On the Reactivity of the Sulfhydryl Groups of Ferredoxin Nicotinamide Adenine Dinucleotide Phosphate Reductase

The chloroplast flavoprotein has by amperometric titration four sulfhydryl groups and one disulfide per molecule. Inactivation by mercurials follows first order kinetics. From the order of this inactivation reaction with respect to mercurial, it is found that integrity of one sulfhydryl group is essential for catalytic activity. NADP affects the rate of inactivation of the flavoprotein by mercurials. This substrate protects against inactivation by polar mercurials, whereas it accelerates the inhibition by nonpolar ones. Reactivation studies show that the enzyme inactivated by mercurials, although completely fluorescent, partially recovers its catalytic activity upon removal of inhibitor.