Powell W S, Gravelle F
Endocrine Laboratory, Royal Victoria Hospital, Montreal, Quebec, Canada.
J Biol Chem. 1988 Feb 15;263(5):2170-7.
The major dihydroxy metabolites of arachidonic acid formed by human polymorphonuclear leukocytes (PMNL) are leukotriene B4 (LTB4), 6-trans-LTB4, and 12-epi-6-trans-LTB4. LTB4, and to a lesser extent its 6-trans isomers, are metabolized to 20-hydroxy products by a hydroxylase in PMNL. We have recently reported the existence of a second pathway involving a reductase which, combined with the hydroxylase, results in the conversion of 6-trans-LTB4 to dihydro-6-trans-LTB4. We have now investigated some of the characteristics of this novel triene reductase pathway in human PMNL and have characterized some of the products and their mechanism of formation. At low substrate concentrations, the major pathway for the initial metabolism of both 6-trans-LTB4 and 12-epi-6-trans-LTB4 is reduction of the conjugated triene chromophore to give dihydro products with single absorption maxima at about 230 nm. Dihydro-6-trans-LTB4 is rapidly converted to its 20-hydroxy metabolite by LTB4 20-hydroxylase. However, 20-hydroxy-6-trans-LTB4 is not a substrate for the reductase. Neither 12-epi-6-trans-LTB4 nor its dihydro metabolite, 5,12-dihydroxy-7,9,14-eicosatrienoic acid, which was identified by gas chromatography-mass spectrometry, were very good substrates for the hydroxylase. The dihydro metabolites of 6-trans-LTB4 and 12-epi-6-trans-LTB4 were formed rapidly during the initial phase of the reaction, whereas the corresponding dihydro-20-hydroxy metabolites were formed only after a lag phase. Experiments utilizing deuterium-labeled 12-epi-6-trans-LTB4 indicated that a hydrogen atom is lost from the 5-position of the substrate, suggesting that the initial step in the formation of the dihydro products is the formation of a 5-oxo intermediate. LTB4 is metabolized very rapidly by LTB4 20-hydroxylase in PMNL, and we have not yet identified dihydro products derived from this substance. However, LTB4 strongly inhibits the conversion of 12-epi-6-trans-LTB4 to dihydro products, suggesting that it may also interact with the reductase.
人类多形核白细胞（PMNL）形成的花生四烯酸主要二羟基代谢产物是白三烯B4（LTB4）、6-反式-LTB4和12-表-6-反式-LTB4。LTB4及其6-反式异构体在较小程度上可被PMNL中的一种羟化酶代谢为20-羟基产物。我们最近报道存在第二条途径，涉及一种还原酶，该还原酶与羟化酶共同作用，可将6-反式-LTB4转化为二氢-6-反式-LTB4。我们现在研究了人类PMNL中这种新型三烯还原酶途径的一些特性，并对一些产物及其形成机制进行了表征。在低底物浓度下，6-反式-LTB4和12-表-6-反式-LTB4初始代谢的主要途径是共轭三烯发色团的还原，生成在约230nm处有单一吸收最大值的二氢产物。二氢-6-反式-LTB4可被LTB4 20-羟化酶迅速转化为其20-羟基代谢产物。然而，20-羟基-6-反式-LTB4不是还原酶的底物。12-表-6-反式-LTB4及其二氢代谢产物5,12-二羟基-7,9,14-二十碳三烯酸（通过气相色谱-质谱法鉴定）都不是羟化酶的良好底物。6-反式-LTB4和12-表-6-反式-LTB4的二氢代谢产物在反应初始阶段迅速形成，而相应的二氢-20-羟基代谢产物仅在延迟期后形成。利用氘标记的12-表-6-反式-LTB4进行的实验表明，底物的5位失去一个氢原子，这表明二氢产物形成的初始步骤是形成一个5-氧代中间体。LTB4在PMNL中可被LTB4 20-羟化酶非常迅速地代谢，我们尚未鉴定出源自该物质的二氢产物。然而，LTB4强烈抑制12-表-6-反式-LTB4向二氢产物的转化，这表明它也可能与还原酶相互作用。
