The effect of pepsin solubilization on platelet aggregation by types I and III collagens

The effect of pepsin solubilization on the platelet aggregating activity of type I collagen and type III collagen was examined. Pepsin-digested type I collagen was unable to initiate platelet aggregation in either soluble form or as pre-formed fibrils. In contrast, pepsin-digested type III collagen was active in soluble form or as preformed fibrils. Mixtures of type I and type III collagen were assayed for platelet aggregating activity. In soluble form, these mixtures demonstrated elevated activity with increasing type III concentration. When the mixtures were tested as pre-formed fibrils, the rate of aggregation was relatively constant with the combination 75% type III and 25% type I manifesting the highest activity. The lag time for onset of aggregation was also minimized for this same type III/type I ratio. The combinations of the two collagen types formed fibrils which reflected the amounts of types I and III collagens in solution.