Xia Qingsu, Chou Ming W, Kadlubar Fred F, Chan Po-Cheun, Fu Peter P
National Center for Toxicological Research, Jefferson, Arkansas 72079, USA.
Chem Res Toxicol. 2003 Jan;16(1):66-73. doi: 10.1021/tx025605i.
Riddelliine, a widespread naturally occurring genotoxic pyrrolizidine alkaloid, induced liver tumors in rats and mice in an NTP 2-year carcinogenicity bioassay. We have determined that riddelliine induces liver tumors in rats through a genotoxic mechanism involving the formation of (+/-)-6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP), which reacts with DNA to form a set of eight DNA adducts. To determine the relevance to humans of the results obtained in experimental animals, the metabolism of riddelliine was conducted using human liver microsomes. As with rat liver microsomes, DHP and riddelliine N-oxide were major metabolites in incubations conducted with human liver microsomes. The levels of DHP and riddelliine N-oxide were 0.20-0.62 and 0.03-0.15 nmol/min/mg protein, respectively, which are comparable to those obtained from rat liver microsomal metabolism. When metabolism was conducted in the presence of calf thymus DNA, the same set of eight DHP-derived DNA adducts was formed. Both the metabolism pattern and DNA adduct profile were very similar to those obtained from rat liver microsomes. When metabolism was conducted in the presence of the P450 3A4 enzyme inhibitor triacetyleandomycin, the formation of DHP and riddelliine N-oxide was reduced 84 and 92%, respectively. For DHP formation, the Km values were determined to be 0.37 +/- 0.05 and 0.66 +/- 0.08 mM from female rats and female humans; the Vmax values from female rat and human liver microsomal metabolism were 0.48 +/- 0.03 and 1.70 +/- 0.09 nmol/min/mg protein, respectively. These results strongly indicate the mechanistic data on liver tumor induction obtained for riddelliine in laboratory rodents is highly relevant to humans.
瑞德利碱是一种广泛存在的天然遗传毒性吡咯里西啶生物碱，在一项美国国家毒理学计划（NTP）的2年致癌性生物测定中，可诱发大鼠和小鼠的肝脏肿瘤。我们已经确定，瑞德利碱通过一种遗传毒性机制在大鼠中诱发肝脏肿瘤，该机制涉及（±）-6,7-二氢-7-羟基-1-羟甲基-5H-吡咯里嗪（DHP）的形成，DHP与DNA反应形成一组八个DNA加合物。为了确定实验动物中获得的结果与人类的相关性，使用人肝微粒体对瑞德利碱进行了代谢研究。与大鼠肝微粒体一样，DHP和瑞德利碱N-氧化物是人肝微粒体孵育中的主要代谢产物。DHP和瑞德利碱N-氧化物的水平分别为0.20 - 0.62和0.03 - 0.15 nmol/分钟/毫克蛋白质，这与大鼠肝微粒体代谢得到的水平相当。当在小牛胸腺DNA存在下进行代谢时，形成了相同的一组八个源自DHP的DNA加合物。代谢模式和DNA加合物谱与大鼠肝微粒体得到的非常相似。当在细胞色素P450 3A4酶抑制剂三乙酰夹竹桃霉素存在下进行代谢时，DHP和瑞德利碱N-氧化物的形成分别减少了84%和92%。对于DHP的形成，从雌性大鼠和女性人体中确定的Km值分别为0.37±0.05和0.66±0.08 mM；雌性大鼠和人肝微粒体代谢的Vmax值分别为0.48±0.03和1.70±0.09 nmol/分钟/毫克蛋白质。这些结果有力地表明，在实验室啮齿动物中获得的关于瑞德利碱诱发肝脏肿瘤的机制数据与人类高度相关。
