DIAPHORASE ACTIVITY OF FERREDOXIN - NADP OXIDOREDUCTASE IN THE PRESENCE OF DIBROMOTHYMOQUINONE

In the presence of dibromothymoquinone (10 mu M) NADPH- and ferredoxin:NADP oxidoreductase-dependent oxygen uptake was observed in buffered (40 mM Tris-HCl buffer) medium. The rate of O-2 uptake depended on the pH of the reaction mixture and was about two orders of magnitude faster at pH 8.7 than at pH 6.7. In alkaline medium the rate was about 0.2 mu mol O-2 min(-1) cm(-3) (or 9.3 nmol O-2 min(-1) mg(-1) FNR) at the time of tracing, whereas at a lower pH (7.7 or 6.7) oxygen consumption was 1.5-2.0-fold faster during the first minute of monitoring than during the second minute. Ferredoxin was not an obligatory component involved in this process. However, at a lower pH (7.7, 6.7) oxygen consumption by the reaction mixture was stimulated 2-8-fold by adding ferredoxin (0.6 mu M). The v(2) value was also enhanced by ca 36% in the presence of 0.2 M NaCl, whereas inorganic pyrophosphate caused reduction of v(1) and v(2) to about 62 and 3%, respectively, of the control. Under anaerobic conditions the Michaelis constant (K-m) and the pseudo-first-order rate constant (k) for the FNR-catalysed DBMIB reduction was about 16 mu M and 0.34 s(-1), respectively. The results indicate that DBMIB can accept electrons from NADPH-ferredoxin: NADP oxidoreductase, and the reduced DBMIB (presumably in the unprotonated form) can be reoxidized by molecular oxygen.