Characterisation of nitrate reductase in the marine macroalga Kappaphycus alvarezii (Rhodophyta)

Nitrate reductase (NR) of the marine red macroalga Kappaphycus alvarezii (Doty) Doty ex P. Silva was examined regarding the biochemical characteristics and daily fluctuations in activity. NR activity was maximum at pH 8.0 and stable over a wide range of temperatures, with an optimum at 23 degreesC. The highest specific activity measured in a crude extract was 0.16 U mg(-1) protein. Apical parts of the algal thallus showed 1.5 times higher activity compared to basal parts. The NR was NADH-specific (EC 1.6.6.1), since no activity was found when NADPH was used as an electron donor. The apparent Michaelis-Menten (K-M) constants for nitrate and NADH were 30 and 5 muM, respectively. The stability of NR was assessed by storing the crude extract in different temperatures. Stored at 4 degreesC, the crude extract maintained high NR activity for 1 week. NR activity exhibited a daily rhythm in algae grown under either a light-dark cycle of 55 mumol photons m(-2) s(-1) or in constant light of 100 mumol photons m(-2) s(-1), with maximal activity during the midday phase. The results thus indicate that the expression of NR activity may be under circadian control. (C) 2004 Published by Elsevier B.V.