In vitro apatite induction by phosphophoryn immobilized on modified collagen fibrils

Noncollagenous phosphoproteins that interact with type I collagen are thought to nucleate the mineral phase to collagen network of mineralized tissues. Previously, we found that phosphophoryn cross-linked to type I collagen was an effective nucleator of apatite. Here, we investigated the potential role of collagen telopeptide structure on this nucleation. We used pepsin and sodium borohydride (NaBH4) to modify the telopeptide region and reducible cross-links in the collagen fibrils and determined the effect on mineral induction by phosphophoryn cross-linked to it. The amount of phosphophoryn cross-linked to NaBH4-reduced collagen fibrils was higher than that to intact (unmodified) collagen fibrils. However, the amount of phosphophoryn cross-linked to collagen that lacked the telopeptides (atelocollagen) was 25% of that cross-linked to intact collagen fibrils. Each preparation was incubated at 37 degrees C in metastable calcium phosphate solutions that did not spontaneously precipitate. Apatite was induced by phosphophoryn cross-linked to intact collagen fibrils at 15.0 h whereas phosphophoryn cross-linked to reduced collagen fibrils induced apatite formation after 10.9 h. Enough phosphophoryn was cross-linked to atelocollagen to induce mineral formation, but it did not. The failure of the phosphophoryn-atelocollagen complex to nucleate mineral might have been caused by a cross-linking pattern in the helical portion of the collagen molecule that did not promote the growth of the calcium-phosphate clusters into nuclei. The present study indicates that the telopeptide domains of type I collagen play a role in the interaction with phosphophoryn, which is critical for the nucleation process.