The pyridine nucleosidase from Bacillus subtilis. Kinetic properties and enzyme-inhibitor interactions.

The interaction between the Bacillus subtilis DPNase and its inhibitor is a time-dependent second order reaction, with a rate constant of 5.3 × 10⁵ M^?1 s^?1 at 28 °C and pH 7.5. The interaction is noncompetitive with the substrate, and the presence of substrate does not affect the rate of interaction. Dissociation of the enzyme-inhibitor complex occurs below pH 4.3. The presence of DPN⁺ does not promote dissociation of the complex at neutral pH values, but does promote dissociation at pH 4.5.The enzyme has a high specificity for the presence of the nicotinamide ring in the substrate. DPN⁺ and TPN⁺ are the only dinucleotides that are hydrolyzed by the DPNase out of a number of DPN⁺ analogs that were tested. The thionicotinamide analog of DPN⁺ is a potent inhibitor of the enzyme.The stability of the DPNase and its inhibitor against heat and acid denaturation was also investigated.