Nitrate reductase activity has been characterized in cell-free extracts of the marine centric diatom Skeletonema costatum. The enzyme can be easily extracted, and shows magnesium and phosphate activation, with an optimum pH at 7.9 Kmvalues for nitrate and NADH have been found to be, respectively, 0.24 mM and 20 μM. NADPH can substitute for NADH as obligatory electron donor although with much smaller efficiency. Neither ammonium nor amino acids affect in vitro activity. However, ammonium exerts a rapid in vivo inactivation of the NADH-nitrate reductase and associated activities. The enzyme appears to be more ammonium-repressible than nitrate-inducible. © 1978.
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