COMPARISON OF CARBON-MONOXIDE, NITRIC-OXIDE, AND NITRITE AS INHIBITORS OF THE NITROGENASE FROM CLOSTRIDIUM-PASTEURIANUM

A comparative study of CO, NO and nitrite as inhibitors of C. pasteurianum nitrogenase was done. Confirming previous studies, CO inhibited acetylene reduction, but not H2 evolution nor ATP hydrolysis. NO and nitrate both inhibited acetylene reduction, H2 evolution and ATP hydrolysis. Nitrogenase inhibition by CO is readily reversible; the effects of NO and nitrate are irreversible. No rapidly and irreversibly inactivated the FE protein, but not the MO-Fe protein. In the presence of NO, part of the iron of the Fe protein is complexed by bathophenanthrolinedisulfonate, which suggests that NO disrupts the FE4S4 cluster present in the protein. Like NO, nitrite reacts preferentially with the Fe protein, and it also induces complexation of the Fe by bathophenanthrolinedisulfonate. Under the conditions normally used for the assay of nitrogenase, nitrite is reduced by dithionite. Even though the latter reaction proceeds at a very low rate, enough NO is evolved to inhibit nitrogenase. In view of the striking similarities between the inhibitory effects of NO and nitrite, nitrogenase may be inhibited not by nitrite itself, but rather by the nitric oxide produced by the reduction of nitrite.