In vitro apatite induction by phosphophoryn immobilized on modified collagen fibrils

Noncollagenous phosphoproteins that interact with type I collagen are thoug ht to nucleate the mineral phase to collagen network of mineralized tissues . Previously, we found that phosphophoryn cross-linked to type I collagen w as an effective nucleator of apatite, Here, we investigated the potential r ole of collagen telopeptide structure on this nucleation. We used pepsin an d sodium borohydride (NaBH4) to modify the telopeptide region and reducible cross-links in the collagen fibrils and determined the effect on mineral i nduction by phosphophoryn cross-linked to it. The amount of phosphophoryn c ross-linked to NaBH4-reduced collagen fibrils was higher than that to intac t (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 incubat ed at 37 degrees C in metastable calcium phosphate solutions that did not s pontaneously 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 ph osphophoryn was cross-linked to atelocollagen to induce mineral formation, but it did not, The failure of the phosphophoryn-atelocollagen complex to n ucleate mineral might have been caused by a cross-linking pattern in the he lical portion of the collagen molecule that did not promote the growth of t he 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.