Determination of beta-isomerized aspartic acid as the corresponding alcohol

The age-related formation of succinimides in proteins, through spontaneous deamidation of asparagine, and through cyclization of aspartic acid, is thought to be followed by the hydrolysis of the succinimide ring, yielding a mixture of "normal" aspartic acid sites and beta-isomerized aspartic acid sites (isoaspartic acid). The chemical reduction of an isoaspartyl site to the corresponding amino acid alcohol, isohomoserine, has now been investigated as a general approach to measuring the accumulation of isomerized residues in aging proteins. The methods employed were based on conditions previously found to be successful in reducing protein aspartic acid to homoserine. Borane was employed as the reducing agent, and was found to produce the expected amino acid alcohols in reactions with model peptides. In addition, amino acid analysis revealed a complex pattern of unknown products of these reduction reactions, some of which were also evident when a much stronger reducing agent, lithium aluminum hydride, was used. The correlation of some of these side-products with the isomerization of the peptide suggests, unexpectedly, that the reactivity of reducing agents toward aspartyl residues and perhaps other sites in the peptide may be influenced by steric factors related to aspartyl isomerization. The borane reduction method was also applied to proteins. No detectable isohomoserine was formed either in ovalbumin, a model aged protein, or in human lens proteins of advanced age, with conditions that fully reduced normal aspartyl residues to homoserine. These tests thus indicate that the percentage of aspartic acid in the isomerized form in these proteins is below the limit of detectability (below approximately 5%). These results complement previous experimental results that have indicated a low bulk isoaspartyl content in most natural proteins.