RAT PLATELETS CONTAIN GLYCOSYLATED AND NONGLYCOSYLATED FORMS OF PLATELET FACTOR-4 - IDENTIFICATION AND CHARACTERIZATION BY MASS-SPECTROMETRY

Platelet factor 4 is a heparin-binding protein released from the a. gr anules of activated platelets. This study describes the purification a nd identification of two forms of rat platelet factor 4, the previousl y characterized non-glycosylated form of 7 kDa and an additional glyco sylated form of molecular mass 9 kDa. The two proteins both neutralize d the antithrombin-III-dependent inhibitory activity of heparin. Altho ugh their amino acid composition was found to be the same, in the N-te rminal sequence of the 9-kDa protein, the second threonine residue cou ld not be detected and a difference of 976Da was determined by mass sp ectrometry. After digestion with O-glycanase and sialidase, the two pr oteins showed the same molecular mass. Overall consideration of these data led to identification of the higher-molecular-mass protein as a g lycosylated form of rat platelet factor 4 with O-glycosylation at the second N-terminal amino acid, while the structure of the oligosacchari de core was established by mass spectrometry and sugar differentiation with lectins. The two forms of platelet factor 4 are both present in platelets and secreted after platelet activation.