Decomposition rates of isothiocyanate conjugates determine their activity as inhibitors of cytochrome P450 enzymes
Decomposition rates of isothiocyanate conjugates determine their activity as inhibitors of cytochrome P450 enzymes
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DOI：
10.1021/tx010029w
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发表时间：
2001-09-01
影响因子：
4.1
通讯作者：
Chung, FL
中科院分区：
文献类型：
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作者：
Conaway, CC;Krzeminski, J;Chung, FL
异硫氰酸酯的硫醇缀合物（硫醇-ITCs）是哺乳动物中在巯基尿酸途径中形成的ITCs的代谢物。它们在小鼠肺肿瘤生物测定和其他模型中是有效的化学预防剂。硫醇-ITC是P450的抑制剂，但尚未确定P450抑制是由于缀合物本身还是由于通过去缀合反应释放的母体ITC。在硫醇-ITC的化学预防作用机制的研究中，Cys、GSH和异硫氰酸苄酯（BITC）、异硫氰酸苯乙酯（PEITC）、异硫氰酸6-苯基己酯（PHITC）和莱菔硫烷（SFN）的N-乙酰基-L-半胱氨酸（NAC）缀合物的解缀合速率，表示为一级速率常数k（1）和分解半衰期Dt（1/2），在pH 7.4和37度的水溶液中测量。Cys偶联物的Dt（1/2）S比相应GSH偶联物的Dt（1/2）S短几倍，而NAC偶联物的Dt（1/2）S最长。巯基缀合物的裂解是pH依赖性的，在酸性条件下比在pH 7.4下慢得多。使用PROD（戊氧基试卤灵O-脱烷基化）（P450 2B 1）和EROD（乙氧基试卤灵O-脱烷基化）（P450 1A 1）跟踪硫醇-ITC对P450酶的抑制。PROD和EROD的抑制水性硫醇-ITCs增加预孵育时间，并大致平行的共轭物的分解的程度已经发生，表明这两个各自的母ITC和还原裂解率的共轭物的效力影响酶抑制。在250-1000 μ M GSH的存在下，与生理水平相当，巯基-ITCs的去缀合速率显著降低; PROD的抑制也成比例地降低。预计在血浆和组织中的硫醇-ITCs的分解速度缓慢，这表明从硫醇-ITCs释放的ITCs对参与致癌物活化的P450酶的抑制可能不是其肿瘤抑制活性的主要机制;其他机制可能有助于其化学预防活性。
Thiol conjugates of isothiocyanates (thiol-ITCs) are metabolites of ITCs formed in the mercapturic acid pathway in mammals. They are effective chemopreventive agents in mouse lung tumor bioassays and in other models. Thiol-ITCs are inhibitors of P450s, but it has not been determined if P450 inhibition is due to conjugates themselves or to parent ITCs released by deconjugation reactions. In studies of mechanism of chemopreventive action of thiol-ITCs, rates of deconjugation of Cys, GSH, and N-acetyl-L-cysteine (NAC) conjugates of benzyl isothiocyanate (BITC), phenethyl isothiocyanate (PEITC), 6-phenylhexyl isothiocyanate, (PHITC), and sulforaphane (SFN), expressed as the first-order rate constant k(1) and the half-life of decomposition Dt(1/2), were measured in aqueous solutions at pH 7.4 and 37 degrees. The Dt(1/2)S for the Cys conjugates were severalfold shorter than the Dt(1/2)s for respective GSH conjugates, while the Dt(1/2)S for the NAC conjugates were the longest. Cleavage of thiol conjugates was pH dependent, much slower under acidic conditions than at pH 7.4. Inhibition of P450 enzymes by thiol-ITCs was followed using PROD (pentoxyresorufin O-dealkylation) for P450 2B1 and EROD (ethoxyresorufin O-dealkylation) for P450 1A1. The inhibition of PROD and EROD by aqueous thiol-ITCs increased with preincubation time and was roughly parallel to the extent of decomposition of the conjugate that had occurred, indicating that both potency of the respective parent ITC and the rate of reductive cleavage of the conjugate influenced enzyme inhibition. In the presence of 250-1000 muM GSH, comparable to physiological levels, rates of deconjugation of thiol-ITCs were markedly reduced; inhibition of PROD was also proportionately reduced. Slow rates of decomposition of thiol-ITCs anticipated in plasma and tissues suggests that inhibition of P450 enzymes involved in carcinogen activation by ITCs released from thiol-ITCs may not be a principal mechanism for their tumor inhibitory activity; other mechanisms probably contribute to their chemopreventive activity.