Fig. 2. Comparative rates of 4-hydroxypropofol formation in vitro by human liver microsomes derived from 34 different donors incubated in the presence of propofol and an NADPH-regenerating system. Representative livers that were selected for further study are indicated with an asterisk. Insets show the frequency distribution of the activities in this set of livers measured at 5 (upper ) and 50 (lower ) μm propofol.