V79 (Chinese hamster lung fibroblast) cell lines expressing a functional recombinant phenobarbital-inducible rat liver UDP-glucuronosyltransferase (UGT) i.e., UGT2B1 were established. Western blot analysis of positive colonies, using anti-rat liver UGT antibodies, revealed the presence of an immunoreactive polypeptide of the expected molecular mass of 52 kDa. The substrate specificity of the recombinant enzyme toward >100 compounds was determined. Phenolic and alcoholic substrates included 4-methylumbelliferone, 4-hydroxybiphenyl, chloramphenicol, and testosterone, but a range of carboxylic acids of both endogenous (medium-chain saturated fatty acids, long-chain polyunsaturated fatty acids, and bile acids) and exogenous (profen nonsteroidal anti-inflammatory drugs, fibrate hypolipidemic agents, and sodium valproate) origin were also accepted, indicating that the enzyme was capable of forming both ether- and ester-type glucuronides from various structurally unrelated compounds. Determination of apparent kinetic constants for the glucuronidation by UGT2B1 of selected aglycones revealed a high maximal velocity toward the 3-position of morphine (49.3 ± 2.2 nmol/min/mg of protein), compared with other known substrates such as 4- methylumbelliferone (2.67 ± 0.11 nmol/min/mg of protein) or clofibric acid (0.06 ± 0.02 nmol/min/mg of protein). To gain a better insight into the mechanisms underlying the apparently wide substrate specificity of UGT2B1, series of structurally related compounds were tested as potential substrates. The rate of glucuronidation of unbranched saturated fatty acids and ω,ω,ω- triphenylalkanoic acids increased progressively with increasing alkyl chain length and then declined, with the best substrates in these two homologous series being decanoic acid and 4,4,4-triphenylbutanoic acid, respectively. Glucuronidation of para-substituted phenols always proceeded at a higher rate than that of the corresponding para-substituted benzoic acids. This could mean that the aglycon hydroxyl group was better positioned in the enzyme active site in the case of phenols. Alternatively, if the initial interaction with the enzyme required the aglycon to be in the protonated uncharged form, then the observation could be explained by the difference in ionization between phenols and benzoic acids at the incubation pH used. The introduction of a bulky alkyl group into the para-position led to increases of up to 300- fold in the rate of glucuronidation, probably as a result of the increased aglycon lipophilicity. Finally, the enzyme showed a degree of stereo- and regiospecificity, preferring (S)-ibuprofen to the R-enantiomer (V(max)/K(m), 3.06 and 1.10 μl/min/mg of protein, respectively) and glucuronidating lithocholic acid but not hyodeoxycholic acid, which differs by only a single hydroxyl group. Given the wide substrate specificity of the recombinant enzyme, these transfected V79 cell lines may represent a useful in vitro model for investigating the molecular mechanisms of drug glucuronidation.
|Number of pages||9|
|Publication status||Published - 1 Jan 1994|