On the problem of covalent linkages between phosphoproteins and collagen in bovine dentin and bone

The majority of phosphoproteins in bovine bone and dentin are insoluble in EDTA and guanidine hydrochloride (Gu · HCl) at 2°C. After removal of EDTA and Gu · HCl-soluble proteins at 2°C, collagen α-chains and α-chain polymers were extracted from bovine bone and dentin in Gu · HCl at elevated temperatures and purified by several chromatographic techniques and SDS-PAGE. Small amounts of O-phosphoserine were found in all collagen components. In contrast, O-phosphoserine was not detected in the purified collagen components soluble in EDTA or Gu · HCl at 2°C nor was hydroxyproline detected in the EDTA-soluble phosphoproteins. In contrast, although the vast majority of EDTA-insoluble collagen and phosphoprotein molecules can be readily dissociated by a variety of molecular sieving and ion-exchange chromatographic procedures, a small number are very strongly associated or covalently cross-linked. These results are consistent with the findings that both hydroxyproline and hydroxylysine are present in purified phosphoprotein components released from the EDTA-insoluble tissue by bacterial collagenase. The hydroxylysine/100 hydroxyproline ratios in the phosphoprotein-collagen complexes are much higher than those in dentin or bone collagens.