Nitric oxide (NO) is generated in some biological systems. Due to its radical character, it exhibits high reactivity, but biological system can manage NO without sustaining any damage to bio-compounds in the cell. As a model system to understand how the NO dynamics is controlled in the cell, we have been studying denitrification of microbial respiration, in which NO is generated as an intermediate product. In denitrification, it was found that NO produced by the NO-generating enzyme (NiR: nitrite reductase) can be smoothly transferred to the NO-decomposing enzyme (NOR: nitric oxide reductase) by making a complex of the two enzymes. The chemical mechanism of the NO decomposition by NOR was also revealed by the time-resolved spectroscopic techniques.