As an initial step in the analysis of bone Gla protein (BGP; osteocalcin) function in lower vertebrates, we have developed a simple and rapid method for the isolation of BGP from bone and have applied this to the isolation of BGP from the African clawed toad Xenopus laevis and the fish Sparus aurata. We have also determined the complete amino-acid sequence of Sparus and Xenopus BGP, including the identification of the sites of y-carboxylation. Since the addition of Xenopus and Sparus BGP sequences significantly extends the range of species whose BGP structures are known, we have compared the 18 presently known BGP sequences. Twelve amino acids are invariant in these 18 BGP sequences and are therefore presumably critical to BGP conformation or function. Eight of these 12 invariant amino acids are also invariant in all presently known matrix Gla protein sequences (shark, mouse, rat, cow, human), an observation which strongly supports the evolutionary relationship between these two vitamin K-dependent bone proteins and suggests that the proteins may adapt similar tertiary structures.

http://www.ncbi.nlm.nih.gov/pubmed/8567186?dopt=Abstract