BIPHASIC KINETICS OF QUATERNARY AMMONIUM GLUCURONIDE FORMATION FROM AMITRIPTYLINE AND DIPHENHYDRAMINE IN HUMAN LIVER-MICROSOMES

The tricyclic antidepressant amitriptyline and the H-1-receptor antago nist diphenhydramine are conjugated in human liver microsomes fortifie d with UDP-glucuronic acid at their tertiary amino groups with the for mation of quaternary ammonium glucuronides. The kinetics of the reacti ons were found to be biphasic with apparent K-M1 and K-M2 values of 1. 4 mu M and 311 mu M for amitriptyline and 2.6 mu M and 1180 mu M for d iphenhydramine in four liver samples. V-max1 values varied between 2 a nd 17 pmol . mg protein(-1). min(-1) for the two substrates and V-max2 values between 80 and 740 pmol . mg protein(-1). min(-1). A close cor relation existed between amitriptyline and diphenhydramine glucuronida tion rates in microsomes from seven livers at concentrations correspon ding to 10-40% of K-M2 . At low concentrations, diphenhydramine compet itively inhibited the glucuronidation of amitriptyline. V-max/K-M valu es of the high-affinity UDP-glucuronosyltransferase(s) (UGTs) exceed t hose of the low-affinity enzyme(s) severalfold, such that the former s hould make the major contribution to N-glucuronidation of the drugs at therapeutic concentrations in vivo.