PROTEOLYTIC CLEAVAGE OF FIBRINOGEN - AMPLIFICATION OF ITS SURFACTANT INHIBITORY CAPACITY

Severe deterioration of surfactant function is noted under conditions of plasma protein leakage into the alveolar space; moreover, fibrinogen has previously been reported to possess strong surfactant inhibitory capacity. Dissolution of alveolar deposits of fibrinogen and fibrin (e.g., hyaline membranes) requires enzymatic degradation by the plasminogen/plasmin system or by leukocyte-derived proteases. We investigated the surfactant inhibitory properties of differently prepared sets of fibrinogen cleavage products. Proteolysis was performed with plasmin, with predominant split products D (mol wt 85,000) and E (mol wt 50,000). In addition, fibrinogen was cleaved by leukocyte elastase and trypsin, with fragments ranging mainly between mol wt of 30,000 and 50,000. To provide split products of even lower molecular weight, fibrinogen was incubated sequentially with trypsin and endoproteinase (split products < mol wt 25,000). Natural surfactant extracts used in clinical replacement studies (CLSE, Alveofact, Curosurf, Survanta) as well as an apoprotein-based phospholipid mixture (PLM-C/B; DPPC:PG:PA = 68.5:22.5:9 with 2% [wt/wt] nonpalmitoylated recombinant human SP-C and 1% [wt/wt] natural bovine SP-B) were employed. Experiments were performed in a pulsating bubble surfactometer (standard phospholipid concentration 2 mg/ml) with assessment of surfactant activity measuring adsorption and dynamic surface tension. Fibrinogen caused dose-dependent, severe deterioration of the surface activities of Curosurf and Survanta, whereas CLSE, Alveofact, and PLM-C/B were only moderately affected up to protein-surfactant ratios of 4:1. Plasmin-, elastase-, and trypsin-derived cleavage products of fibrinogen exhibited markedly increased inhibitory capacities on all surfactant preparations used, with extensive loss of surface activities at protein-surfactant ratios above 1:1. Within the plasmic fragments, split product D, but not E, represented the predominant inhibitory compound. Further cleavage of tryptic fragments by endoproteinases to low molecular weight split products resulted in a decrease of surfactant inhibitory capacity. We conclude that differently prepared sets of fibrinogen cleavage products with mol wt > 30,000 exhibit consistently increased surfactant inhibitory properties. These findings suggest that prolonged deterioration of surfactant function may be expected during fibrinolytic lung repair processes.