Distribution of γ-carboxyglutamic acid in calcified tissues

γ-Carboxyglutamic acid, previously identified in the vertebrate mineralized tissues of bone and dentin, is not detectable in the calcified skeletons of six invertabrate species representing five phyla. Its absence in all analyzed invertebrate tissues (including calcitic, aragonitic, and apatitic mineral phases) indicates that matrix protein-bound γ-carboxyglutamic acid is not obligatory for the calcification process in the invertebrates. Further, these data raise the possibility that invertebrates as a group may lack the enzymatic capability for biosynthesizing γ-carboxyglutamic acid. In contrast, the distribution of γ-carboxyglutamic acid in the vertebrates has been further extended by this study to include an aptitic shark tooth and an aragonitic fish otolith. No γ-carnoxyglutamic acid was detected, however, in the organic matrix of the calcitic hen egg shell.