Evolutionary considerations of D-aspartyl endopeptidases and its substrates.

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D-isomer of Asp (D-Asp) residue is often detected in abnormally aggregated proteins. Therefore it is strongly suggested that formation of D-Asp in proteins is potentially noxious for metabolic turnover. The D-aspartyl endopeptidase (DAEP), which we have discovered from mammals, stereoselectively degrades its substrate at the internal D-Asp residue, and seems to physiologically serve as a kind of quality-control system to maintain the protein homeostasis. On the other hand, a distribution of DAEP in living things is not clear. In African clawed frog (Xenopus laevis), high DAEP activity was detectable in its testes, ovaries and unfertilized eggs, though the frog naturally thrives around 20ºC at the most. In order to elucidate physiological functions of DAEP, we therefore examined comparison of properties of DAEPs, purified from aquatic animals: African clawed frogs and Japanese green sea urchins, with mammalian DAEP.