Simultaneous Stereoinversion and Isomerization at Specific Aspartic Acid Residues in αA-Crystallin from Human Lens

Abstract

We characterized the primary structure of αA-crystallin from the lens of the human eye by detailed analyses of the amino acid sequence, mass, and stereoisomers, and found the biologically uncommon o- and β-aspartic acid (Asp) residues in the protein. The stereocon-figuration of the Asp¹⁵¹ and Asp⁵⁸ residues in αA-crystallin from old subjects (mean age: 80 years) was inverted to the D-isomer, and the residues were simultaneously isomerized to β-aspartyl residues, which may occur via a succinimide intermediate. This is thought to be the first observation of stereoinversion of amino acids in protein in vivo. It is noteworthy that similar stereoinversion was observed in the same residues of αA-crystallin from young subjects (age: 11 months), with simultaneous isomerization, although the extent of isomerization was low compared with that in the aged. The conversion may take place in the early life of the lens and the resulting isomers may accumulate with aging. We also found that terminal serine¹⁷³ was cleaved in aged αA-crystallin. Since the cleavage was not observed in young αA-crystallin, it may result from the in vivo aging process. The present findings also make it necessary to revise the amino acid sequence of human αA-crystallin presented previously: human αA-crystallin is composed of 173 (formerly reported as 172) amino acid residues, and the sequence from residues 153 to 155 is THA (formerly reported as -HT).