15N tracing studies on in vitro reactions of ferredoxin-dependent nitrite reductase and glutamate synthase using reconstituted electron donation systems

Abstract

It is known that plants contain ferredoxin (Fd)-dependent nitrite reductase (NiR) and glutamate synthase (GOGAT). The Fd-NiR reaction produces ammonia from nitrite, and the activity is usually measured by nitrite disappearance. The Fd-GOGAT reaction forms two glutamates of different origin, from glutamine and 2-oxoglutarate, and the activity is measured by the oxidation of reductant (NADPH) or by formation of total glutamate. Here, a quantitative probe of the products and efficiency of the process was conducted using 15N tracing techniques on these reactions in vitro. We quantified the reduction of 15N-labeled NO2- to [15N]NH4+ and the formation of [15N]glutamate and [14N]glutamate from [5-15N-amide]glutamine plus 2-oxoglutarate by NiR and GOGAT, respectively, with the reductant-Fd-NADP+ oxidoreductase (FNR)-Fd system as the sequential electron donors. The supply of dithionite or NADPH to recombinant cyanobacterial NiR led to electron donation system-dependent formation of [15N]ammonium from [15N]nitrite. Addition of 20 mM NaCl and 20 mM Na-ascorbate accelerated nitrite reduction under high concentrations of NADPH. A sufficient supply of NADPH to recombinant Zea mays Fd-GOGAT generated complete GOGAT activity (transferring the [5-15N]amide of glutamine to 2-oxoglutarate to form [15N]glutamate), whereas a shortage of NADPH resulted in glutaminase activity only, which removed the amide from glutamine and released ammonia and [14N]glutamate. We conclude that although the recombinant Fd-GOGAT enzyme has two forms of glutamate synthesis, the first by glutaminase (ammonia release by glutamine amidotransferase) and the second by glutamate synthase (coupling of the ammonia and exogenously applied 2-oxoglutarate), the first works without NADPH, while the second is strictly dependent on NADPH availability.