Subcellular distribution of 51Cr and characterization of its binding sites in human platelets.

T HE MOST common radioactive marker for life-span determination of circulating blood elements has been 51chromium. In the two decades since its introduction by Gray and Sterling,’ a considerable number of other isotopic substances both radioactive and nonradioactive have made their appearance, but none achieved the popularity of 51Cr. This substance in its hexavalent form, as chromate ion, lends itself equally well to the determination of red cell and platelet survival times. In spite of the extensive usage of this labeling material, very little is known of its distribution and sites of binding within circulating blood cells. Thile it is established that hemoglobin, particularly the /?-polypeptide chains of the molecule bind chromate specifically in erythrocytes,2 nothing is known of its sites of attachment in the blood platelets. In view of recent experiments utilizing 51Cr release from platelets as a measure of platelet autoantibodies,3 it appeared of importance to examine distribution and binding sites of chromate in blood platelets.