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Activated FXII of both the α-(MW 80000) and the β-(MW 28000) form was tested with a large number of chromogenic tri- and tetrapeptides. The activities of the two enzymes correlated well (r=0.84, n=30). The α-form appeared to have a somewhat higher specificity.  The plasma kallikrein substrate S-2302 and the FXa substrate S-2222 have earlier been shown to be the commercially available substrates most sensitive to the β-form of activated FXII . The specific activities of both the α-and β-forms are the same with S-2302 in the system tris buffer pH 8.0 and I 0.05 at 37°C. At the enzyme concentration of 4 nmol/1, the change in absorbance per min was 0.02 which is in the suitable measuring range. This concentration is approximately equivalent to fully activated plasma diluted 1:100.  From these findings, it could be confirmed that when plasma prekallikrein is assayed via contact activation with S-2302, there is a definite but small contribution to the hydrolysis of the substrate from activated FXII.  A more selective substrate for activated FXII with regard to plasma kallikrein would be preferable in a plasma assay of FXII. S-2222 has these characteristics. At the substrate concentration of 0.33 mmol/1, the β-form of activated FXII splits S-2222 essentially as efficiently as S-2302 while plasma kallikrein is only 5% as efficient with S-2222 as with S-2302.  Both types of substrates are insensitive to FXIa at concentrations obtainable in plasma samples suitably diluted for the assaying of FXII and plasma prekallikrein.

Chromogenic Substrates And Activated Forms Of FXII: Comparison Between The α- And β-Forms And Selectivity With Regard To Plasma Kallikrein And FXIa