The activity and kinetic stability of a keratinolytic subtilisin-like protease from Bacillus sp. P45 was investigated in 100 mM Tris-HCl buffer (pH 8.0; control) and in buffer with addition of Ca(2+) or Mg(2+) (1-10 mM), at different temperatures. Addition of 3 mM Ca(2+) or 4 mM Mg(2+) resulted in a 26% increment on enzyme activity towards azocasein when compared to the control (100%; without added Ca(2+) or Mg(2+)) at 55 A degrees C. Optimal temperature for activity in the control (55 A degrees C) was similar with Mg(2+); however, temperature optimum was increased to 60 A degrees C with 3 mM Ca(2+), displaying an enhancement of 42% in comparison to the control at 55 A degrees C. Stability of protease P45 in control buffer and with Mg(2+) addition was assayed at 40-50 A degrees C, and at 55-62 A degrees C with Ca(2+) addition. Data were fitted to six kinetic inactivation models, and a first-order equation was accepted as the best model to describe the inactivation of protease P45 with and without metal ions. The kinetic and thermodynamic parameters obtained showed the crucial role of calcium ions for enzyme stability. As biocatalyst stability is fundamental for commercial/industrial purposes, the stabilising effect of calcium could be exploited aiming the application of protease P45 in protein hydrolysis.