Biotransformation of caffeine, paraxanthine, theophylline, and theobromine by polycyclic aromatic hydrocarbon-inducible cytochrome(s) P-450 in human liver microsomes.
Biotransformation of caffeine, paraxanthine, theophylline, and theobromine by polycyclic aromatic hydrocarbon-inducible cytochrome(s) P-450 in human liver microsomes.
复制标题
DOI：
--
复制
发表时间：
1987-03
期刊：
影响因子：
--
通讯作者：
M. Campbell;D. Grant;T. Inaba;W. Kalow
中科院分区：
文献类型：
--
作者：
M. Campbell;D. Grant;T. Inaba;W. Kalow
在15个不同的人类肝脏中研究了咖啡因及其主要二甲基黄嘌呤代谢产物副黄嘌呤、茶碱和可可碱的微粒体代谢，包括来自两个已知的非吸烟者和一个已知的吸烟者的肝脏。至少有两种不同的酶具有不同的底物亲和力有可能催化大多数甲基黄嘌呤N-去甲基化和C8-羟基化在体外，然而，在低甲基黄嘌呤浓度经常遇到在体内，参与高亲和力网站预计占主导地位。基于7-乙氧基试卤灵和苯并[a]芘的竞争性抑制，以及基于7-乙氧基试卤灵-O-脱乙基化和甲基黄嘌呤脱甲基化速率之间的显著相关性（p <0.001），高亲和力酶似乎是细胞色素P-450的多环芳烃诱导同工酶。在两个高活性的肝脏中，α-萘酚酮抑制了超过80%的所有甲基黄嘌呤去甲基化，而8-羟基化通常抑制较少。在四个不同的肝脏制剂中，副黄嘌呤7-去甲基化的动力学分析导致相似的Km值为1.2 +/- 0.5 mM（平均值+/- SD），而Vmax值变化8倍，与相同的高亲和力同工酶的参与相容。值得注意的是代谢率的高度肝间变异，已知吸烟者在副黄嘌呤去甲基化率的20倍范围内显示出第二高的活性，与多环芳烃相关酶诱导一致。最大抑制副黄嘌呤8-羟基化的α-萘酚酮留下类似的残留活性在15个肝脏制剂，表明存在的酶活性，这是不可诱导的。此外，在低活性的肝脏，80%以上的副黄嘌呤8-羟基化介导的细胞色素P-450的同工酶不敏感的抑制α-萘酚酮。我们的体外数据表明，脱甲基相对于羟基化产物的副黄嘌呤的比例与7-乙氧基试卤灵O-脱乙基化速率相关。综上所述，这些数据提供了一个潜在的体内标记的多环芳烃诱导细胞色素P-450活性的基础上，尿中代谢产物的比例副黄嘌呤7-去甲基化8-羟基化产物摄入咖啡因。
The microsomal metabolism of caffeine and its primary dimethylxanthine metabolites, paraxanthine, theophylline, and theobromine, was investigated in 15 different human livers, including those from two known nonsmokers and one known smoker. At least two distinct enzymes with differing substrate affinities have the potential to catalyze most methylxanthine N-demethylations and C8-hydroxylations in vitro; however, at the low methylxanthine concentrations routinely encountered in vivo, participation by the high affinity site is expected to predominate. It appears that the high affinity enzyme is a polycyclic aromatic hydrocarbon-inducible isozyme of cytochrome P-450, based on competitive inhibition by 7-ethoxyresorufin and benzo[a]pyrene, and based on a significant (p less than 0.001) correlation between 7-ethoxyresorufin-O-deethylation and methylxanthine demethylation rates. alpha-Naphthoflavone inhibited all methylxanthine demethylations in excess of 80% in two high activity livers, whereas 8-hydroxylations were generally inhibited less. Kinetic analysis of paraxanthine 7-demethylation in four different liver preparations resulted in similar Km values of 1.2 +/- 0.5 mM (mean +/- SD), whereas Vmax values varied 8-fold, compatible with participation by the same high affinity isozyme. Notable was the high degree of inter-liver variation in metabolic rates, with the known smoker showing the second highest activity among a 20-fold range in paraxanthine demethylation rates, consistent with polycyclic aromatic hydrocarbon-related enzyme induction. Maximal inhibition of paraxanthine 8-hydroxylation by alpha-naphthoflavone left similar residual activities in the 15 liver preparations, indicating the presence of an enzyme activity that was not inducible. Furthermore, in low activity livers, more than 80% of paraxanthine 8-hydroxylation was mediated by an isozyme of cytochrome P-450 insensitive to inhibition by alpha-naphthoflavone. Our in vitro data show that the proportion of demethylation relative to hydroxylation products of paraxanthine correlate with 7-ethoxyresorufin O-deethylation rates. Taken together, the data provide a rationale for a potential in vivo marker of polycyclic aromatic hydrocarbon-inducible cytochrome P-450 activity based on a urinary metabolite ratio of paraxanthine 7-demethylation to 8-hydroxylation products after caffeine intake.