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The role of S‐mephenytoin 4'‐hydroxylase in imipramine metabolism by human liver microsomes: a two‐enzyme kinetic analysis of N‐ demethylation and 2‐hydroxylation.
Author(s) -
Chiba K,
Saitoh A,
Koyama E,
Tani M,
Hayashi M,
Ishizaki T
Publication year - 1994
Publication title -
british journal of clinical pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.216
H-Index - 146
eISSN - 1365-2125
pISSN - 0306-5251
DOI - 10.1111/j.1365-2125.1994.tb04269.x
Subject(s) - mephenytoin , imipramine , demethylation , hydroxylation , chemistry , microsome , metabolism , enzyme , microsoma , cytochrome p450 , biochemistry , medicine , gene expression , alternative medicine , pathology , cyp2c19 , dna methylation , gene
1. The metabolism of imipramine (N‐demethylation and 2‐hydroxylation) was studied in relation to the activity of S‐mephenytoin 4'‐hydroxylase in human liver microsomes. 2. Eadie‐Hofstee plots for the formation of despiramine and 2‐hydroxyimipramine were biphasic, suggesting that at least two enzymes are involved in both the N‐demethylation and 2‐ hydroxylation of imipramine by human liver microsomes. 3. The respective mean (+/‐ s.d.) kinetic parameters for the N‐demethylation and 2‐hydroxylation of imipramine derived from a two‐enzyme kinetic analysis were: Km1 = 1.1 +/‐ 0.4 and 1.6 +/‐ 0.6 microM, Vmax1 = 0.11 +/‐ 0.03 and 0.15 +/‐ 0.07 nmol mg‐1 min‐1, and Vmax1/Km1 = 0.10 +/‐ 0.02 and 0.09 +/‐ 0.04 ml mg‐1 min‐1; Km2 = 214 +/‐ 84 and 257 +/‐ 148 microM, Vmax2 = 2.22 +/‐ 0.69 and 0.53 +/‐ 0.15 nmol mg‐1 min‐1, and Vmax2/Km2 = 0.011 +/‐ 0.001 and 0.003 +/‐ 0.002 ml mg‐1 min‐1. 4. With regard to imipramine N‐demethylation and 2‐hydroxylation at 2 microM (representing high‐affinity reactions) and at 400 microM (representing low‐affinity reactions), only N‐demethylation at 2 microM showed a close correlation with the 4'‐hydroxylation of S‐mephenytoin (rs = 0.952, P < 0.01; n = 10 livers). 5. Concentrations up to 250 microM S‐ mephenytoin inhibited the N‐demethylation of imipramine (2 microM), but no further inhibition was observed using concentrations from 250 to 750 microM. 6. Imipramine inhibited S‐mephenytoin 4'‐hydroxylation competitively with a Ki value of 12.5 microM.(ABSTRACT TRUNCATED AT 250 WORDS)