Trapping succinimides in aged polypeptides by chemical reduction.

Cyclization of aspartic acid and asparagine to succinimides is thought to be a common spontaneous aging reaction in proteins, but the instability of the succinimide ring has made it difficult to directly measure this structure. Chemical reduction has now been tested as a means of trapping succinimides as stable derivatives, homoserine and isohomoserine. Two succinimide-containing compounds were tested in this manner. First, polysuccinimide was reduced by sodium borohydride to a derivative that contained homoserine and isohomoserine in amounts that were consistent with the content of succinimide determined independently by quantitative hydrolysis. The identity of isohomoserine was confirmed by its resistance to degradation by L-amino acid oxidase, and through its synthesis by an alternate route involving borane reduction of asparagine. Second, in a test of this approach on a peptide mixture with only a trace-content of succinimide, isohomoserine and homoserine were formed as reduction products in amounts equivalent to the trace content of succinimide in the mixture. Detection of the products of the chemical reduction of polypeptides is therefore diagnostic of succinimides, and can be successfully applied at the trace sensitivity necessary for studies of naturally aging proteins. A related study of the reduction of aspartyl and beta-aspartyl residues to, respectively, homoserine and isohomoserine, is described in the accompanying manuscript (Carter and McFadden, 1994).