Enzymatic detection of L-isoaspartyl residues in food proteins and the protective properties of trehalose

Protein L-isoaspartate (D-aspartate) O-methyltransferase (PCMT) is a ubiquitous enzyme with substrate specificity toward proteins and peptides containing altered aspartyl residues. These are generated from the spontaneous deamidation of normal L-asparaginyl residues and/or isomerization of L-aspartyl residues. Such degradation reactions occur both in vivo and in purified protein molecules. Because of its peculiar substrate specificity, PCMT has been proposed as a tool for identifying and characterizing the L-isoaspartyl-containing byproducts of natural and recombinant proteins. Data presented here show the usefulness of PCMT for the identification of these altered residues in food proteins and the occurrence of these at detectable levels in several proteins from various animal sources. In addition, we noted that ovalbumin, either as a purified protein or in the whole chicken egg, becomes a better substrate for this enzyme during prolonged storage, thus indicating a progressive accumulation of the altered amino acid residues over time. The protective effects of trehalose against the occurrence of such specific protein damage in ovalbumin stored for several days were also assessed.

Protein L-isoaspartate (D-aspartate) O-methyltransferase (PCMT) is a ubiquitous enzyme with substrate specificity toward proteins and peptides containing altered aspartyl residues. These are generated from the spontaneous deamidation of normal L-asparaginyl residues and/or isomerization of L-aspartyl residues. Such degradation reactions occur both in vivo and in purified protein molecules. Because of its peculiar substrate specificity, PCMT has been proposed as a tool for identifying and characterizing the L-isoaspartyl-containing byproducts of natural and recombinant proteins. Data presented here show the usefulness of PCMT for the identification of these altered residues in food proteins and the occurence of these at detectable levels in several proteins from various various animal sources. In addition, we noted that ovalbumin, either as a purified protein or in the whole chicken egg, becomes a better substrate for this enzyme during prolonged storage, thus indicating a progressive accumulation of the altered amino acid residues over time. The protective effects of trehalose against the occurence of such specific protein damage in ovalbumin stored for several days were also assessed.