Regulation of in vitro and in vivo CaCO3 crystallization by fractions of oyster shell organic matrix

Correlations between structural properties of bivalve shell organic matrix and its proposed functions in the regulation of biomineralization were examined using proteinaceous fractions obtained from the shell of the oyster Crassostrea virginica (Gmelin) following dissolution of the mineral with ethylenediaminetetraacetate (EDTA). Matrix isolated in this way contains a continuous size distribution of proteins ranging from relatively small molecular weight (M_r) soluble matrix (SM) components to insoluble matrix (IM) components. Regulation of mineralization by these components was determined primarily by their reduction of crystal growth rate in an in vitro assay. For all fractions tested, there was an inverse correlation between M_r and regulatory activity. However, matrix properties other than molecular size may be important in regulation of crystal growth in vivo in that the larger but less acidic of two soluble matrix proteins was a more effective inhibitor of spicule formation by sea urchin embryos. Base treatment of IM and high molecular weight SM fractions that had little or no inhibitory activity when untreated, resulted in constituents that had a molecular weight distribution and in vitro inhibitory activity in the same range as whole SM. These results, combined with the finding that a major fraction of IM has an amino acid composition very similar to the highly charged SM fractions, suggest that much of the matrix is made up of similar molecules, and that their function in crystal growth regulation may change as they interact to form units of increasing size. A second class of IM was isolated which contained dihydroxyphenylalanine (dopa) and a preponderance of hydrophobic amino acids. This material may represent the basic structural framework of the matrix.