Inhibition of wheat leaves nitrate reductase activity by Cibacron Blue

Cibacron Blue F3GA (CB) inhibited the activities of wheat leaves NADH:nitrate reductase and NADH:cytochrome-e reductase in a time-independent and concentration dependent manner. The methyl viologen:nitrate reductase activity of the enzyme was unaffected by various CB concentrations used in the experiment. Inhibition of NADH:nitrate reductase was of mixed type (partial competitive and pure noncompetitive) with respect to NADH and noncompetitive with respect to nitrate. The estimated inhibition constant (K-i) values were 1 mu M for NADH and 8.4 mu M for nitrate. The secondary plots of inhibition with respect to NADH, indicated a dissociation constant (K-I) of 8.8 mu M for the enzyme-NADH-CB complex. This K-I being greater than the K-i suggested that the noncompetitive inhibition is predominant over the competitive inhibition at the NADH binding site.