New protein cross-linking reagents that are cleaved by mild acid

New homo- and heterobifunctional cross-linking reagents have been synthesized. These reagents are based on ortho ester, acetal, and ketal functionalities that undergo acid-catalyzed dissociation but are base stable. The protein-reactive group in all the homobifunctional reagents is a maleimide group; the heterobifunctional acetal cross-linker has a maleimide group at one end and an N-hydroxysuccinimide ester at the other. These reagents have been used to cross-link diphtheria toxin (DT) to itself to give covalently cross-linked DT dimer or to conjugate DT monomer to the anti-CD5 antibody, T101. The hydrolysis of these cross-linked proteins was studied as a function of pH. Cleavage rates vary from minutes to hours at the pH of acidified cellular vesicles (approximately pH 5.4), ortho esters being the fastest, acetals the slowest, and ketals intermediate, but the cross-linked products are approximately 100 times more stable at the vascular pH of 7.4 and 1000 times more stable at a storage pH of 8.4 in all cases. The utility of these reagents in the reversible blockade of a toxic protein functional domain was demonstrated by using cross-linked DT dimer where the blocking and unblocking of toxin binding sites correlates with cellular toxicity. Of the different cross-linkers described, the acetone ketal, bis(maleimidoethoxy)propane (BMEP), appears to be the most promising in the construction of highly efficacious immunotoxins.