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Prior to the emergence of life, it is believed that only L-amino acids were selected for formation of proteins, and that D-amino acids were eliminated on the primitive Earth. Whilst homochirality is essential for life, recently the occurrence of proteins containing D-beta-aspartyl (Asp) residues from various tissues of elderly subjects has been reported. Here, we discuss the presence of D-beta-Asp-containing proteins in the lens, ciliary body, drusen, and sclera of the eye, skin, cardiac muscle, blood vessels of the lung, chief cells of the stomach, longitudinal and circular muscles of the stomach, and small and large intestines. Since the D-beta-Asp residue occurs through a succinimide intermediate, this isomer may potentially be generated in proteins more easily than initially thought. UV Rays and oxidative stress can accelerate the formation of the D-beta-Asp residue in proteins.

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