N-alkylation of the haem moiety of cytochrome P-450 caused by substituted dihydropyridines. Preferential attack of different pyrrole nitrogen atoms after induction of various cytochrome P-450 isoenzymes

Abstract

1. 3,5-Diethoxycarbonyl-4-ethyl-1,4-dihydro-2,6-dimethylpyridine (4-ethyl-DDC) gives rise to N-ethylprotoporphyrin in the liver of rats by donating its 4-ethyl group to one of the pyrrole nitrogen atoms of haem. Four structural isomers are obtained, depending on which pyrrole nitrogen is alkylated. 2. When rats are pretreated with an inducer of cytochrome P-450, the production of N-ethylprotoporphyrin caused by 4-ethyl-DDC is greater, both in the whole animal and in hepatocytes incubated with the drug in vitro. 3. Pre-incubation of hepatocytes with 2-allyl-2-isopropylacetamide decreases the yield of N-ethylprotoporphyrin due to 4-ethyl-DDC, an effect largely reversed by adding exogenous haem. 4. The isomeric composition of N-ethylprotoporphyrin produced in vivo and in vitro depends on the cytochrome P-450 isoenzyme that predominates at the time of treatment, suggesting a role for the apo-cytochrome in directing alkylation on to one of the pyrrole nitrogens.

Differential haemin-mediated restoration of cytochrome P-450 N-demethylases after inactivation by allylisopropylacetamide

“…Such an event is consistent with the concept that the unique spatial orientation of each apoprotein moiety around its own prosthetic haem (i.e. the haem environment) critically determines the regiospecific alkylation of each cytochrome P-450 haem moiety (Kunze et al, 1983;De Matteis et al, 1983). Accordingly, for any given suicide substrate, the regiospecificity of haem alkylation is found to vary with individual cytochrome P-450 isoenzymes (De Matteis et al, 1983), even though their haem moieties are structurally identical.…”

“…the haem environment) critically determines the regiospecific alkylation of each cytochrome P-450 haem moiety (Kunze et al, 1983;De Matteis et al, 1983). Accordingly, for any given suicide substrate, the regiospecificity of haem alkylation is found to vary with individual cytochrome P-450 isoenzymes (De Matteis et al, 1983), even though their haem moieties are structurally identical. Thus, given the particular haem environment of each cytochrome P-450 isoenzyme, it is quite conceivable that some isoenzymes might sustain haem alkylation, whereas others (whose protein moieties protect otherwise-susceptible haem target sites) might incur alkylation of their apoprotein moiety and be irreparably damaged.…”

Synthesis of N-(formylmethyl)protoporphyrin and N-(2-hydroxyethyl)protoporphyrin IX dimethyl esters

“…Reduction of N-(CH2CHO)-PPIXDMEZn(II)Cl (2) to JV-(CH2CH2OH)-PPIXDMEZn(II)Cl (4). LiAlH(0-f-C4H9)3 (34 mg) was added to a solution of ./V-(formylmethyl)-PPIXDMEZn(II)Cl (2) (34 mg) in tetrahydrofuran (20 mL) under argon and the reaction mixture was stirred at 0 °C for 2 h. Saturated aqueous NH4C1 solution was added and the porphyrin was extracted with chloroform.…”