Albumin modulates lateral assembly of fibrin polymers: evidence of enhanced fine fibril formation and of unique synergism with fibrinogen.

Abstract

We identified a new property of human albumin. It enhances formation of fine fibril (or leptofibril) structures during fibrin gelation, and by nephelometric and electron microscopic measurements, this property is independent of and synergistic with that of fibrinogen. We examined fibrin aggregation using physiologic temperatures and pH and albumin:fibrin concentration ratios below those at which the known accelerating effect on fibrin aggregation occurs. An albumin concentration dependent decrease in gel turbidity maxima was consistently demonstrable in buffers containing or lacking (2-5 mM) CaCl2. This decrease was shown to be induced by albumin preparations which had been exposed to 2 mM ethylene-diaminetetraacetate disodium salt (EDTA), dialyzed, and tested in EDTA-free buffer. A delay in the onset of aggregation was also shown in calcium-lacking buffers by use of either reaggregating fibrin or fibrinogen aggregated with low (0.01-0.05 unit/mL) thrombin concentrations. Rates of fibrin aggregation as well as those of fibrinopeptide release were not affected by albumin, and the decrease in gel absorbance was demonstrable when solubilized fibrin was reaggregated at all final fibrin concentrations (0.2-4 microM) examined. Computed from wavelength dependence turbidity measurements (1 microM fibrin, I = 0.20), albumin decreased the average mass:length ratio from 8.24 X 10(11) to 4.26 X 10(11) daltons/cm, or from that of an approximately six to a three protofibril-thick strand. It also decreased the mean fibril radius from 48.5 to 36.4 nm but had no effect on fibril density. Electron microscopic measurements of cross-sectional fibril widths, performed on sections of glutaraldehyde-fixed gels, disclosed differences between albumin-containing and control gels which were significant by chi 2 analysis (P greater than 0.001). Fibril groups of 7-20- and 21-40-nm width together comprised 77% of fibrils formed in the presence of albumin (n = 251) compared to 30% of controls (n = 309). Conversely, coarser fibrils of 41-60- and 61-97-nm width together comprised 23% of fibrils formed in the presence of albumin and 70% of controls. This albumin effect was demonstrable by use of different monomeric albumin preparations including defatted, undefatted (unexposed and exposed to 60 degrees C, 10 h), chromatographically [gel exclusion and (diethylaminoethyl)cellulose] pure, S-(carboxymethyl)albumin, and S-(N-ethylsuccinimidyl)albumin. Chromatographically isolated albumin oligomers lacked this property, suggesting that a specific site(s) on albumin was (were) required.(ABSTRACT TRUNCATED AT 400 WORDS)