Effect of salts on the activity of nitrate reductase from the photosynthetic bacteriumRhodobacter sphaeroides SW

The effect of different salts on the nitrate reductase (NR) activity of Rhodobacter sphaeroides SW was studied. An increase of the activity in the presence of monovalent cations was observed (90, 80 and 77% with Na+, K+ and NH, respectively), whereas divalent cations inhibited the enzyme activity (30 and 85% with Ca2+ and Mg2+). Both activation or inhibition of the NR activity were reversible. These results could be explained by the interaction of cations with negatively charged amino acids. On the other hand, several anions were also effective in stimulating NR activity (Cl− > NO > Br− > I− > SCN−), suggesting that the degree of hydration of the enzyme molecule might influence its activity. Finally, the effect of salts on the NR activity of R. sphaeroides SW can be atributed to both electrostatic and hydrophobic interactions.