KINETIC-PARAMETERS OF LYMPHOCYTE MICROSOMAL EPOXIDE HYDROLASE IN CARBAMAZEPINE HYPERSENSITIVE PATIENTS - ASSESSMENT BY RADIOMETRIC HPLC

Idiosyncratic hypersensitivity reactions with carbamazepine have been postulated to be due to a deficiency of microsomal epoxide hydrolase (HYL1), although this is based on indirect evidence. Using H-3-cis stilbene oxide (0.5 Gi/mmol) as a substrate, we have developed a radiometric HPLC assay sensitive enough to measure the kinetic parameters of HYL1 in lymphocytes. The intra-assay coefficient of variation was 8%. Enzyme activity has been measured in lymphocytes from six carbamazepine hypersensitive patients, six patients on carbamazepine without any adverse effects, and twelve drug-naive healthy volunteers. No significant difference was observed in three kinetic parameters of the enzyme among these three groups. The values for K-m, V-max, and intrinsic clearance ranged from 6.1-89.9 mu M, 3.0-23.2 pmoles diol formed/min/mg protein, and 0.147-0.493 mu l/min/mg protein. There was no difference in enzyme activity between patients currently on carbamazepine and healthy volunteers, indicating a lack of induction of lymphocyte HYL1 by carbamazepine. Go-incubation of lymphocytes with 1,1,1-trichloropropene oxide, an inhibitor of hepatic HYL1, resulted in an 82% inhibition of activity, similar to that observed with the hepatic enzyme. The healthy volunteers were genotyped as being either GSTM1 positive (n = 6) or GSTM1 negative (n = 6). This did not affect the kinetic parameters of lymphocyte microsomal epoxide hydrolase. Our results suggest that there is normal HYL1 activity in lymphocytes of hypersensitive patients using cis-stilbene oxide as a substrate.