Smith T J, Guo Z, Li C, Ning S M, Thomas P E, Yang C S
Laboratory for Cancer Research, College of Pharmacy, Rutgers University, Piscataway, New Jersey 08855-0789.
Cancer Res. 1993 Jul 15;53(14):3276-82.
The tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1- butanone (NNK), induces lung tumors in mice, rats, and hamsters. Phenethyl isothiocyanate (PEITC), which occurs as gluconasturtiin in cruciferous vegetables, is a potent inhibitor of NNK-induced carcinogenesis. The present study investigated the enzymatic basis for the bioactivation of NNK and the mechanisms of the inhibition of this process by dietary PEITC in mice. The apparent Km for the formation of keto aldehyde, keto alcohol, and NNK-N-oxide in lung microsomes was 4.9, 2.6, and 1.8 microM and, in liver microsomes, 5.5, 5.1, and 8.8 microM, respectively. Immunoinhibition studies suggested that cytochrome P450s (P450s) 2A1 and 2B1 or related forms are the major enzymes involved in the oxidative metabolism of NNK in mouse lung microsomes. When female A/J mice were fed diets containing 0, 1, or 3 mumol of PEITC/g of diet for 4 wk, the dietary PEITC had no significant effects on the food consumption and body weight of the mice. NNK oxidation in the lung microsomes of mice consuming the 1 or 3 mumol of PEITC/g of diet was decreased by 13 to 27% or 30 to 50%, respectively. In liver microsomes, whose NNK oxidative metabolism rates were about twice those of lung microsomes on a per mg of protein basis, the activities were decreased by 14 to 31% by the 3 mumol of PEITC/g of diet. The apparent Km remained unchanged, and the apparent Vmax decreased in the lung and liver microsomes of PEITC-fed mice, suggesting a noncompetitive nature of the inhibition. When added to the incubation mixture, PEITC decreased NNK metabolism in a concentration-dependent manner and exhibited a competitive inhibition with apparent Ki values of 51 to 93 nM. Dietary PEITC decreased the hepatic P450 content by 25%, but increased (2-fold) the O-dealkylase activities of 7-pentoxyresorufin (indicative of P450 2B1) and 7-ethoxyresorufin (indicative of P450 1A) in the liver microsomes of mice consuming the 3 mumol of PEITC/g of diet. The P450 2B level was increased in liver microsomes but slightly decreased in the lung microsomes. The p450 2E1 level was increased by dietary PEITC by 1.2- and 1.6-fold in the liver and lung microsomes, respectively. The activities of glutathione S-transferase and NAD(P)H-quinone oxidoreductase in liver and lung microsomes were not affected appreciably by the dietary PEITC treatment. The results suggest that chronic consumption of PEITC decreases the rate of metabolic activation of NNK by chemical inactivation and competitive inhibition of the enzyme(s) responsible for NNK oxidation.
烟草特异性亚硝胺4-(甲基亚硝基氨基)-1-(3-吡啶基)-1-丁酮(NNK)可在小鼠、大鼠和仓鼠体内诱发肺部肿瘤。异硫氰酸苯乙酯(PEITC)以葡糖芥苷的形式存在于十字花科蔬菜中，是NNK诱导致癌作用的有效抑制剂。本研究调查了NNK生物活化的酶学基础以及膳食PEITC在小鼠体内抑制这一过程的机制。肺微粒体中酮醛、酮醇和NNK - N -氧化物形成的表观Km分别为4.9、2.6和1.8微摩尔，肝微粒体中分别为5.5、5.1和8.8微摩尔。免疫抑制研究表明，细胞色素P450(P450)2A1和2B1或相关形式是参与小鼠肺微粒体中NNK氧化代谢的主要酶。当雌性A/J小鼠喂食含0、1或3微摩尔PEITC/克饲料4周时，膳食PEITC对小鼠的食物摄入量和体重没有显著影响。食用1或3微摩尔PEITC/克饲料的小鼠肺微粒体中NNK氧化分别降低了13%至27%或30%至50%。在肝微粒体中，以每毫克蛋白质计算，其NNK氧化代谢率约为肺微粒体的两倍，3微摩尔PEITC/克饲料可使肝微粒体中的活性降低14%至31%。表观Km保持不变，喂食PEITC的小鼠肺和肝微粒体中的表观Vmax降低，表明这种抑制具有非竞争性。当添加到孵育混合物中时，PEITC以浓度依赖的方式降低NNK代谢，并表现出竞争性抑制，表观Ki值为51至93纳摩尔。膳食PEITC使肝脏P450含量降低25%，但使食用3微摩尔PEITC/克饲料的小鼠肝微粒体中7 -戊氧基试卤灵(指示P450 2B1)和7 -乙氧基试卤灵(指示P450 1A)的O -脱烷基酶活性增加(2倍)。肝微粒体中P450 2B水平升高，但肺微粒体中略有降低。膳食PEITC使肝和肺微粒体中的p450 2E1水平分别升高1.2倍和1.6倍。膳食PEITC处理对肝和肺微粒体中谷胱甘肽S -转移酶和NAD(P)H -醌氧化还原酶的活性没有明显影响。结果表明，长期食用PEITC可通过化学失活和对负责NNK氧化的酶的竞争性抑制来降低NNK的代谢活化率。
