A sensitive fluorometric assay for plasminogen, plasmin, and streptokinase

A rapid, convenient, and highly sensitive fluorometric assay for plasmin (P), plasminogen (Pg), and streptokinase (SK) as the activator complex (SK·P) is described. These assays are based on the measurement of the fluorescence of β-naphthol (βN) released from α-N-methyl α-N-tosyl-l-lysine β-naphthol ester (MTLNE) by the P present or generated during the assay. The rate of βN release may be followed by direct recording or determined subsequently, following termination of the enzyme reaction at a fixed digestion time, by the addition of p-nitrophenyl-p′-guanidinobenzoate. The latter method may be readily automated. The K_m and V values for the hydrolysis of MTLNE by P or SK·P are equivalent. The Pg activator activity of P was shown to be very small (less than 0.2% of that of SK·P).