Xba I-digested pcDNA expression vector. Protein content was measured according to the method of Lowry et al. The mixtures were centrifuged at 12,000 g for 10 min. The mixture was incubated and UGT1A6 activity was determined as described above, except that the reaction was initiated by the addition of the naphthol-troglitazone mixture. The mode of troglitazone inhibition of UGT1A6 activity was determined from the Lineweaver-Burk plots, and the K i value was derived from replots of slope versus inhibitor concentration.
All data shown are the means of duplicate determinations.
When discussing drug inhibition of UGT, it is important to consider the concentration of the drug in the vicinity of the enzyme. Generally, that concentration is approximated by the concentration in the liver. When, as a result of inherently reduced UGT1A6 activity, the plasma concentration of a drug approaches a threshold of toxicity, co-administration of troglitazone might carry that concentration into the toxic range. We would like to thank M. Suzaki of Central Research Laboratory, Shiga University of Medical Science, for his technical assistance.