Protein carboxyl methyltransferase facilitates conversion of atypical L-isoaspartyl peptides to normal L-aspartyl peptides.

Prolonged incubation of L-isoaspartate-containing forms of lactate dehydrogenase (231-242), sperm activating peptide, and adrenocorticotropin (22-27) at 37 degrees C, pH 7.4, with S-adenosyl-L-methionine and protein carboxyl methyltransferase from bovine brain leads to extensive conversion of the atypical isopeptide bond to a normal peptide bond. For the lactate dehydrogenase-related peptide, conversion was 80% complete after 24 h. For the other two peptides, conversion reached a level of approximately 65% after 48 h. The mechanism of conversion involves (i) rapid enzymatic methylation of the alpha-carboxyl of the L-iso-Asp residue; (ii) nonenzymatic demethylation resulting in formation of an L-aspartyl cyclic imide; and (iii) a slow, nonenzymatic hydrolysis of the cyclic imide to form a mixture of 15-25% normal L-Asp peptide and 75-85% L-iso-Asp peptide. The regenerated L-iso-Asp peptide is remethylated and the cycle is repeated. The extent of conversion is limited by a competing side reaction wherein the L-imide slowly racemizes, leading to the formation of mainly D-iso-Asp peptide, which is not a substrate for the methyltransferase. The ability of protein carboxyl methyltransferase to initiate conversion of L-iso-Asp residues to normal L-Asp suggests a possible role for this enzyme in facilitating the repair or degradation of deamidated proteins in vivo.