IPF(2α)-I: An index of lipid peroxidation in humans

Isoprostanes are prostaglandin isomers produced from arachidonic acid by a free radical-catalyzed mechanism. Urinary excretion of 8-iso-prostaglandin F(2α), an isomer of the PGG/H synthase (cyclooxygenase or COX) enzyme product, prostaglandin F(2α) (PGF(2α)), has exhibited promise as an index of oxidant stress in vivo. We have developed a quantitative method to measure isoprostane F(2α)-I, (IPF(2α)-I) a class I isomer (8-iso-PGF(2α) is class IV), using gas chromatography/mass spectrometry. IPF(2α)-I is severalfold as abundant in human urine as 8-iso-PGF(2α), with mean values of 737 ± 20.6 pg/mg creatinine. Both isoprostanes are formed in a free radical-dependent manner in low density lipoprotein oxidized by copper in vitro. However, IPF(2α)-I, unlike 8-iso-PGF(2α), is not formed in a COX-dependent manner by platelets activated by thrombin or collagen in vitro. Similarly, COX inhibition in vivo has no effect on IPF(2α)-I. Neither serum IPF(2α)-I, an index of cellular capacity to generate the isoprostane, nor urinary excretion of IPF(2α)-I, an index of actual generation in vivo, is depressed by aspirin or indomethacin. In contrast, both serum thromboxane B(2) and urinary excretion of its 11-dehydro metabolite are depressed by the COX inhibitors. Although serum 8-iso-PGF(2α) formation is substantially depressed by COX inhibitors, urinary excretion of the compound is unaffected. Urinary IPF(2α)-I is elevated in cigarette smokers compared with controls (1525 ± 180 versus 740 ± 40 pg/mg creatinine; P < 0.01) and is highly correlated with urinary 8-iso-PGF(2α) (r = 0.9; P < 0.001). Urinary IPF(2α)-I is a novel index of lipid peroxidation in vivo, which can be measured with precision and sensitivity. It is an abundant F(2)-isoprostane formed in a free radical- but not COX-dependent manner. Although 8-iso-PGF(2α) may be formed as a minor product of COX, this pathway contributes trivially, if at all, to levels in urine. Urinary excretion of both isoprostanes is elevated in cigarette smokers.

异前列腺素（Isoprostanes）是由花生四烯酸经自由基催化机制生成的前列腺素异构体。作为前列腺素G/H合酶（cyclooxygenase/COX，环氧合酶）的酶促产物前列腺素F(2α)（PGF(2α)）的异构体，尿液中8-异前列腺素F(2α)的排泄量已被证实可作为体内氧化应激的有效检测指标。本研究建立了一种采用气相色谱/质谱联用法（gas chromatography/mass spectrometry）定量检测异前列腺素F(2α)-I（IPF(2α)-I）的方法；该物质属于I类异前列腺素，而8-异前列腺素F(2α)则归为IV类。人体尿液中IPF(2α)-I的含量为8-异前列腺素F(2α)的数倍，平均水平为737±20.6 pg/mg肌酐。两种异前列腺素均可在体外经铜离子氧化的低密度脂蛋白中以自由基依赖的方式生成。然而，与8-异前列腺素F(2α)不同，IPF(2α)-I无法在凝血酶或胶原蛋白激活的体外血小板中以COX依赖的方式生成。同理，体内COX抑制对IPF(2α)-I水平无显著影响。无论是反映细胞异前列腺素生成能力的血清IPF(2α)-I水平，还是反映体内实际生成量的尿液IPF(2α)-I排泄量，均不会被阿司匹林或吲哚美辛所抑制。与之形成鲜明对比的是，COX抑制剂可显著降低血清血栓素B2及其11-脱氢代谢物的尿液排泄量。尽管血清中8-异前列腺素F(2α)的生成会被COX抑制剂大幅抑制，但该物质的尿液排泄量不受其影响。与健康对照组相比，吸烟者的尿液IPF(2α)-I水平显著升高（1525±180 vs 740±40 pg/mg肌酐；P<0.01），且其与尿液中8-异前列腺素F(2α)的水平高度相关（r=0.9；P<0.001）。尿液IPF(2α)-I是一种新型的体内脂质过氧化检测指标，可实现高精度、高灵敏度的定量检测。该物质属于丰度较高的F2类异前列腺素，通过自由基依赖而非COX依赖的途径生成。尽管8-异前列腺素F(2α)可能作为COX的次要产物生成，但该途径对尿液中该物质的水平贡献极小，甚至可忽略不计。两类异前列腺素的尿液排泄量在吸烟者中均显著升高。