[Interaction of platelets with 125I-labelled collagen type III. The necessity for forming a fibrillary structure].

The interaction of type III collagen (CIII) with washed human platelets was studied, using a CIII preparation from human placenta. CIII was labeled with 125I, and the monomeric and fibrillar forms of 125I-CIII (125I-CIIIm and 125I-CIIIf, respectively) were incubated with the platelets at room temperature. The platelet-associated and free labels were separated by centrifugation through 20% sucrose. The binding of 125I-CIIIf was unsaturable, linearly dependent on the label concentration and made up to 28 +/- 3% of the added protein. In comparison with CIIIf, the binding of 125I-CIIIm was minimal, i.e., only 0.9 +/- 0.2% of the added protein; so it did not significantly increase the background level (label sedimented through 20% sucrose in the absence of platelets). Although the level of 125I-CIIIm was very low, the binding was also unsaturable and linearly dependent on the concentration of the labeled protein. Platelet activation did not influence the level of CIIIf binding, nor did it stimulate the binding of CIIIm. The binding of 125I-CIIIf was not inhibited by the unlabeled CIIIm. The data obtained testify to the absence of high affinity platelet collagen receptors and support the hypothesis on multiple low affinity interactions between collagen fibrils and platelet surface. The binding of CIIIf to platelets was characterized by very fast kinetics; the level of binding reached a plateau within the range of 1 min and was similar in the presence of Ca2+/Mg2+ and EDTA.(ABSTRACT TRUNCATED AT 250 WORDS)