STUDIES ON PLATELET PLASMA-MEMBRANES .1. CHARACTERIZATION OF SURFACE PROTEINS OF HUMAN PLATELETS LABELED WITH DIAZOTIZED (DIIODOSULFANILIC-I-125 ACID

Abstract

A polar, nonpenetrating compound of high specific activity, diazotized (125I)-diiodosulfanilic acid (DD125ISA), was developed as a label for exposed proteins of the human platelet plasma membrane, and platelet proteins and the pattern of labeling were studied with sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). DD125ISA bound specifically to membrane proteins as demonstrated by the specific activity of isolated membrane protein was 5-7 times that of whole platelet protein and no proteins of intact platelets were labeled which were not represented in the isolated plasma membrane. The DD125ISA-labeled membrane proteins were exposed on the cell surface because DD125ISA-labeled proteins were altered by trypsin treatment of intact, labeled platelets and the pattern of labeling produced by reaction of isolated membranes with DD125ISA was quite different from that produced by the labeling of intact platelets. Analysis of platelet membrane proteins by SDS-PAGE demonstrated the glycoproteins previously described at 150,000 daltons (termed glycoprotein I) and 92,000 daltons (glycoprotein III) but apparently distinct glycoproteins were discriminated in the intermediate region (IIa: 125,000 daltons and II: 118,000 daltons). Glycoproteins I and III were constant as IIa was clearly visible only in unreduced samples and II was predominant in reduced samples. Reaction of DD125ISA with intact platelets resulted in equal labeling of 3 of these 4 membrane glycoproteins (IIa, II and III). The pattern of exposed proteins on the platelet surface labeled by DD125ISA was different from lactoperoxidase-131I, which labeled predominantly the 92,000 dalton glycoprotein, as demonstrated by simultaneous SDS-PAGE analysis. Three glycoproteins of the human platelet plasma membrane were exposed to a radioisotope probe on the platelet surface and were accessible for contact interactions.