The effects of heparin on the physicochemical properties of reconstituted collagen

Pepsin-solubilized bovine dermal collagen was reconstituted in 0.02 M sodium phosphate (pH 7.2), concentrated to 30-40 mg/ml, and adjusted to physiological ionic strength by addition of sodium chloride. These preparations, at 4-15 degrees C, are fibrillar suspensions composed of fibrils of varying diameters and nonassociated molecules. Addition of heparin to these suspensions promoted a dose-dependent increase in average fibril diameter as measured by turbidimetry and electron microscopic analyses. These effects were relatively specific for heparin and heparin-like glycosaminoglycans. Chondroitin sulfate and hyaluronic acid had little or no effect on fibrillar diameters under these conditions, whereas dermatan sulfate had an intermediate effect on fibrillar reorganization. Differential scanning calorimetry revealed that addition of optimal concentrations of heparin generated fibrils of higher stability and that this effect was associated with the disappearance of structures of lower stability, including nonassociated molecules and thin fibrils. Light microscopic analyses of the fibrillar collagen/heparin matrix showed it to be a more open network of distinct collagen fibers than was observed with the fibrillar collagen preparation alone. Binding experiments indicated that heparin bound to fibrillar collagen in a saturable fashion with a Kd of approximately 4 X 10(-7) M. Creep experiments provided evidence that the addition of heparin to fibrillar collagen suspensions greatly reduces the gelation phenomenon that is normally observed when such suspensions are warmed to 37 degrees C. These differences in fibrillar architecture may be in part responsible for differences noted in the biological response to fibrillar collagen and fibrillar collagen/heparin implants in vivo (McPherson et al., 1988).