Boersma Marelle G, van der Woude Hester, Bogaards Jan, Boeren Sjef, Vervoort Jacques, Cnubben Nicole H P, van Iersel Marlou L P S, van Bladeren Peter J, Rietjens Ivonne M C M
Division of Toxicology, Wageningen University, Tuinlaan 5, 6703 HE, The Netherlands.
Chem Res Toxicol. 2002 May;15(5):662-70. doi: 10.1021/tx0101705.
The regioselectivity of phase II conjugation of flavonoids is expected to be of importance for their biological activity. In the present study, the regioselectivity of phase II biotransformation of the model flavonoids luteolin and quercetin by UDP-glucuronosyltransferases was investigated. Identification of the metabolites formed in microsomal incubations with luteolin or quercetin was done using HPLC, LC-MS, and (1)H NMR. The results obtained demonstrate the major sites for glucuronidation to be the 7-, 3-, 3'-, or 4'-hydroxyl moiety. Using these unequivocal identifications, the regioselectivity of the glucuronidation of luteolin and quercetin by microsomal samples from different origin, i.e., rat and human intestine and liver, as well as by various individual human UDP-glucuronosyltransferase isoenzymes was characterized. The results obtained reveal that regioselectivity is dependent on the model flavonoid of interest, glucuronidation of luteolin and quercetin not following the same pattern, depending on the isoenzyme of UDP-glucuronosyltransferases (UGT) involved. Human UGT1A1, UGT1A8, and UGT1A9 were shown to be especially active in conjugation of both flavonoids, whereas UGT1A4 and UGT1A10 and the isoenzymes from the UGTB family, UGT2B7 and UGT2B15, were less efficient. Due to the different regioselectivity and activity displayed by the various UDP-glucuronosyltransferases, regioselectivity and rate of flavonoid conjugation varies with species and organ. Qualitative comparison of the regioselectivities of glucuronidation obtained with human intestine and liver microsomes to those obtained with human UGT isoenzymes indicates that, in human liver, especially UGT1A9 and, in intestine, UGT1A1 and UGT1A8 are involved in glucuronidation of quercetin and luteolin. Taking into account the fact that the anti-oxidant action as well as the pro-oxidant toxicity of these catechol-type flavonoids is especially related to their 3',4'-dihydroxyl moiety, it is of interest to note that the human intestine UGT's appear to be especially effective in conjugating this 3',4' catechol unit. This would imply that upon glucuronidation along the transport across the intestinal border, the flavonoids loose a significant part of these biological activities.
黄酮类化合物II相共轭反应的区域选择性预期对其生物活性具有重要意义。在本研究中，对模型黄酮类化合物木犀草素和槲皮素经UDP - 葡萄糖醛酸基转移酶进行II相生物转化的区域选择性进行了研究。使用高效液相色谱（HPLC）、液相色谱 - 质谱联用（LC - MS）和核磁共振氢谱（¹H NMR）对用木犀草素或槲皮素进行微粒体孵育所形成的代谢产物进行鉴定。所获得的结果表明葡萄糖醛酸化的主要位点是7 -、3 -、3'-或4'-羟基部分。利用这些明确的鉴定结果，对来自不同来源（即大鼠和人类的肠道及肝脏）的微粒体样品以及各种个体人类UDP - 葡萄糖醛酸基转移酶同工酶催化木犀草素和槲皮素葡萄糖醛酸化的区域选择性进行了表征。所获得的结果显示区域选择性取决于所关注的模型黄酮类化合物，木犀草素和槲皮素的葡萄糖醛酸化并不遵循相同模式，这取决于所涉及的UDP - 葡萄糖醛酸基转移酶（UGT）同工酶。已表明人类UGT1A1、UGT1A8和UGT1A9在这两种黄酮类化合物的共轭反应中特别活跃，而UGT1A4和UGT1A10以及UGTB家族的同工酶UGT2B7和UGT2B15效率较低。由于各种UDP - 葡萄糖醛酸基转移酶表现出不同的区域选择性和活性，黄酮类化合物共轭反应的区域选择性和速率随物种和器官而变化。对人类肠道和肝脏微粒体获得的葡萄糖醛酸化区域选择性与人类UGT同工酶获得的区域选择性进行定性比较表明，在人类肝脏中，特别是UGT1A9参与了槲皮素和木犀草素的葡萄糖醛酸化，而在肠道中，UGT1A1和UGT1A8参与其中。考虑到这些儿茶酚型黄酮类化合物的抗氧化作用以及促氧化毒性尤其与其3'，4'-二羟基部分相关，值得注意的是人类肠道UGT似乎在共轭这个3'，4'儿茶酚单元方面特别有效。这意味着在沿着肠道边界转运过程中进行葡萄糖醛酸化时，黄酮类化合物会丧失这些生物活性的很大一部分。
