Newer knowledge of non-collagenous protein in dentin and cortical bone matrix

The current state of knowledge of the composition of the NCM components of bone and dentin has been summarized at the end of the appropriate sections. It is significant that increasing interest in the chemistry of hard tissue matrices has coincided with the development and refinement of a wide range of separation techniques, resulting in the isolation of an unexpectedly large number of components. The most sophisticated techniques, such as iso-electric focusing and isotachophoresis, give rise to discrete fractions often of very similar composition, particularly in terms of amino acid content. Such components might best be considered in groups, especially should such groups be identified in terms of common immunochemical properties. Dickson has used such an approach in a recent study of the proteins of sheep cortical bone. The Liverpool group has approached the problem on a broad front and thus directed attention to the number and diversity of NCM components, but it will be seen that those studies directed to the isolation of a specific component have invariably revealed the presence of several other fractions, set aside while attention was directed to the component under investigation. It is clear that the major proportion of bone and dentin NCM consists of glycoproteins of the less-acidic and anionic types. Several of the anionic components contain phosphate, levels being higher in those derived from dentin. Glycosaminoglycans, the first class of non-collagenous compounds to be identified in hard tissue matrices, are now known to comprise only about 5-7 per cent of the NCM. Precise details of the glycosaminoglycan fraction of human dentin and considerable information concerning that of bovine bone are now available. The major component in each tissue is chondroitin-4-sulfate, which exists in the form of proteoglycan, the protein moieties of those from bovine bone and human dentin being very different. Although greater interest is currently being shown in the glycoprotein fractions, several studies have been made of the calcium-binding properties of proteoglycan preparations and of individual glycosaminoglycans. Attempts to relate differences in chemical composition and properties to specific bone sites may not prove to be the best approach to the study of the precise chemistry of mineralization. It is clear, from the earlier work of Lindenbaum and Kuettner that mineralization takes place in a very narrow layer which will normally represent only a fraction of a typical zone prepared for analysis...