Collagen biochemistry of the two layers of carious dentin.

The amino acid composition and intermolecular cross links of collagen fibers in the two layers of carious dentin, differentiated by a 0.5% basic fuchsin-propylene glycol-stain, were biochemically investigated. No difference in the pattern of amino acid composition of collagen fibers was found between the first and second layers of carious dentin and the sound dentin. However, obvious differences in the intermolecular cross links of collagen fibers were found between the three layers. Namely, compared with the sound dentin, the second layer of carious dentin had decreased cross links and increased precursors. This change is considered to be reversible. Contrastingly, in the first layer, both the cross links and the precursors remarkably decreased. In addition, the hexitollysines (protein-saccharide compounds probably related to bacterial metabolism) were found and several peaks of unknown materials appeared. This indicates irreversible destruction of cross-linkage in the first layer of carious dentin.