Macromolecules Released from Platelet Storage Organelles

Much attention has been paid to the low molecular weight substances released from platelets since the release reaction was first discovered. The release of macromolecules on the other hand has received much less attention, and of the published work on platelet macromolecules most has been devoted to the known plasma protein eauivalents which are either synthesized de-novo or adsorbed by the platelet prior to their subsequent release. Whereas it is fairly easy to show that the physical and/or immunological properties of these molecules (notably albumin, fibrinogen and IgG) are slightly but reproducibly different from those of plasma, it is very difficult (if not impossible) to prove that they are truly synthesized within the platelet as such, rather than undergoing modulation during the release reaction or isolation procedure. Whilst these “plasma equivalent” proteins are of academic interest, they have less practical application than those which are “platelet specific” i.e. are not normally found in native, platelet free plasma. Unfortunately, there are several technical problems in the study of such proteins, and we have chosen to study them by a combination of techniques, namely biological activity, immunological activity and physico-chemical properties. Each approach has some drawbacks, but in combination, much progress is possible and we can now identify at least four antigens, four biological activities, and two physically distinct proteins, which are not necessarily related. With the recent interest in the biological activity of released macromolecules, it is now desirable that the various groups exchange samples and antisera in order to identify each antigen and biological activity.