UNLABELLED: Essentials Elevated lipoproteinp(a) is an independent and causal risk factor for atherothrombotic diseases. rs3798220 (Ile/Met substitution in apo(a) protease-like domain) is associated with disease risk. Recombinant I4399M apo(a) altered clot structure to accelerate coagulation/delay fibrinolysis. Evidence was found for increased solvent exposure and oxidation of Met residue. SUMMARY: Background Lipoprotein(a) (Lp[a]) is a causal risk factor for a variety of cardiovascular diseases. Apolipoprotein(a) (apo[a]), the distinguishing component of Lp(a), is homologous with plasminogen, suggesting that Lp(a) can interfere with the normal fibrinolytic functions of plasminogen. This has implications for the persistence of fibrin clots in the vasculature and hence for atherothrombotic diseases. A single-nucleotide polymorphism (SNP) (rs3798220) in the gene encoding apo(a) has been reported that results in an Ile→Met substitution in the protease-like domain (I4399M variant). In population studies, the I4399M variant has been correlated with elevated plasma Lp(a) levels and higher coronary heart disease risk, and carriers of the SNP had increased cardiovascular benefit from aspirin therapy. In vitro studies suggested an antifibrinolytic role for Lp(a) containing this variant. Objectives We performed a series of experiments to assess the effect of the Ile→Met substitution on fibrin clot formation and lysis, and on the architecture of the clots. Results We found that the Met variant decreased coagulation time and increased fibrin clot lysis time as compared with wild-type apo(a). Furthermore, we observed that the presence of the Met variant significantly increased fibrin fiber width in plasma clots formed ex vivo, while having no effect on fiber density. Mass spectrometry analysis of a recombinant apo(a) species containing the Met variant revealed sulfoxide modification of the Met residue. Conclusions Our data suggest that the I4399M variant differs structurally from wild-type apo(a), which may underlie key differences related to its effects on fibrin clot architecture and fibrinolysis.