Nuclease P1 is a zinc-dependent endonuclease from the mold Penicillium citrinum. Nuclease P1 exhibits an increase in activity in the presence of low concentrations of urea or GuHCl. At 0.05 M GuHCl or 1 M urea the enzyme activity enhanced maximally by 3- and 3.9-fold. The kinetic parameters indicate a decrease in K-m with an increase in catalytic constant. The K-m values for control and in presence of 0.05 M GuHC1 or 1 M urea are 1.11 mg, 0.86 mg and 0.60 mg, respectively. In presence of metal ions such as Cu2+ and Co2+, urea or GuHCl treated enzyme still maintains the activity enhancement up to different extent. The far UV-CD results point to a minor conformational change and fluorescence spectra reveal increase in the relative fluorescence intensity at lower concentration. Thermal denaturation studies reveal increase in apparent T-m from 75 degrees C for control to 80 degrees C and 77 degrees C in presence of 0.1 M GuHCl or 0.5 M urea, respectively. The above results show that the activation of nuclease PI in presence of low concentrations of denaturants is mainly because of the increase in the catalytic constant suggesting that activation is due to a more open and flexible conformation of the activated enzyme with increased catalytic efficiency. (c) 2008 Elsevier Inc. All rights reserved.