Studies on the procedure to measure accurately the binding properties of benzo[a]pyrene to cytochrome P-450/P-448

where [E]/[E], is fractional enzyme activity, and t is reaction time. The constants C and k were determined graphically (Defares & Sneddon, 1960). The results of the effect of diethyl pyrocarbonate on DEAE-cellulose column-chromatography fractions obtained from two samples of normal gastric mucosa and three samples of stomach adenocarcinoma are presented in summary form in Table I. It will be seen from Table 1 that, in some cases, the value for C, is negative and also is greater than unity. In this form, eqn. (1) describes a process of enzyme activation followed by inactivation. The maximum proteinase activation by diethyl pyrocarbonate was 40&500% of initial proteinase activity. It will also be seen that the peak containing the diethyl pyrocarbonate-activated enzyme appeared, if at all, at around 580ml of emuent produced from the column. In experiments with enzyme from normal human mucosa it was found that fractions that could be activated with diethyl pyrocarbonate could also be activated by exposure to pH3.5 (T. B. Malliopoulou & E. T. Rakitzis, unpublished work). Proteolytic activity at pH 3.5 may be due to pepsin I1 produced from pepsinogen I1 (Becker & Rapp, 1979), and it is therefore probable that the activation effect produced by diethyl pyrocarbonate on fractions obtained from normal human gastric mucosa and stomach carcinoma is due to an effect of this compound on pepsinogen 11.