Interaction of disopyramide enantiomers for sites on plasma protein

The binding of disopyramide enantiomers to donor plasma to which had been added human alpha-1 acid glycoprotein (AAG) was characterized alone and in the presence of the opposite enantiomer. At pre-dialysis concentrations of 10(-5) M, S(+)-disopyramide increased the percent of R(-)-disopyramide free (unbound) 2.6-fold from 11 to 30%. At similar pre-dialysis concentrations, R(-)-disopyramide increased the percent of S(+)-disopyramide free 2-fold from 4.1 to 9.0%. Differences in the binding of one enantiomer due to the presence of the other were due to apparent changes in association constant; no changes in capacity to bind the enantiomers were observed. It is concluded that the enantiomers of disopyramide compete with each other for one site on AAG.