A daily rhythm in the activity of nitrate reductase (NR: EC 1.6.6.1) isolated from the marine red algae Gracilaria tenuistipitata is shown to be attributable to changes in amounts of the protein. The enzyme was purified in four steps: ion exchange Q-Sepharose separation, ammonium sulfate precipitation, gel filtration on Sephacryl S-300, and affinity chromatography on Affigel-blue resin. This purification procedure yielded an active purified NR of about 500-fold with a recovery of 85%. The SDS-PAGE silver staining of purified NR revealed a 110kDa single band. Non-denaturated protein showed a molecular mass of 440 kDa on gel filtration comparing with SDS-PAGE, the enzyme is apparently composed of four identical subunits. In extracts of algae grown under either constant dim light or a light-dark cycle, the activity of NR exhibited a daily rhythm, peaking at midday phase as does photosynthesis. Staining with monoclonal antibodies, raised against NR from Porphyra yezoensis, showed that the amount of protein changes by a factor of about 12, with a maximum occurring in the midday phase. (C) 2002 Elsevier Science (USA). All rights reserved.