Haemin affects platelet aggregation and lymphocyte mitogenicity in whole blood incubations

The effect of haemin on platelet aggregation induced by epinephrine, ADP, collagen and arachidonic acid was examined, as well as on lymphocyte mitogenicity induced by phytohaemagglutinin (PHA). The receptor mediated platelet inducers, such as epinephrine and ADP were enhanced to maximal aggregation capacity by exogenous haemin, whereas non-receptor inducers, i.e. collagen and arachidonic acid, showed a marked decrease of platelet aggregation. The results indicate the involvement of haemin in receptor mediated platelet activation. PHA-induced lymphocytes treated with haemin showed a reduced rate of 3H-thymidine incorporation. Inhibition of lymphocyte mitogenic activity was dose-dependent on haemin concentration. Ultrastructural localization of haemin binding sites was performed by the osmiophilic reaction of benzidine. Haemin was shown to bind to the cell membrane and to the granule membrane of the platelets. The lymphocyte membrane was in practice masked by a layer of haemin.