Activation of urinary inactive kallikrein by an extract from the rat kidney cortex

Activation of purified urinary inactive kallikrein by an extract from the rat kidney cortex was investigated. The extract produced a dose-dependent activation of the inactive kallikrein and the optimum pH for this activation was 5.0. Marked depression of the activation was observed when the extract was pre-incubated with E-64, p-CMB and iodoacetate, but not with DFP, PMSF or pepstatin A. The molecular weight of the inactive kallikrein (Mr 44,000) was reduced to 38,000 by treatment with the extract, this molecular weight value being identical with that of urinary active kallikrein. These results indicate that the rat kidney cortex contains a protease catalyzing conversion of urinary inactive kallikrein into its active form, and that the protease has properties compatible with those of a thiol protease, but not of trypsin which has been used as a tool for the activation of urinary inactive kallikrein. The thiol protease is probably one of regulators of the kallikrein-kinin system in the kidney.