Lactoperoxidase-catalyzed125I iodination and sodium dodecyl sulphate-polyacrylamide gel electrohoresis have been performed on whole, washed platelets as well as on isolated platelet membranes and granules. Electrophoresis of the whole platelets demonstrated two major radioactive peaks, corresponding to glycopolypeptides of estimated molecular weights of 120 000 and 100 000. A small, but consistent amount of radioactivity was also associated with a 147 000 dalton glycopolypeptide. The membranes showed the same pattern of radioactivity as the whole plateltes, whereas only negligible amounts of labeled material was found in the soluble and granule fractions. Practically all the polypeptides were labeled in membranes iodinated after their isolation. A glycopolypeptide of 147 000 molecular weight was observed also in the soluble and the granule fractions, but no radioactivity was associated with these substances. In unreduced form, the granule glycopolypeptide penetrated only slightly into the polyacrylamide gel. Thrombin induced the release of this granule-located substance from whole platelets, as observed by gel electrophoresis of the supernatant after release reaction (secretion). The granule glycoproteins were only partly exposed on the granule membrane since about 50% of the acid-hydrolyzable sialic acid could be liberated by neuraminidase treatment of isolated granules. In whole, iodinated granules the bulk of the radioactivity was associated with a polypeptide of estimated molecular weight 46 000 (possibly actin). This polypeptide was not seen in the supernatant after removal of the thrombin-degranulated platelets by centrifugation, which indicates that the granule membrane is retained with the platelets during the secretion process. © 1976.
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