Hydrolysis of p-nitrophenyl phenylphosphonate catalysed by bovine pancreatic deoxyribonuclease

Abstract

The ability of bovine pancreatic DNAase to hydrolyse the synthetic substrate p-nitrophenyl phenyl phosphonate (NPPP) is intrinsic and is not due to the contamination of the DNAase preparation by non-specific phosphodiesterases because the activities of DNA and NPPP hydrolysis are co-eluted from a DEAE-cellulose column with use of the Ca2+-affinity elution method and because the two activities are decreased simultaneously when the purified enzyme is treated with Cu2+/iodoacetate, an active-site-labelling agent for DNAase. NPPP hydrolysis is facilitated by the metal ion-DNAase. At relatively high Na+ concentrations, where the metal ion-DNA interaction is weak, DNA hydrolysis is also facilitated by the metal ion-DNAase. With NPPP as substrate the Michaelis constants are K(m) 3.7 mM for Mn2+ and K(m) 49 mM for Mg2+ in 0.2 M-Tris/HCl buffer, pH 7.2 Ca2+ competes with Mn2+, with K(i) 64 mM. Free Cu2+ ions non-competitively inhibit DNAase-catalysed DNA or NPPP hydrolysis in the presence of Mn2+ or Mg2+ and the inhibition is not relieved by Ca2+. The affinity of Ca2+ for free DNAase is higher than that for Mn2+-DNAase. Mn2+ in not bound to DNAase via a simple ionic interaction, as Mn2+ remains bound in the presence of relatively high Na+ concentrations and induces a near-u.v. difference absorption spectrum. The kinetics of NPPP hydrolysis catalysed by Mn2+-DNAase are sigmoidal. From the Hill equation, h = 2.0 is obtained, suggesting that more than two NPPP molecules are bound per molecule of DNAase with a certain amount of co-operativity. Because DNAase in solution is a monomer with a single catalytic site, the multiple NPPP molecules on a single protein molecule are probably in one location, resulting in a co-operative interaction that may resemble that in the stacked base-pairs of double-helical DNA.