Nitric oxide, substrate of Euphorbia characias peroxidase, switches off the CN− inhibitory effect

The oxidation of nitric oxide (NO) by Euphorbia characias latex peroxidase (ELP‐FeIII), in the presence or in the absence of added calcium, has been investigated. The addition of hydrogen peroxide to the native enzyme leads to the formation of Compound I and serves to catalyse the NO oxidation. The addition of NO to Compound I leads to the formation of Compound II and, afterwards, to the native enzyme spectrum. Under anaerobic conditions, the incubation of the native enzyme (ELP‐FeIII)with NO leads to the formation of the stable complex, showing a characteristic absorption spectrum (ELP‐FeII–NO+). The rate of the formation of this complex is slower in the presence of calcium than in its absence, and the same applies to the rate of the formation of Compound II from Compound I, using NO as substrate. Finally, we demonstrate that NO protects ELP from the inactivation caused by CN− via a mechanism presumably requiring the formation of an enzyme‐nitrosyl cyanide complex.