Structural and chromatographic heterogeneity of normal plasma fibrinogen associated with the presence of three γ-chain types with distinct molecular weights

Abstract

Three different γ chains have been identified in fibrinogen isolated form normal human plasma with apparent molecular weights of 50000 (γ 50), 55000 (γ 55) and 57500 (γ 57.5), as shown by SDS-polyacrylamide gel electrophoresis. Plasma fibrinogen was separated by ion-exchange chromatography on DEAE-Sephacel into three populations of molecules, each differing in γ-chain composition. The first peak contained 87% of the total fibrinogen and was composed of molecules containing only γ 50 chains; the second peak included 3% of the fibrinogen and contained one γ 50 and one γ 55.5 chain and the third peak had 10% of the total which containedone γ 50 and one γ 57.5 chain, Cross-linked fibrin obtained from fibrinogen with only γ 50 chains contained γγ dimers exclusively of M r 100000, representing a uniform γ 50γ 50 dimer composition. The γγ dimers from purified fibrinogen of γ 50γ 55 type had molecular weights of 111000, 105000 and 100000, while dimers from γ 50γ 57.5 fibrinogen were M r 115000, 108000 and 100000. The relative proportions of γγ dimers from each purified fibrinogen population were consistent with random crosslinking of the γ monomers. The γ-chain identity of the variants was established by their conversion to covalent dimeric species after clotting by thrombin in the presence of Factor XIII, their incorporation of the fluorescent lysine analog dansyl cadaverine, by the staining intensity of monomeric and dimeric forms with the periodic acid-Schiff reagent after electrophoretic separation, and by plasmic degradation of all monomeric and dimeric forms in a pattern that was characteristic for γ chains. Individual γγ dimers were purified by preparative gel electrophoresis, and tyrosine was demonstrated to be the amino-terminal residue of all three γ chain types. We conclude that normal human fibrinogen can be separated into three populations of molecules due to molecular weight heterogeneity of their γ chains.