N-Glucuronidation of some 4-arylalkyl-1H-imidazoles by rat, dog, and human liver microsomes.
N-Glucuronidation in vitro of six 4-arylalkyl-1H-imidazoles (both enantiomers of medetomidine, detomidine, atipamezole, and two other closely related compounds) by rat, dog, and human liver microsomes and by four expressed human UDP-glucuronosyltransferase isoenzymes was studied. Human liver microsomes formed N-glucuronides of 4-arylalkyl-1H-imidazoles with high activity, with apparent V(max) values ranging from 0.59 to 1.89 nmol/min/mg of protein. In comparison, apparent V(max) values for two model compounds forming the N-glucuronides 4-aminobiphenyl and amitriptyline were 5.07 and 0.56 nmol/min/mg of protein, respectively. Atipamezole showed an exceptionally low apparent K(m) value of 4.0 microM and a high specificity constant (V(max)/K(m)) of 256 compared with 4-aminobiphenyl (K(m), 265 microM; V(max)/K(m), 19) and amitriptyline (K(m), 728 microM; V(max)/K(m), 0.8). N-Glucuronidation of medetomidine was highly enantioselective in human liver microsomes; levomedetomidine exhibited a 60-fold V(max)/K(m) value compared with dexmedetomidine. Furthermore, two isomeric imidazole N-glucuronides were formed from dexmedetomidine, but only one was formed from levomedetomidine. Dog liver microsomes formed N-glucuronides of 4-arylalkyl-1H-imidazoles at a low rate and affinity, with apparent V(max) values ranging from 0.29 to 0.73 nmol/min/mg of protein and apparent K(m) values from 279 to 1640 microM. Rat liver microsomes glucuronidated these compounds at a barely detectable rate. Four expressed human UDP-glucuronosyltransferase isoenzymes (UGT1A3, UGT1A4, UGT1A6, and UGT1A9) were studied for 4-arylalkyl-1H-imidazole-conjugating activity. Only UGT1A4 glucuronidated these compounds at an activity of about 5% of that measured for 4-aminobiphenyl. The observed activity of UGT1A4 does not explain the high efficiency of glucuronidation of 4-arylalkyl-1H-imidazoles in human liver microsomes.
Kaivosaari
Sanna
S
Department of Pharmacy, Division of Pharmaceutical Chemistry, University of Helsinki, Finland.
Salonen
Jarmo S
JS
Taskinen
Jyrki
J
D000818:Animals; D002851:Chromatography, High Pressure Liquid; D003902:Detergents / Q000494:pharmacology; D004285:Dogs; D004789:Enzyme Activation / Q000187:drug effects; D014453:Glucuronosyltransferase / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007093:Imidazoles / Q000378:metabolism* / Q000493:pharmacokinetics; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000235:genetics / Q000378:metabolism; D007700:Kinetics; D008297:Male; D020926:Medetomidine / Q000378:metabolism / Q000493:pharmacokinetics; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism*; D017830:Octoxynol / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D013045:Species Specificity; D013237:Stereoisomerism; D013329:Structure-Activity Relationship
D003902:Detergents; D007093:Imidazoles; D007527:Isoenzymes; C050701:atipamezole; C041255:detomidine; D017830:Octoxynol; D014453:Glucuronosyltransferase; D020926:Medetomidine