Molecular defect in coagulation factor XFriuli results from a substitution of serine for proline at position 343.

ACTOR X IS A SERINE protease zymogen that plays F a pivotal role in blood coagulation.’ It can be activated physiologically by the extrinsic (factor VIIa/tissue factor complex) and intrinsic (factor IXa/factor VIIIa complex) pathways in the presence of calcium and phospholipid and nonphysiologically by an enzyme in Russell’s viper venom (RW). In all cases, activation to factor Xa entails cleavage of the same arginine-isoleucine bond near the aminoterminal of the heavy chain of factor X.’ Factor Xa combines with factor Va on cell surfaces in the presence of calcium to form the macromolecular enzyme complex for prothrombin activation. The various molecular interactions defined for factor X/Xa have served as a means of grouping molecular aberrations associated with deviant factor X molecule^.^^^ One type is exemplified by observations in a population of patients in the Friuli region of Italy who exhibit a moderate bleeding tendency associated with a homozygous factor X deficiency defined by prolonged prothrombin and partial thromboplastin clotting times, which exhibited activity levels of 4% to 9% of normal, but near-normal clotting times in the presence of RVV.’ Their plasma contains normal levels of factor X antigen.333‘ Three related patients with the homozygous deficiency had had bleeding since early childhood, with epitaxis, bleeding from the gums, posttraumatic hemarthroses, and bleeding after tooth extractions and other surgical procedures. Those with the heterozygous factor XFrluh defect possess factor X levels varying from 32% to 55% of normal and are usually asymptomatic. No gross structural differences were observed by molecular weight or immunochemical analyses between the isolated variant and normal protein.6 Purified factor X,,,,,, was activated at a near-normal rate by RW, but activation was reduced by 98% and 75% in the extrinsic and intrinsic activation pathways, respectively. Small chromogenic substrates are not cleaved efficiently by activated factor XFnui,,6.7 and fluorescence polarization studies suggested that a bound dansylated inhibitor of factor Xa was not properly oriented within the active site of the variant Factor XaFrlu,, generated thrombin from prothrombin at only about one third the rate observed for normal factor Xa. Peptide zymogen and the active site of the enzyme and explains the previous observations of discrete biochemical and functional differences between factor XFriuli and normal factor X. The mutation abolished an HgiCl restriction site present in the normal factor X gene, and this change constitutes the basis for a convenient method for screening individuals carrying this molecular defect. ProlineMJ is in conserved region 5 of the serine protease superfamily to which factor X belongs and is part of a 14-residue L**t**P****t*C motif that occurs in at least 16 other enzymes. Computer analysis suggests that the motif may be an essential aspect of conformational features important to functional properties of factor X as well as other serine proteases. o 1991 by The American Society of Hematology.