Abstract
The spectrophotometric titration curves (λ max: 295 nm) of the tyrosyl residues (Tyr) of MW 55000 (H-UK) and 36000 (L-UK) forms of human urinary urokinase and their pH-esterase activity curves with N-acetyl glycyl-L-lysine methyl ester as a substrate showed that most of the activity was lost by alkali with the ionization of only 2 and 1 Tyr of all the 19 and 12 Tyr, respectively. The alkalized H- and L-UK showed difference CD spectra (λ max: 250 nm) attributed to ionized Tyr. H-UK had the non-ionized Tyr (4.1) much more than those of L-UK (0.4), and the rapidly (11.8) and slowly (3.1) ionized Tyr as many as those of L-UK (7.7 and 3.9). Most of the activity decreased gradually as 14 and 10 Tyr of H- and L-UK reacted with cyanuric fluoride (CyF), respectively. For H-UK, the activity drastically decreased after 14 Tyr reacted with CyF, suggesting that some of 5 CyF-non-reactive Tyr including 4 Tyr of the non-ionized type stabilized the conformation near the active site.
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