N-GLUCOSYLATION OF 5,5-DISUBSTITUTED BARBITURATES BY MOUSE-LIVER MICROSOMES

The structure-substrate relationships of the imide N-glucosylation pathway were investigated in mouse liver microsomes by testing 5,5-disubstituted barbiturates (amobarbital, aprobarbital, barbital, barbituric acid, butabarbital, butalbital, pentobarbital, and secobarbital) for their ability to inhibit phenobarbital N-glucosylation. After inhibition of phenobarbital N-glucosylation was demonstrated, evidence for formation of new barbiturate N-glucosides was obtained. Aprobarbital and secobarbital N-glucosides were synthesized to verify their formation by mouse liver microsomes. The apparent K-m and V-max values for formation of amobarbital, aprobarbital, (1'R,S)-secobarbital, (1'R,S)-, (1'R)-, and (1'S)-pentobarbital N-glucoside conjugates were obtained. Increasingly lipophilic barbiturates exhibited increasing inhibition of phenobarbital N-glucosylation and this was associated with a lower K-m for N-glucosylation of the respective barbiturate. Changing the C-5 ethyl to a C-5 allyl had no effect on the efficiency of the conjugation reaction (V-max/K-m). Product enantioselectivity was observed for all of the barbiturate N-glucosides for which separation of the diastereomers was achieved. Approximately a two-fold substrate enantioselectivity for N-glucosylation was observed for (R)-pentobarbital over (S)-pentobarbital and the product enantioselectivity was different for each isomer. The formation of N-glucoside conjugates appears to be a general enzymatic pathway for the biotransformation of 5,5-disubstituted barbiturates in mouse liver microsomes.