Factor V (FV) deficiency, also known as parahemophilia, is a rare bleeding disorder. Herein we investigate the first reported missense mutation associated with FV deficiency, Ala221Val, assigned as FV New Brunswick. To elucidate the molecular pathology associated with the Ala221Val substitution, the mutation was recreated in a recombinant system together with 3 FV mutants (Ala221Gly, Glu275Gln, and Cys220Ala/Cys301Ala) designed to help explain the Ala221Val phenotype. The expression pattern was analyzed by pulse-chase experiments and an FV-specific enzyme-linked immunosorbent assay (ELISA), the results suggesting the Ala221Val mutation not to interfere with the synthesis or secretion. The functional properties of the recombinant FV New Brunswick were evaluated in both plasma clotting and purified systems. The Ala221Val mutation did not affect the factor Xa (FXa) cofactor function; nor did it interfere with the activated protein C (APC)-mediated down-regulation of activated FV (FVa) activity. However, FV New Brunswick demonstrated reduced stability at 37 degrees C due to an increased rate of dissociation of light and heavy chains of FVa. In conclusion, this in vitro study of FV New Brunswick suggests the Ala221Val mutation not to impair synthesis and expression of procoagulant activity, indicating overall proper folding of the mutant molecule. Rather, the Ala221Val substitution appears to interfere with the stability of the activated FVa mutant, the reduced stability possibly explaining the deficiency symptoms associated with the mutation.