DIRECT ELECTROCHEMISTRY OF THE FLAVIN DOMAIN OF ASSIMILATORY NITRATE REDUCTASE - EFFECTS OF NAD(+) AND NAD(+) ANALOGS

Direct electrochemical studies, utilizing two voltammetric methods-squ are-wave voltammetry (SWV) and cyclic voltammetry (CV)-have been perfo rmed on recombinant forms of the flavin domain of spinach assimilatory nitrate reductase in the presence of NAD(+) analogs. The reduction po tentials (E degrees') of the flavin domains have been determined at an edge pyrolytic graphite electrode utilizing MgCl2 as a redox-inactive promoter. Under identical experimental conditions (pH 7.0, 25 degrees C), the two electron reduction potential for the FAD/FADH(2) couple h as been determined to be -274 and -257 mV by SWV and CV, respectively. In contrast, the reduction potentials of free FAD have been determine d to be -234 and -227 mV by SWV and CV, respectively. The reduction po tentials of the complex formed between the FAD prosthetic group in the recombinant flavin domain and various NAD(+) analogs have been determ ined to be as follows: NAD(+) (E degrees' = -192 mV), 5'-ADP ribose (E degrees' = -199 mV), ADP (E degrees' = -154 mV), AMP (E degrees' = -1 96 mV), adenosine (E degrees' = -192 mV), adenine (E degrees' = -220 m V), and NMN (E degrees' = -208 mV). In contrast to these positive shif ts in reduction potential, nicotinamide (E degrees' = -268 mV) had ver y little effect on the reduction potential of this flavin complex. Mor eover, addition of NAD(+) to the FAD prosthetic group in a variety of mutant forms of the recombinant flavin domain resulted in positive shi fts in the reduction potential of the complex, although the magnitude of the shifts varied from a minimum of 6 mV obtained for the C240A mut ant to a maximum of 79 mV obtained for the C62S mutant. These results represent the first extensive application of direct electrochemistry t o examine the redox properties of assimilatory nitrate reductase and i ndicate that complex formation with NAD(+), or various NAD(+) analogs, results in a positive shift in the flavin reduction potential, with t he magnitude of the shift correlating well with the efficiency of the inhibitor. (C) 1997 Academic Press.