Neutral lipids facilitate transfer of bone morphogenetic proteins and other noncollagenous proteins

In the process of separation of bone morphogenetic proteins from bone matrix, lipids were found in unexpected amounts closely associated with noncollagenous proteins soluble in guanidine hydrochloride. Lipids representing 33.7–49.9% by weight were recovered with various solvents. Composites of noncollagenous proteins and lipids soluble in either chloroform- methanol or acetone implanted in the hindquarter muscles of mice induced the formation of large deposits of heterotopic bone. The protein-lipid aggregates formed microspherules which were stained by Sudan Black B. Implants of bone morphogenetic proteins and noncollagenous proteins-lipid microspherules stained with Sudan Black B induced bone development in the same manner as unstained delipidized bone morphogenetic proteins associated with noncollagenous proteins. Lipid-free osteocalcein, osteonectin, albumin and other bone matrix proteins did not induce bone formation or bind Sudan Black B. The more highly purified the noncollagenous proteins, with or without activity of bone morphogenetic proteins, the lower the level of binding with Sudan Black B. Acetone-soluble bone matrix lipids consisted chiefly of triglycerides, cholesterol and saturated short chain fatty acids, and included little or no phospholipids or monounsaturated fatty acids. Composites of recombinant bone morphogenetic proteins-2 and acetone-soluble lipids induced larger deposits of bone than implants of recombinant bone morphogenetic proteins-2 without acetone-soluble lipids. The hypothesis that an association of bone lipids with protein facilitates the local transport of bone morphogenetic proteins warrants further investigation.