Plasma membrane as target of alkylating agents

N mustard resistant Walker cells exhibit the same frequency of DNA interstrand cross-links and the same rate of cross-link removal as the sensitive parental line. Employing cytostatically active concentrations of chlorambucil covalently bound to polyethyleneimine, the extent of DNA cross-linking is reduced to levels observed in the presence of nontoxic concentrations of free chlorambucil. It is concluded, therefore, that DNA cross-links alone are not sufficient to explain the inhibition of cell multiplication by alkylating agents and that additional mechanisms have to be considered. Evidence for an interference of alkylating agents with several enzymes of the plasma membrane is presented. An inhibition by N mustard of the furosemide-sensitive Na+/K+/Cl- -cotransport and the Na+/H+-antiport is described in greater detail. Considering the fact that the enzymes which are affected by alkylating agents are controlled by growth factors it was investigated whether a synergism between inhibitors of early growth-factor-controlled reactions and alkylating agents is to be seen. It is demonstrated that mepacrine, an inhibitor of phospholipase C, and the calmodulin binding drugs, chlorpromazine and flunarizine, amplify the action of N mustard.