Spontaneous alterations in the covalent structure of synapsin I during in vitro aging.

Synapsin I purified from bovine brain was incubated for 30 days at pH 7.4 and 37 degrees C. Samples were taken at various times and assayed for isoaspartate content using protein-L-isoaspartyl methyltransferase. During the first 22 days, synapsin accumulated isoaspartyl sites at a rate of > or = 6 sites per day per 100 molecules of synapsin. Concomitant with isoaspartate formation, synapsin underwent two other types of modification: a substantial degree of spontaneous intermolecular cross-linking via the formation of disulfide bonds, and a second, less pronounced, irreversible aggregation. The irreversible aggregation apparently results from covalent cross-linking of a non-disulfide nature or possibly a strong hydrophobic interaction. Isoaspartate accumulated in both aggregated and non-aggregated forms of synapsin during in vitro aging. These findings demonstrate that synapsin is capable of significant spontaneous covalent alteration under physiological conditions. These modifications may play a role in the function of, or limit the lifetime of, synapsin in vivo.