TRENDS in Endocrinology & Metabolism Vol.12 No.6 August 2001 http://tem.trends.com 1043-2760/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S1043-2760(01)00411-8 243 Review Isoprostanes are members of a newly described family of prostaglandin isomers that are produced from oxidative modification of polyunsaturated fatty acids via a free radical-catalyzed mechanism. The formation of prostaglandin-like structures in vitro from auto-oxidation of fatty acids was first demonstrated almost 25 years ago 1,2 . However, only in the early 1990s were these compounds also characterized in vivo 3 . Since then, a large body of evidence has accumulated to suggest that measurement of isoprostanes can provide a sensitive and specific assessment of lipid peroxidation (LPO) in vitro and in vivo. M echanism of formation and nomenclature of isoprostanes Isoprostanes are generated initially at the site of a free radical attack of esterified arachidonate in cell membranes from which they are cleaved, presumably by phospholipases; they then circulate in plasma and are excreted in urine 4 (Fig. 1). Compounds analogous to the isoprostanes can be formed from other fatty acids, including eicosapentaenoic acid and docosahexaenoic acid 5,6 . Free radical-derived isomers of other prostaglandins (PGs), leukotrienes and thromboxanes have been also reported 7–10 . However, the most extensively studied compounds are isomeric to PGF 2α , and for this reason are called F 2 -isoprostanes. Because of their structural complexity 11–13 , and the increasing interest in these compounds as indices of LPO, a systematic nomenclature that can also account for isoprostanes originating from all the common polyunsaturated fatty acid precursors seemed desirable. Initially, it was proposed that isoprostanes should be abbreviated to ‘IsoPs’, to avoid confusion with the definition of IP for inositol phosphate or the prostacyclin receptor 14 . Recently, we proposed a new nomenclature based on the ω-system of counting polyunsaturated fatty acid double bonds, which accounts for variations in chain length and the position of double bonds. We suggested that the acronym ‘iP’ should be used for isoprostane; the letters D, E, F, G and H should be used to qualify the type of cyclopentane ring of an isoprostane; and roman numerals I–VI should refer to the six types of isoprostanes derived from eicosapentaenoic acid and four types from arachidonic acid (III–VI) 15 . More details on this nomenclature system can be found in Ref. 15. For example, the most studied F 2 -isoprostane (F 2 -iP), 8-epiPGF 2α or 8-isoPGF 2α , is now known as iPF 2α -III, and the former IPF 2α -I is now known as iPF 2α -VI (Refs 15,16). In the present review, we shall adhere to this nomenclature. Isoprostane measurement F 2 -iPs have been measured as indices of LPO in body fluids such as urine, blood, bile 17,18 , pericardial 19 and cerebrospinal fluid 20,21 and lung condensate 22 , in addition to tissues such as vasculature 10,23 , brain 20 and liver 24 . Since their first description, different analytical approaches to the measurement of F 2 -iPs have been applied. GC–MS assays Originally, Morrow and Roberts described gas chromatography (GC)/electron capture/negative chemical ionization mass spectrometry (MS) assays 3 for quantifying levels of ‘total free or esterified F 2 -iPs’ using commercially available isotope-labeled PGF 2α as internal standard. This method had some limitations owing to the absence of an iP internal standard and because of imperfect resolution of iP peaks on the GC. Recently, this group has started to use a deuterium-labeled iPF 2α -III as internal standard 25 . Another group proposed a simplified version of this method 26 . However, it also could not resolve iPF 2α -III from the other isomers. For example, both assays misassigned iPF 2α -III as a major F 2 -iP isomer 27 . We initially developed an assay for the iPF 2α -III isomer using a GC–MS assay, employing an isotope labeled iPF 2α -III as internal standard 28 . Next, we synthesized an internal standard for a class VI isomer, iPF 2α -VI (formerly The isoprostanes in biology and medicine Domenico Praticò, John A. Lawson, Joshua Rokach and Garret A. FitzGerald Domenico Praticò* John A. Law son Garret A. FitzGerald The Center for Experimental Therapeutics, University of Pennsylvania, School of Medicine, Philadelphia, PA 19104-6100, USA. *e-mail: domenico@ spirit.gcrc.upenn.edu Joshua Rokach Claude Pepper Institute, Dept of Chemistry, Florida Institute of Technology, Melbourne, FL, USA. Isoprostanes are a new class of lipids, isomers of the conventional enzymatically derived prostaglandins, which are produced in vivo primarily by a free radical-catalyzed peroxidation of polyunsaturated fatty acids. F 2 -isoprostanes, isomers of the enzyme-derived prostaglandin F 2α , are the most studied species, but analogous isomers of other prostaglandins and leukotrienes have been described. Because of their mechanism of formation, specific structural features that distinguish them from other free radical- generated products and chemical stability, they can provide a reliable index for the oxidant component of several diseases in vivo. Consistent data suggest that formation of F 2 -isoprostanes is altered in a variety of clinical settings putatively associated with oxidant stress. Moreover, measurement of F 2 -isoprostanes might provide a sensitive biochemical basis for dose-selection in studies of natural and synthetic antioxidants. Finally, some F 2 -isoprostanes possess potent biological activities in vitro and in vivo, suggesting that they may also act as mediators of the cellular effects of oxidative stress.