EFFECT OF ADJACENT HISTIDINE AND CYSTEINE RESIDUES ON THE SPONTANEOUSDEGRADATION OF ASPARAGINYL-CONTAINING AND ASPARTYL-CONTAINING PEPTIDES

Aspartate and asparagine residues in polypeptides are subject to nonen zymatic reactions that lead to deamidation, isomerization, peptide bon d cleavage and racemization. Much of this reactivity is due to the pro pensity for the initial formation of a cyclic succinimide intermediate . We have been interested in determining the effect of the side chains of neighboring histidine and cysteine residues in facilitating these reactions, particularly in the possibility that they can act as genera l acids and bases. In this study, we found little or no effect of hist idine residues preceding an asparagine residue in hexapeptides derived from the sequence of adrenocorticotropic hormone, while a histidine r esidue preceding an aspartic acid residue was found to increase the ra te of succinimide formation 8- to 11-fold. The presence of a histidine residue following either an asparagine or aspartic acid residue did n ot effect the rate of succinimide formation by peptide-bond nitrogen a ttack, but did increase the rate of the competing side-chain nitrogen attack leading to cleavage in the asparaginyl-containing peptide. We f ound that the effect of a cysteine residue following an asparagine or aspartic acid residue was in general similar to that of a serine resid ue, although the cleavage reaction appeared to be enhanced. These resu lts suggest that His-Asp sequences may be particularly labile to spont aneous degradation in proteins and peptides, possibly owing to the abi lity of the histidine residue to facilitate succinimide formation by p rotonating the OH- leaving group on the side chain carboxylic acid of the aspartic acid residue. Finally, we have also utilized these result s, along with previously accumulated data on succinimide formation in related peptides, to correlate the rate of succinimide formation with the predicted acidity of the peptide bond nitrogen atom that is involv ed in the initial nucleophilic attack. (C) Munksgaard 1995.