the opportunities and challenges of COX inhibitors as therapeutic targets in airway inflammation. The review covers literature from the past 20 years.

EXPERT OPINION

Current literature favors COX inhibitors as potential targets for airway diseases. However, from the information available, it is not clear whether the COX enzyme by itself can serve as a target in drug development for asthma and COPD. Therefore, additional research is required to elucidate the mechanisms of action of COX metabolites before it can be considered as a target.

Cyclooxygenase mechanisms

Several advances have occurred in the past year in our understanding of cyclooxygenase catalysis. The role of specific heme oxidation states in the formation of catalytically competent tyrosyl radicals has been defined; the identity of physiological hydroperoxide activators has been established; and the participation of individual amino acids in substrate binding and oxygenation has been elucidated.

Synthesis and applications of stereospecifically (3)H-labeled arachidonic acids as mechanistic probes for lipoxygenase and cyclooxygenase catalysis

Stereospecifically (3)H-labeled substrates are useful tools in studying the mechanism of hydrogen abstractions involved in the oxygenation of polyunsaturated fatty acids. Here, we describe modified methods for the synthesis of arachidonic acids labeled with a single chiral tritium on the methylene groups at carbons 10 or 13. The appropriate starting material is a ketooctadecanoic acid which is prepared from an unsaturated C18 fatty acid precursor or by total synthesis. The (3)H label is introduced by NaB(3)H(4) reduction and the resulting tritiated hydroxy fatty acid then is tosylated, separated into the enantiomers by chiral phase HPLC, and subsequently transformed into stearic acids. A variety of stereospecifically labeled unsaturated fatty acids are obtained using literature methods of microbial transformation with the fungus Saprolegnia parasitica. Two applications are described: (i) In incubations of [10S-(3)H]- and [10R-(3)H]arachidonic acids in human psoriatic scales we show that a 12R-lipoxygenase accounts not only for synthesis of the major product 12R-HETE, but it contributes also, through subsequent isomerization, to the minor amounts of 12S-HETE. (ii) The [10R-(3)H]- and [10S-(3)H]arachidonic acids were also used to demonstrate that prostaglandin ring formation by cyclooxygenases does not involve carbocation formation at C-10 of arachidonic acid as was hypothesized recently.