The platelet reactivity of collagen type VI.

Collagen type VI in native (undenatured or triple-helical) form has been shown, like collagen types I-V, alpha 1(I) trimer and alpha 2(I) trimer, to possess platelet reactivity provided that essential quaternary structural needs are first satisfied. Thus platelet aggregation was induced by the collagenous domain of collagen type VI, isolated free of the non-collagenous elements, when this entity was presented to platelets in fibrillar form. This implies that platelet recognition sites in collagen type VI are located in the collagenous sequence of the molecule. Aggregation of platelets was also induced, although a higher concentration was required, by the intact, "parent" collagen following its polymerisation by random association of molecules with the aid of a cross-linking agent (glutaraldehyde) to yield an amorphous polymer. This permits the suggestion that the more ordered molecular assembly of collagen type VI thought to occur in vivo, to yield a microfibrillar form, is likely to be associated with significant platelet reactivity. Our results support the notion that any collagenous species may be reactive towards platelets provided that essential tertiary and quaternary structural requirements are met and in this sense, therefore, they favour more the idea of multiple platelet-reactive sites in collagen of relatively low structural specificity and low affinity.