The distribution of soluble, mineral-bound, and matrix-bound proteins in osteoporotic and normal bones

Abstract

Very little is known about the composition of noncollagenous protein (NCP) in osteoporotic bone. We have obtained osteoporotic (OP) cancellous bone samples from 2–4 cm below the surface of human femoral heads and compared the protein profiles to age-matched (old controls, OC), as well as young autopsy controls (YC). The proteins were analyzed using a serial extraction method which liberates proteins from distinct pools; first, proteins were extracted in 4.0 M GuHCl (G1), followed by the extraction of mineral-bound protein with 0.5 M EDTA (E), and finally the protein previously masked by the mineral was released by a second 4.0 M GuHCl extraction (G2). The tissue residue was digested with bacterial collagenase liberating proteins bound to the collagenous matrix. Uniform bone preparations of finely ground bone powder were used for our study. The protein content of the four groups was determined together with the protein distribution using amino acid analysis and gel-electrophoresis. Our data indicate that the noncollagenous protein content is considerably reduced in osteoporotic bone when compared with age-matched or young controls, whereas the collagen content is unaffected. When bone proteins were investigated using gel-electrophoresis we found a similar distribution of proteins in the G1 and E fractions of all three groups, but the osteoporotic bone samples, using densitometric analysis of the gels, showed quantitative differences in a number of the proteins. The greatest differences were observed in the G2 extract, where the NCP of osteoporotic bone was limited to a few low-molecular-weight bands when compared with the OCs or YCs. With the use of Western immunoblot analysis, we have identified and confirmed the presence of several bone proteins present in the above extracts. Electrophoretic analysis of the collagenase digest showed that adult bone has very little NCP bound to the collagenous matrix. Collagen quantity was unchanged by age or disease but its extractability decreased. Our study supports the concept that the disease of osteoporosis involves complex changes in the organic matrix of bone.