PATTERNS OF PRODUCT INHIBITION FOR BACTERIAL NITRITE REDUCTASE

Product inhibition was examined in the turnover kinetics of cytochrome cd1 from Pseudomonas aeruginosa (ATCC 19429) and from Paracoccus denitrificans (ATCC 13456). A common characteristic was a decrease in rate during the time course of assays that was not due to substrate depletion or irreversible inactivation. The product of nitrite reduction, nitric oxide (NO), acted as a product inhibitor in anaerobic assays with an apparent Ki of 0.2 .mu.M, but only if the enzyme was first preincubated with NO for 15 min. The enzyme was inhibited by the oxidized form of electron donors and this could account for the decrease in rate during an assay. For the donors hydroquinone, ascorbate, tetramethylphenylenediamine and azurin, measured values of the Ki were at least 10-fold lower than measured Km. Cytochromes c as donors demonstrated a complex pattern of product inhibition by the ferric form. Although numerical values of Ki in these cases were not obtained, trends indicated that apparent values would be less than Km.