Delta crystallin, the principal crystallin in embryonic lens, is found only in birds and reptiles. This protein also functions as an enzymatically active argininosuccinate lyase. 2-(N(omega)-L-arginino)succinate = fumarate + L-arginine Amino-acid biosynthesis; L-arginine biosynthesis; L-arginine from L-ornithine and carbamoyl phosphate: step 3/3. Homotetramer. Eye lens. Each of the four substrate-binding sites present in the homotetrameric assembly are shared between three of the four subunits. Belongs to the lyase 1 family. Argininosuccinate lyase subfamily.