Distribution of gamma-carboxyglutamic acid in calcified tissues.

gamma-Carboxyglutamic acid, previously identified in the vertebrate mineralized tissues of bone and dentin, is not detectable in the calcified skeletons of six invertebrate species representing five phyla. Its absence in all analyzed invertebrate tissues (including calcitic, aragonitic, and apatitic mineral phases) indicates that matrix protein-bound gamma-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 gamma-carboxyglutamic acid. In contrast, the distribution of gamma-carboxyglutamic acid in the vertebrates has been further extended by this study to include an apatitic shark tooth and an aragonitic fish otolith. No gamma-carboxyglutamic acid was detected, however, in the organic matrix of the calcitic hen egg shell.