Collagen fibrillogenesis in vitro: a characterization of fibril quality as a function of assembly conditions.

Pepsin-solubilized bovine corium collagen was reconstituted by rapid neutralization in dilute phosphate buffer at temperatures ranging from 10 degrees C-25 degrees C. The resultant fibrils were harvested by centrifugation and resuspended in physiological buffer to a constant protein concentration. The optical density of such suspensions, measured at 410 nm in a 1 mm path length cuvette, exhibited a strong inverse correlation with temperature of fibrillogenesis. The absorbance values of fibrillar suspensions prepared from intact collagen were greater than those observed with suspensions prepared from pepsin-solubilized collagen under similar conditions and demonstrated a reduced dependence on temperature of fibril assembly. The nature of the variation in opacity of fibrillar suspensions prepared from pepsin-solubilized material was further investigated using transmission electron microscopy, trypsin sensitivity, SDS gel electrophoresis and polarimetry. Reconstitution conditions that favored more rapid precipitation (e.g., higher incubation temperatures) tended to produce fibril suspensions of lower opacity (translucent). These translucent suspensions exhibited fibrils that were small in diameter when compared to fibril suspensions of higher opacity. Translucent preparations also contained higher levels of a trypsin sensitive, early melting component and displayed a higher proportion of peptides migrating faster than alpha 2(I) on SDS polyacrylamide gels. Collagen preparations depleted of the early melting component continued to demonstrate the correlation between increased temperature and decreased fibrillar opacity, suggesting that the two phenomena were independent. It is proposed that the unstable components are nicked or shortened collagen helices, presumably generated by pepsinization or the action of endogenous proteases of the bovine corium, which are differentially incorporated into fibrils depending on the conditions of fibril assembly.