Synthesis, purification, and characterization of an Arg152 .fwdarw. Glu site-directed mutant of recombinant human blood clotting factor VII

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Abstract
Coagulation factor VII circulates in blood as a single-chain zymogen of a serine protease and is converted to its activated two-chain form, factor VIIa, by cleavage of an internal peptide bond located at Arg152-Ile153. Previous studies using serine protease active-site inhibitors suggest that zymogen factor VII may possess sufficient proteolytic activity to initiate the extrinsic pathway of blood coagulation. In order to assess the putative intrinsic proteolytic activity of single-chain factor VII, we have constructed a site-specific mutant of recombinant human factor VII in which arginine-1523 has been replaced with a glutamic acid residue. Mutant factor VII was purified in a single step from culture supernatants of baby hamster kidney cells transfected with a plasmid containing the sequence for Arg152 .fwdarw. Glu factor VII using a calcium-dependent, murine anti-factor VII monoclonal antibody column. Purified mutant factor VII was indistinguishable from plasma-derived or recombinant wild-type factor VII by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and migrated as a single band with an apparent molecular weight of 50,000. The average specific activity of several mutant factor VII preparations was 0.00025 unit/.mu.g, or 0.01% of that observed for recombinant wild-type factor factor VII preparations. The clotting activity of mutant factor VII was, however, completely inhibited following incubation with dansyl-Glu-Gly-Arg chloromethyl ketone, suggesting that the apparent clotting activity of mutant factor VII was due to a contaminating serine protease. Immunoblots of mutants factor VII incubated with human factor IXa revealed no cleavage, whereas incubation of mutant factor VII with human factor Xa resulted in cleavage of mutant factor VII and the formation of a lower molecular weight degradation product migrating at Mr .apprxeq. 40,000. Incubation of mutant factor VII with Staphylococcus aureus V8 protease resulted in the proteolytic activation of mutants factor VII followed by a progressive decline in activity as a result of proteolytic degradation. By comparison, incubation of recombinant wild-type factor VII with V8 protease resulted in the proteolytic degradation of factor VII and loss of coagulant activity without an apparent transient increase in activity. Our results are consistent with the proposal that zymogen factor VII possesses no intrinsic proteolytic activityy toward factor X or Factor IX.