Studies of nitrate reductase in the fresh water dinoflagellatePeridinium cinctum

Nitrate reduction was studied in the dinoflagellatePeridinium cinctum collected from extensive algal blooms in Lake Kinneret (Israel).Among several methods tested for the preparation of cell free extracts, only the use of a ground-glass tissue culture homogenizer was found to be efficient. The assimilatory nitrate reductase ofP. cinctum was located in a particulate fraction. In this respect,P. cinctum did not behave like other eukaryotes, such as green algae, but as a prokaryote. Nitrite reductase activity was found in the soluble fraction.Nitrate reductase used NADH as a preferable electron donor; it reacted also with NADPH but only to give 16.5% of the NADH dependent rate. Methyl viologen and benzyl viologen could also serve as electron donors, with rates higher than the NADH dependent activity (3–6 times and 1.5–3 times, respectively). The Km of nitrate reductase for NADH was 2.8×10^–4 M and for NO₃-1.9×10^–4 M. Flavins did not stimulate the activity, nor was ferricyanide able to activate it. Carboxylic anions stimulated nitrate reductase activity 3–4 fold, an effect which was not mimicked by other anions.Chlorate, azide and cyanide were competitive inhibitors ofP. cinctum, nitrate reductase withK _i values of 1.79×10^–3 M, 2.1×10^–5 M and 8.9×10^–6 M respectively.