Evidence that an Arg79-->Gln substitution in human factor VII is not associated with a reduction in coagulant activity.

A recent report hypothesized that an Arg79-->Gln mutation in the first epidermal growth factor-like domain of human factor VII is the molecular basis for a severe (< 1%) factor VII functional deficiency. In the present study, a site-specific mutant human factor VII cDNA (Arg79-->Gln) was constructed, subcloned and expressed in baby hamster kidney cells. Mutant factor VII was purified to homogeneity and characterized with respect to gamma-carboxyglutamic acid content, ability to activate, tissue factor-dependent amidolytic activity and expression of factor VIIa proteolytic activity on tissue factor-bearing cells. Mutant factor VII was fully carboxylated and exhibited the same molecular weight and coagulant activity as plasma factor VII. Mutant factor VII was activated by factor Xa at the same rate, and to the same extent, as plasma factor VII. In the presence of tissue factor, mutant factor VII was converted to factor VIIa in an autocatalytic manner at a rate indistinguishable from that observed with plasma factor VII. In addition, the amidolytic activities of mutant factor VIIa and plasma factor VIIa towards S-2288 in the presence of relipidated tissue factor were identical. Finally, following complex formation with cell surface tissue factor, mutant factor VIIa activated factor X at essentially the same rate as plasma factor VIIa under comparable conditions. These results are not consistent with the notion that the arginine-79 residue in the first epidermal growth factor-like domain of human factor VII is essential for the expression of tissue factor-dependent factor VIIa proteolytic activity.