Characterisation of theophylline metabolism by human liver microsomes. Inhibition and immunochemical studies.
Anti-human NADPH-cytochrome P-450 reductase inhibited all theophylline metabolic pathways confirming the involvement of cytochrome P-450 isozymes in the metabolism of theophylline. Tolbutamide, debrisoquine, mephenytoin, theobromine, phenylbutazone, sulphaphenazole and sulphinpyrazone did not inhibit theophylline metabolism by human liver microsomes. Verapamil and dextropropoxyphene were non-selective competitive inhibitors of theophylline metabolism. Cimetidine and caffeine selectively inhibited the two demethylations as Ki values for these two pathways were lower than for the 8-hydroxylation pathway. The effects of nifedipine, propranolol and alpha-naphthoflavone were atypical. The degree of inhibition by propranolol reached a plateau, which was greater for the two demethylations than for the 8-hydroxylation. Alpha-naphthoflavone (ANF) at low concentrations inhibited the demethylations to a greater extent than the 8-hydroxylation. At higher concentrations ANF activated all pathways, with this effect being most marked for the 8-hydroxylation. Nifedipine inhibited the theophylline demethylations but not the 8-hydroxylation. In some livers the 8-hydroxylation was markedly activated. The results confirm that there are at least two distinct cytochrome P-450 isozymes involved in theophylline metabolism, one isozyme being involved with the demethylations and a different isozyme involved in the 8-hydroxylation pathway. Preliminary correlation studies suggest that the human orthologue to the rabbit polycyclic hydrocarbon inducible P-450 Form 4 may be involved in the N-demethylations of theophylline.
Robson
R A
RA
Department of Clinical Pharmacology, Flinders Medical Centre, Bedford Park, South Australia.
Miners
J O
JO
Matthews
A P
AP
Stupans
I
I
Meller
D
D
McManus
M E
ME
Birkett
D J
DJ
D000328:Adult; D001571:Benzoflavones / Q000494:pharmacology; D002110:Caffeine / Q000494:pharmacology; D002927:Cimetidine / Q000494:pharmacology; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D011431:Dextropropoxyphene / Q000494:pharmacology; D006801:Humans; D007158:Immunologic Techniques; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000378:metabolism; D007700:Kinetics; D008862:Microsomes, Liver / Q000378:metabolism*; D008875:Middle Aged; D006899:Mixed Function Oxygenases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D013806:Theophylline / Q000378:metabolism*; D014700:Verapamil / Q000494:pharmacology
D001571:Benzoflavones; D065607:Cytochrome P-450 Enzyme Inhibitors; D007527:Isoenzymes; D002110:Caffeine; C011512:alpha-naphthoflavone; D002927:Cimetidine; D003577:Cytochrome P-450 Enzyme System; D013806:Theophylline; D014700:Verapamil; D006899:Mixed Function Oxygenases; D011431:Dextropropoxyphene