人肝细胞色素P450 2C8/9、2E1比活性测定

目的　建立甲苯磺丁脲羟化酶 (CYP2C8/ 9)和氯羟苯口恶唑 6 羟化酶 (CYP2E1)比活性的测定方法 ,为深入研究CYP45 0在药物代谢中的作用奠定基础。方法　从成人肝细胞中提取微粒体 ,测定其蛋白含量 ,以甲苯磺丁脲、氯羟苯口恶唑为底物 ,用HPLC以梯度洗脱法测定其代谢产物羟基甲苯磺丁脲及 6 羟基氯羟苯 口恶唑生成量 ,据此计算人肝细胞色素P45 0 (CYP45 0 )同工酶甲苯磺丁脲羟化酶 (CYP2C8/9)和氯羟苯 口恶唑 6 羟化酶 (CYP2E1)比活性。结果　于不同时间反复测定的CYP2C8/ 9、CYP2E1比活性无差异。结论　CYP2C8/ 9、CYP2E1的测定方法较为简单、稳定、重复性好 ,可用于新药筛选、安全性评价及肝脏病理学、毒理学研究。     

人肝细胞色素P450 2C8/9、2E1比活性测定

目的建立甲苯磺丁脲羟化酶(CYP2C8/9)和氯羟苯口恶唑6-羟化酶(CYP2E1)比活性的测定方法,为深入研究CYP450在药物代谢中的作用奠定基础.方法从成人肝细胞中提取微粒体,测定其蛋白含量,以甲苯磺丁脲、氯羟苯口恶唑为底物,用HPLC以梯度洗脱法测定其代谢产物羟基甲苯磺丁脲及6-羟基氯羟苯口恶唑生成量,据此计算人肝细胞色素P450(CYP450)同工酶甲苯磺丁脲羟化酶(CYP2C8/9)和氯羟苯口恶唑6-羟化酶(CYP2E1)比活性.结果于不同时间反复测定的CYP2C8/9、CYP2E1比活性无差异.结论 CYP2C8/9、CYP2E1的测定方法较为简单、稳定、重复性好,可用于新药筛选、安全性评价及肝脏病理学、毒理学研究.     

rh-EGF对恒河猴肝细胞色素P450s的影响

目的研究重组人表皮生长因子(recombinant human epidermal growth factor,rh-EGF)对恒河猴肝细胞色素P450s同工酶活性的影响.方法恒河猴口服rh-EGF 16,32,160μ*kg-1*d-1,qd,连续给药26周,于停药时及停药8周后,各解剖一半动物,制备肝微粒体,进行肝细胞色素P450s总量、肝微粒体蛋白含量测定,用荧光分光光度法和HPLC法进行肝细胞色素P450s同工酶中7-乙氧基试卤灵脱乙基酶(CYP1A1)、香豆素7-羟化酶(CYP2A6)、甲苯磺丁脲羟化酶(CYP2C8/9)、S-(+)-美芬妥英羟化酶(CYP2C19)、丁呋洛尔1-羟化酶(CYP2D6)、氯唑沙宗羟化酶(CYP2E1)比活性测定.结果给药26周和停药8周后,rh-EGF对恒河猴肝细胞色素P450s总量、微粒体蛋白含量无明显影响,肝细胞色素P450s同工酶(CYP1A1,CYP2A6,CYP2C8/9,CYP2C19,CYP2D6,CYP2E1)活性无显著变化.结论本试验建立的恒河猴肝细胞色素P450s测定方法快速、简便,可广泛用于药物开发早期以及预测药物间相互作用和药物毒性的研究等.rh-EGF对恒河猴肝P450s部分同工酶无影响.     

人类细胞色素P450s研究概况及其在新药安全性评价中的应用

人类细胞色素P450s(CYP450s)对许多内源、外源性化合物在体内的I相生物转化有重要作用。人类编码CYP450s的基因已基本明确，为55个基因，29个假基因。目前欧美各国已经把CYP450s及其同工酶测定用于新药的筛选及安全性评价。本实验室建立了不同种属动物肝细胞色素P450s同工酶的测定方法，并选择来氟米特、重组人白细胞介素－11（rhIL-11)及重组人表皮生长因子（rh-EGF)，检测其对不同动物CYP450s的影响。实验结果表明：①来氟米特对Beagle犬肝微粒体蛋白含量、CYP450s总量、7－乙氧基试卤灵脱乙基酶（CYP1A1）、香豆素7－羟化酶（CYP2A6／CYP2A2）、甲苯磺丁脲羟化酶（CYP2C8／9）、氯唑沙宗羟化酶（CYP2E1）的比活性均无明显影响。②大鼠皮下注射rhIL-11后，肝微粒体中CYP1A1及CYP2A2比活性明显低于对照组，而肝微粒体蛋白含量、CYP450总量及CYP2E1比活性无明显变化。③rh-EGF对猕猴肝微粒体蛋白含量、CYP450s总量、CYP1A1、CYP2A6、CYP2C8／9、S－（＋）－美芬妥英羟化酶（CYP2C19）、丁呋洛尔1－羟化酶（CYP2D6）、CYP2E1比活性均无明显影响。本实验建立的犬、大鼠及猴肝CYP450s测定方法具有快速、简单、重复性好等特点，可广泛用于药物筛选及临床前安全性评价。除合成药物外，在基因工程药物安全性评价研究中，也应重视药物对CYP450s的影响。     

丹参注射液对人肝微粒体酶CYP2C9,CYP2C19,CYP2D6体外抑制作用的研究

目的 评价丹参注射液在体外对人肝微粒体酶CYP2C9、CYP2C19、CYP2D6活性的影响。方法 将丹参注射液与CYP450酶3种亚型(CYP2C9、CYP2C19、CYP2D6)的特异性探针底物甲苯磺丁脲、奥美拉唑、右美沙芬与大鼠肝微粒体进行孵育,采用LC-MS/MS法测定对应3种代谢产物4-羟基甲苯磺丁脲、5-羟基奥美拉唑、右啡烷的浓度,求算出IC₅₀。结果 丹参注射液对CYP2C9、CYP2C19和CYP2D6的IC₅₀值均>50 μg/mL。结论 丹参注射液对人肝微粒体酶CYP2C9、CYP2C19、CYP2D6没有抑制作用。     

艾瑞昔布对CYP2C9酶活性及mRNA和蛋白表达的影响

目的:评价艾瑞昔布对CYP2C9酶活性及mRNA和蛋白表达的影响。方法:采用超高效液相色谱(UPLC),色谱柱：Acquity UPLC BEH-C₁₈柱(2.1 mm×50 mm, 1.7μm);流动相为水-乙腈(76∶24,V/V);流速0.4 mL·min^-1;检测波长235 nm;柱温30℃;进样量2μL;内标为卡马西平。选择甲苯磺丁脲为CYP2C9酶特异性探针药物,与艾瑞昔布在肝微粒体中共同孵育,UPLC测定代谢产物4-羟基甲苯磺丁脲的生成量,考察艾瑞昔布对CYP2C9酶活性的影响。Wistar大鼠给予艾瑞昔布,RT-PCR和Western Blot测定肝组织中CYP2C9酶活性及mRNA和蛋白表达。结果:随着艾瑞昔布浓度的升高,肝微粒体孵育体系中4-羟基甲苯磺丁脲生成量降低,艾瑞昔布对CYP2C9活性呈剂量依赖性抑制作用,IC₅₀值为74.77μmol·L^-1;随着给药时间延长,大鼠肝脏CYP2C9酶的mRNA和蛋白表达减少(P<0.05)。结论:艾瑞昔布可降低CYP2C9酶的mR...     

甲基原薯蓣皂苷对人肝微粒体中7种CYP450酶活性的影响

目的研究甲基原薯蓣皂苷对CYP450酶的7种亚型酶活性的影响。方法将MPD和CYP450酶7种亚型的特异性探针底物咖啡因(CYP1A2)、右美沙芬(CYP2D6)、甲苯磺丁脲(CYP2C9)、s-美芬妥因(CYP2C19)、氯唑沙宗(CYP2E1)、香豆素(CYP2A6)及咪达唑仑(CYP3A4)与人肝微粒体进行孵化反应,采用HPLC和LC-MS/MS法测定对应的7种代谢产物(1,7-二甲基黄嘌呤、去甲右美沙芬、4-羟基甲苯磺丁脲、4-羟基美芬妥因、6-羟基氯唑沙宗、7-羟基香豆素和1-羟基咪达唑仑)的浓度,通过与对照组比较,确定MPD对以上7种酶活性的影响。结果MPD在1~10μmol.L-1时对7种酶均无明显抑制作用,在100μmol.L-1时对CYP2D6有抑制趋势,但对其他6种酶无抑制作用,均无统计学意义(P>0.05)。结论MPD在与以上6种酶(CYP1A2、CYP2E1、CYP2C19、CYP3A4、CYP2C9和CYP2A6)代谢的药物联合用药时,发生药物相互作用的可能性较小。     

探针药物法评价艾瑞昔布对大鼠CYP2C9酶的影响

利用探针药物法研究艾瑞昔布对大鼠体内CYP2C9酶的影响。将大鼠随机分为试验组和对照组,每组18只,试验组经口灌胃艾瑞昔布,对照组经口灌胃等量蒸馏水,连续给药7 d(2次/d)。d8,每只大鼠经口灌胃甲苯磺丁脲,并于给药后24 h内眼内眦取血,以HPLC法测定血浆药物浓度,拟合药动学参数,并对两组的药动学参数进行统计学分析。结果表明,试验组的AUC0→24 h、AUC0→∞、tmax、CL和V与对照组相比均有显著性差异(P<0.05)。合用艾瑞昔布后,甲苯磺丁脲的AUC0→24 h和AUC0→∞显著降低,CL明显增加,表明艾瑞昔布诱导CYP2C9酶的活性,加快甲苯磺丁脲的代谢。     

超高效液相串联质谱法快速测定CYP2C9酶活性

目的：建立一种超高效液相串联三重四级杆质谱法快速测定CYP2C9体外酶学活性检测的新方法。方法：色谱柱为ACQUITY UPLCBEH C18柱（100 mm×2.1 mm,1.7μm）;流动相为乙腈-水（含0.1%甲酸和0.5%氨水）（40∶60）,流速为0.2 ml·min^-1,柱温30℃,内标为氯磺丙脲;质谱条件：电喷雾离子化源（ESI）,正离子检测模式;用实验室制备的CYP2C9＊1,＊2,＊3和＊13酶在37℃孵育甲苯磺丁脲后,加入800μl冰乙酸乙酯终止反应,10 000 g离心后取有机层于氮吹仪下吹干并用200μl流动相复溶后上样。结果：4-羟基甲苯磺丁脲的保留时间为1.21 min,线性范围为0.05-5 ng·μ^l-1（r=0.999 8）,最低定量限为0.01 ng·μl^-1,回收率为99.3%-100.3%。4-羟基甲苯磺丁脲的日内、日间RSD均〈5%,孵育体系中的其他内源性物质不干扰测定。CYP2C9＊1,＊2,＊3和＊13孵育甲苯磺丁脲后结果显示突变体CYP2C9＊2,＊3,＊13的体外酶学活性分别为野生型CYP2C9＊1的47.3%,11%和0.3%。结论：该方法快速、稳定,该方法操作简便,适于CYP2C9的快速活性检测及抑制药等的相关性研究。     

短刺小克银汉霉AS 3.910的CYP2C9同工酶抑制作用

目的探索具有药物代谢酶CYP2C9活性的微生物模型对CYP2C9抑制剂的响应。方法选用短刺小克银汉霉AS 3.910为模型菌株， 检测CYP2C9抑制剂对CYP2C9底物特定转化产物产率的影响， 并通过底物间相互影响的程度探索该转化体系中的药物代谢相互作用。 采用液相色谱-多级质谱联用技术检测转化产物。 结果 CYP2C9抑制剂苯溴马隆抑制4′-羟基甲苯磺丁脲的生成，使其产率由100%下降到14.5%；磺胺苯吡唑抑制O-去甲基吲哚美辛的生成，使其产率由75.2%降低到9.9%；丙戊酸抑制4′-羟基双氯芬酸的生成，使其产率由98.6%降低到2.7%。底物药物甲苯磺丁脲、双氯芬酸和吲哚美辛之间存在代谢相互作用，导致生成相应代谢物的产率降低。结论3种CYP2C9抑制剂不同程度地抑制了短刺小克银汉霉的CYP2C9同工酶，而且CYP2C9底物间存在药物代谢相互作用。     

In vitro evaluation of valproic acid as an inhibitor of human cytochrome P450 isoforms: preferential inhibition of cytochrome P450 2C9 (CYP2C9)

AIMS: To evaluate the potency and specificity of valproic acid as an inhibitor of the activity of different human CYP isoforms in liver microsomes. METHODS: Using pooled human liver microsomes, the effects of valproic acid on seven CYP isoform specific marker reactions were measured: phenacetin O-deethylase (CYP1A2), coumarin 7-hydroxylase (CYP2A6), tolbutamide hydroxylase (CYP2C9), S-mephenytoin 4'-hydroxylase (CYP2C19), dextromethorphan O-demethylase (CYP2D6), chlorzoxazone 6-hydroxylase (CYP2E1) and midazolam 1'-hydroxylase (CYP3A4). RESULTS: Valproic acid competitively inhibited CYP2C9 activity with a Ki value of 600 microM. In addition, valproic acid slightly inhibited CYP2C19 activity (Ki = 8553 microM, mixed inhibition) and CYP3A4 activity (Ki = 7975 microM, competitive inhibition). The inhibition of CYP2A6 activity by valproic acid was time-, concentration- and NADPH-dependent (KI = 9150 microM, Kinact=0.048 min(-1)), consistent with mechanism-based inhibition of CYP2A6. However, minimal inhibition of CYP1A2, CYP2D6 and CYP2E1 activities was observed. CONCLUSIONS: Valproic acid inhibits the activity of CYP2C9 at clinically relevant concentrations in human liver microsomes. Inhibition of CYP2C9 can explain some of the effects of valproic acid on the pharmacokinetics of other drugs, such as phenytoin. Co-administration of high doses of valproic acid with drugs that are primarily metabolized by CYP2C9 may result in significant drug interactions.     

Identification of the main human cytochrome P450 enzymes involved in safrole 1'-hydroxylation

Safrole is a natural plant constituent, found in sassafras oil and certain other essential oils. The carcinogenicity of safrole is mediated through 1'-hydroxysafrole formation, followed by sulfonation to an unstable sulfate that reacts to form DNA adducts. To identify the main cytochrome P450 (P450) involved in human hepatic safrole 1'-hydroxylation (SOH), we determined the SOH activities of human liver microsomes and Escherichia coli membranes expressing bicistronic human P450s. Human liver (n = 18) microsomal SOH activities were in the range of 3.5-16.9 nmol/min/mg protein with a mean value of 8.7 +/- 0.7 nmol/min/mg protein. In human liver (n = 3) microsomes, the mean K(m) and V(max) values of SOH were 5.7 +/- 1.2 mM and 0.14 +/- 0.03 micromol/min/nmol P450, respectively. The mean intrinsic clearance (V(max)/K(m)) was 25.3 +/- 2.3 microL/min/nmol P450. SOH was sensitive to the inhibition by a CYP2C9 inhibitor, sulfaphenazole, and CYP2E1 inhibitors, 4-methylpyrazole and diethyldithiocarbamate. The liver microsomal SOH activity showed significant correlations with tolbutamide hydroxylation (r = 0.569) and chlorzoxazone hydroxylation (r = 0.770) activities, which were the model reactions catalyzed by CYP2C9 and CYP2E1, respectively. Human CYP2C9 and CYP2E1 showed SOH activities at least 2-fold higher than the other P450s. CYP2E1 showed an intrinsic clearance 3-fold greater than CYP2C9. These results demonstrated that CYP2C9 and CYP2E1 were the main P450s involved in human hepatic SOH.     

Trimethoprim and sulfamethoxazole are selective inhibitors of CYP2C8 and CYP2C9, respectively.

To evaluate the inhibitory effects of trimethoprim and sulfamethoxazole on cytochrome P450 (P450) isoforms, selective marker reactions for CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4 were examined in human liver microsomes and recombinant CYP2C8 and CYP2C9. The in vivo drug interactions of trimethoprim and sulfamethoxazole were predicted in vitro using [I]/([I] + K(i)) values. With concentrations ranging from 5 to 100 microM, trimethoprim exhibited a selective inhibitory effect on CYP2C8-mediated paclitaxel 6alpha-hydroxylation in human liver microsomes and recombinant CYP2C8, with apparent IC(50) (K(i)) values of 54 microM (32 microM) and 75 microM, respectively. With concentrations ranging from 50 to 500 microM, sulfamethoxazole was a selective inhibitor of CYP2C9-mediated tolbutamide hydroxylation in human liver microsomes and recombinant CYP2C9, with apparent IC(50) (K(i)) values of 544 microM (271 microM) and 456 microM, respectively. With concentrations higher than 100 microM trimethoprim and 500 microM sulfamethoxazole, both drugs lost their selectivity for the P450 isoforms. Based on estimated total hepatic concentrations (or free plasma concentrations) of the drugs and the scaling model, one would expect in vivo in humans 80% (26%) and 13% (24%) inhibition of the metabolic clearance of CYP2C8 and CYP2C9 substrates by trimethoprim and sulfamethoxazole, respectively. In conclusion, trimethoprim and sulfamethoxazole can be used as selective inhibitors of CYP2C8 and CYP2C9 in in vitro studies. In humans, trimethoprim and sulfamethoxazole may inhibit the activities of CYP2C8 and CYP2C9, respectively.     

CYP2C8/9 mediate dapsone N-hydroxylation at clinical concentrations of dapsone.

Using selective cytochrome P450 (CYP) inhibitors and clinical concentrations (4 microM) of dapsone (DDS), we found a major contribution of CYP2C9 and little or no contribution (< or = 10%) of CYP3A4 and CYP2E1 to dapsone N-hydroxylation (DDS-NHY) in human liver microsomes. Sulfaphenazole (2.16 microM) and tolbutamide (500 microM), selective inhibitors of CYP2C9 (or 2C8/9), inhibited DDS-NHY by 48 +/- 14 and 41 +/- 15%, respectively. The apparent Michaelis-Menten Km values for DDS-NHY by cloned CYP2C8, CYP2C9, CYP2C18, and CYP2C19 were 75 microM, 31 microM, 25 microM, and greater than 1 mM, respectively. CYP3A4 and CYP2E1 were incapable of DDS-NHY at 4 microM DDS. S-mephenytoin (360 microM) activated DDS-NHY by human liver microsomes and by CYP2C8 by 43 +/- 36 and 193 +/- 16%, respectively. This activation was cytochrome b5-dependent. In contrast, S-mephenytoin inhibited DDS-NHY by CYP2C9, CYP2C18, and CYP2C19 by 27 +/- 2, 49 +/- 1, and 32 +/- 4%, respectively. Because CYP2C18 and CYP19 are expressed at low concentrations in the human liver, these observations indicate that at clinical DDS concentrations, CYP2C9 is a major and CYP2C8 is a likely minor contributor to DDS-NHY in human liver microsomes.     

灯盏花素注射液对大鼠体外肝微粒体细胞色素P450酶活性的影响

摘 要 目的:研究灯盏花素注射液对大鼠肝微粒体CYP1A2、CYP2C9、CYP2D6、CYP2E1和CYP3A1/2 五个亚型酶活性的影响。方法: 采用大鼠肝微粒体体外孵育法,选用非那西丁（CYP1A2）、甲苯磺丁脲（CYP2C9）、右美沙芬（CYP2D6）、氯唑沙宗（CYP2E1）和睾酮（CYP3A1/2）作为5个亚型酶的探针药物,孵育系统中加入不同浓度的灯盏花素注射液,用HPLC法测定5个亚型酶探针药物代谢产物的生成量,比较空白对照组与不同浓度灯盏花素注射液给药组探针药物的活性,反映灯盏花素注射液对5个亚型酶活性的影响。结果:大鼠肝微粒体体外孵育系统中,灯盏花素注射液对大鼠CYP3A1/2的IC₅₀为29.40μg·ml^-1,K_i为37.78 μg·ml^-1;对CYP1A2、CYP2C9、CYP2D6和CYP2E1的IC₅₀＞200 μg·ml^-1。结论:灯盏花素注射液对大鼠体外肝微粒体CYP3A1/2有弱的抑制作用,对CYP1A2、CYP2C9、CYP2D6和CYP2E1活性无明显影响。     

Characterization of liver microsomal 7-ethoxycoumarin O-deethylation and chlorzoxazone 6-hydroxylation activities in Japanese and Caucasian subjects genotyped for CYP2E1 gene

To determine whether the CYP2E1 genetic polymorphisms cause alterations in protein expression and enzyme catalytic activities, three CYP2E1 genetic polymorphisms, namely RsaI/PstI, DraI, and MspI types, were determined in liver genomic DNA isolated from 39 Japanese and 45 Caucasians. These genotypes were compared with levels of CYP2E1 and activities of 7-ethoxycoumarin O-deethylation and chlorzoxazone 6-hydroxylation in liver microsomes from these human samples. In combination of three types of CYP2E1 polymorphisms, it was classified into seven genotypes in the Japanese population and four in the Caucasian population. The incidence in the occurrence of RsaI/PstI polymorphism or DraI polymorphism was 0.24 and 0.29 for Japanese, and 0.01 and 0.02 for Caucasians. Ethnic difference was also noted in the MspI polymorphism in which frequencies in Japanese and Caucasian populations were 0.15 and 0.02, respectively. Studies with liver microsomes showed that there were no significant differences in the levels of expression of CYP2E1 protein between wild-type (group A) and other 6 genotypes (B, C, D, E, F, and G) in Japanese and other three genotypes (B, D, and F) in Caucasians. Catalytic activities for 7-ethoxycoumarin O-deethylation and chlorzoxazone 6-hydroxylation by liver microsomes were also found to be less significantly affected by mutations in the CYP2E1 gene in human samples examined in this study. These results support the view that RsaI/PstI, DraI, and MspI types of CYP2E1 genetic polymorphisms may not cause significant alterations in protein expression and enzyme catalytic activities of CYP2E1 enzyme in human livers.     

CYP2C19 participates in tolbutamide hydroxylation by human liver microsomes.

Tolbutamide is a sulfonylurea-type oral hypoglycemic agent whose action is terminated by hydroxylation of the tolylsulfonyl methyl moiety catalyzed by cytochrome P-450 (CYP) enzymes of the human CYP2C subfamily. Although most studies have implicated CYP2C9 as the exclusive catalyst of hepatic tolbutamide hydroxylation in humans, there is evidence that other CYP2C enzymes (e.g., CYP2C19) may also participate. To that end, we used an immunochemical approach to assess the role of individual CYP2Cs in microsomal tolbutamide metabolism. Polyclonal antibodies were raised to CYP2C9 purified from human liver, and were then back-adsorbed against recombinant CYP2C19 coupled to a solid-phase support. Western blotting revealed that the absorbed anti-human CYP2C9 preparation reacted with only recombinant CYP2C9 and the corresponding native protein in hepatic microsomes, and no longer recognized CYP2C19 and CYP2C8. Monospecific anti-CYP2C9 not only retained the ability to inhibit CYP2C9-catalyzed reactions, as evidenced by its marked (90%) inhibition of diclofenac 4'-hydroxylation by purified CYP2C9 and by human liver microsomes, but also exhibited metabolic specificity, as indicated by its negligible (<15%) inhibitory effect on S-mephenytoin 4'-hydroxylation by purified CYP2C19 or hepatic microsomes containing CYP2C19. Monospecific anti-CYP2C9 was also found to inhibit rates of tolbutamide hydroxylation by 93 +/- 4 and 78 +/- 6% in CYP2C19-deficient and CYP2C19-containing human liver microsomes, respectively. Taken together, our results indicate that both CYP2C9 and CYP2C19 are involved in tolbutamide hydroxylation by human liver microsomes, and that CYP2C19 underlies at least 14 to 22% of tolbutamide metabolism. Although expression of CYP2C19 in human liver is less than that of CYP2C9, it may play an important role in tolbutamide disposition in subjects expressing either high levels of CYP2C19 or a catalytically deficient CYP2C9 enzyme.     

In vitro inhibition of cytochrome P450 enzymes in human liver microsomes by a potent CYP2A6 inhibitor, trans-2-phenylcyclopropylamine (tranylcypromine), and its nonamine analog, cyclopropylbenzene.

Currently, there are no selective, well characterized inhibitors for CYP2A6. Therefore, the effects of trans-(+/-)-2-phenylcyclopropylamine (tranylcypromine), a potent CYP2A6 inhibitor, on human liver microsomal cytochromes P450 (CYP) were studied to elucidate its selectivity. The IC50 value of tranylcypromine in coumarin 7-hydroxylation (CYP2A6 model activity) was 0.42 +/- 0.07 microM and in chlorzoxazone 6-hydroxylation (CYP2E1 model activity) 3.0 +/- 1.1 microM. The IC50 values for CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4 activities were >10 microM. Potency and selectivity of tranylcypromine were strongly dependent on the amine group, because its nonamine analog cyclopropylbenzene was much less potent inhibitor of CYP1A, CYP2A6, CYP2C19, and CYP2E1 activities and did not inhibit at all CYP2C9, CYP2D6, or CYP3A4 activities. In human liver microsomes tranylcypromine induced type II and cyclopropylbenzene type I difference spectrum. According to the double reciprocal analysis of these spectral responses both tranylcypromine and cyclopropylbenzene may have at least two P450-related binding sites in liver microsomes. The K(a) values of tranylcypromine varied from 4.5 to 15.1 microM and -34.3 to 167 microM in microsomes derived from three different livers and of cyclopropylbenzene from -1.6 to 10.1 microM and -34.6 and 75.2 microM in the same liver microsomes. Based on these results, tranylcypromine seems an adequately selective CYP2A6 inhibitor for in vitro use.     

Inhibition of human CYP1A2 activity in vitro by methylxanthines: potent competitive inhibition by 8-phenyltheophylline

1. Humans are exposed in vivo to methylxanthines by dietary ingestion, as well as from their use as therapeutic agents. The inhibitory effect of a series of these compounds on high-affinity phenacetin O-deethylase activity in the human liver microsomal fraction, a measure of CYP1A2 activity, has been evaluated. 2. Paracetamol, the product of phenacetin O-deethylase activity, was analysed by gas chromatography/negative-ion mass spectrometry using a novel bistrifluoromethylbenzoyl/ trimethylsilyl derivative, and incubation conditions for assessing high-affinity phenacetin O-deethylase activity were examined and optimized. 3. 1-Methylxanthine, caffeine, theophylline, 8-methylxanthine, pentoxyfylline and 3isobutyl-1-methylxanthine caused moderate inhibition with IC50 = 260, 140, 120, 100, 62 and 36 microM respectively. 4. 8-Phenyltheophylline was a potent competitive inhibitor of high-affinity phenacetin O-deethylase activity with an IC50 = 0.7 microM and Ki = 0.11 microM. 5. The specificity of inhibition by 8-phenyltheophylline was assessed by measuring its effect on debrisoquine 4-hydroxylase (CYP2D6), terfenadine hydroxylase (CYP3A4), chlorzoxazone 6-hydroxylase (CYP2E1) and tolbutamide 4-hydroxylase (CYP2C9) activities in human liver microsomal fraction. No inhibition of any of these activities was observed. 6. The potency and specificity of 8-phenyltheophylline as an inhibitor of human hepatic CYP1A2 indicate that the compound may be useful as a chemical inhibitor of this enzyme for further in vitro studies.