Activation of Thalassiosira pseudonana nadh: Nitrate reductase

NADH: nitrate reductase (NR) has been isolated in both active and inactive states, and both could be purified using blue-Sepharose. The state of activation of the enzyme depended on the presence or absence of agents such as cysteine or EDTA during the assay. When NR was assayed, the addition of activator before NADH led to maximum activity. Therefore, the reduced NR appeared to be inactivated during the assay in the absence of activator. Inactivation may have occurred via a mechanism similar to the inactivation of lipoamide dehydrogenase by trace metals, such as CU²⁺. The activation of NR by cysteine or EDTA was interpreted as protection of the reduced enzyme due to chelation of trace metals in the assay solution by the activators.