Nitrate reductase, an old enzyme that produces nitric oxide in plants and algae

Abstract

Nitric oxide (NO) is a free radical that is synthesized in animals by the enzyme NO synthase (NOS). In contrast to our improved knowledge on animal NOS, however, NO producing mechanism in plants has yet been controversial. Here we show that nitrate reductase (NR) is an enzyme responsible for NO production in plants and algae. Although NR normally catalyzes the reduction of nitrate (NO₃^-) to produce nitrite (NO₂^-), the addition of the product nitrite into a solution of purified NR enzyme causes a rapid NO production. This NO producing reaction can be inhibited by NR inhibitors. Under aerobic conditions, NR also produces peroxynitrite that is the most toxic molecule among reactive nitrogen species (RNS). These new findings offer us to consider that the old enzyme yet includes unexplored and important functions other than assimilatory nitrogen metabolism.