Fine powdering exposes the mineral-protected collagen of bone to protease digestion

The method of heat-denaturation and trypsin digestion was used to dissect bone biochemically into mineral-protected and mineral-unprotected pools of collagenous matrix. It was found that varying the particle size of the bone powder had a profound effect on the results. Using mature bovine cortical bone, the observed pool of "unmineralized" (mineral-unprotected) collagen could be varied from 2% to more than 60% of the total bone collagen simply by decreasing the particle size of the bone sample from greater than 1 mm to less than 38 micron. No major differences were seen in the contents of hydroxypyridinium cross-links between the collagens of the trypsin-soluble and trypsin-insoluble pools from the fine powders, contrary to earlier reports. A trend to a higher content of these cross-links was evident, however, in the very small collagen pool extracted from the coarsest bone particles. Similar extraction differences were noted using bacterial collagenase to probe for mineral-protected vs. mineral-unprotected domains of bone collagen. In summary, the biochemical dissection results appear largely to be an artifact of the powdering technique, the shear energies of which presumably destroy the intimate physical relationship between the mineral crystallites and the collagen fibrils at the fractured surfaces of the bone particles. As the fractured surface area increases with decreasing particle size so the fraction of protease degradable collagen increases. Since powdering is routinely adopted for many structural studies on both the mineral and organic phases of bone, the findings on finely powdered bone should be interpreted cautiously.