Platelet interaction with [125I]-labeled collagen type III: requirement of fibrillar structure formation

Interaction of collagen type III (CIII) with washed human platelets was studied using CIII preparation from human placenta. CIII was labeled with [125I] and [125I]-CIII in monomeric and fibrillar form [( 125I]-CIIIm and [125I]-CIIIf respectively) were incubated with platelets at room temperature. Platelet-associated and free labels were separated by centrifugation through 20% sucrose. Binding of [125I]-CIIIf was unsaturable, linearly dependent on the concentration of label and represented 28 +/- 3% of the added protein. In comparison with CIIIf, binding of [125I]-CIIIm was minimal and represents only 0.9 +/- 0.2% of the added protein. The binding of [125]-CIIIm was also nonsaturable and linearly depend on the concentration of the labeled protein. Platelet activation neither increases the CIIIf binding, nor stimulates the binding of CIIIm. The binding of [125I]-CIIIf was not inhibited by the excess of the unlabeled CIIIm. The data obtained suggests the absence of high-affinity collagen receptors in platelets and corroborates the hypothesis of multiple low-affinity interactions between collagen fibrils and platelet surface. Binding of CIIIf was very fast--the level of binding reached a plateau within 1 min, and was similar in the presence of Ca2+/Mg2+ and EDTA. Spectrophotometrically undetectable microfibril formation during the lag phase of fibrillogenesis was sufficient for nearly the same as with large fibrils binding of CIII to platelets. Unlike platelets red blood cells (RBC) fail to bind significant amounts of [125I]-CIIIf.