Diurnal fluctuation of nitrate reductase activity in the marine red alga Gracilaria tenuistipitata (Rhodophyta)

The biochemical characteristics and diurnal changes in activity of the enzyme nitrate reductase (NR; EC 1.6.6.1) from the marine red alga Gracilaria tenuistipitata var. liui Zhang et Xia are described. Different assay conditions were tested to determine the stability of NR. The crude extract of G. tenuistipitata has a NR specific activity of 10.2 U . mg(-1), which is higher than the NR activities found for other algae, plants, and fungi. This NR is highly active at a slightly alkaline pH and is stable over a wide range of temperature, with an optimal activity at 20 degrees C. The apical portions of the thallus contain 64.9 +/- 6.6% of the total NR specific activity. The apparent Michaelis-Menten (K-M) constant found for KNO3 was 197 mu M, and it was 95 mu M for NADH. The NR from G. tenuistipitata can be included in the NADH-specific group, because no activity was found when NADPH was used as an electron donor. In extracts of algae grown under either continuously dim light or a light-dark cycle the activity of NR exhibits a daily rhythm, peaking at the middle of the light phase, when activity is 30-fold higher than during the night phase.