Nitrate and nitrite reductases in Platymonas striata Butcher (Prasinophyceae)

Abstract

The Michaelis constants for nitrate reductase (E.C. 1.6.6.1.) and nitrite reductase (E.C. 1.6.6.4.) were 3·1 × 10^-2 m for nitrate and 2·4 × 10^-5 m for nitrite respectively in broken cell preparations of Platymonas striata Butcher. The activities of both enzymes disappeared rapidly after treatment with ammonium ion in vivo, but were unaffected by in vitro treatment. It is suggested that this loss of enzyme activity was not due to degradation of the enzymes. The absence of light also caused complete loss of enzyme activity; which was regained in light. Nitrate ions induced increases in the activities of both enzymes, but was not essential for their synthesis, since this occurred in nitrogen-starved cells in light.