Nitrate reductase Subject

The enzymatic mechanisms of nitrate bioactivation have long been the subject of debate. The glutathione-S-transferase and cytochrome P-450 systems were thought to be involved [8], while recently a nitrate reductase that specifically catalyzes the formation of 1,2-glyceryl dinitrate from glyceryl trinitrate was purified and identified as a mitochondrial aldehyde dehydrogenase [46]. Interestingly, this... 

In the early 1970s Nason and co-workers showed that extracts of nit-1 mutants of Neurospora crassa exhibited nitrate reductase activity when mixed with solutions of other molybdenum enzymes that had been subjected to denaturing conditions (20-23). The isolation and... 

Five proteins containing molybdenum are known nitrate reductase, nit-rogenase, xanthine oxidase, aldehyde oxidase and sulphite oxidase. They also contain iron, and the first four are best classified as multi-enzyme systems. Early studies on xanthine oxidase used a number of important ESR techniques, particularly rapid freeze kinetic methods and isotopic substitution in metalloproteins. This work has been reviewed [38, 39], Nitrogenase is the subject of considerable recent interest since it contains detectable iron-sulphur centres but as there is some disagreement at present concerning the interpretations of the results readers are referred to the original literature [40-42]. 

This, the largest and most diverse family of Mo and W enzymes, has been the subject of numerous XAS investigations. One early application of the method to this family was to Escherichia coli nitrate reductase, which gave the first hints of an active site that had four sulfur donors, and that difficulty in analysis might arise from sample heterogeneity, both harbingers of the complexity that was later to become apparent for this family of enzymes. 

Substrate induction is subject to a temporal control of an unknown kind. Substrate induction anddi temporal control of substrate induction of a higher order have been discovered in a few further instances, such as different kinds of nitrate reductase and an enzyme involved in anthocyanin synthesis in Petunia hybrida. In addition, there is quite a number of enzymes for which substrate induction ora temporal control of synthesis of a higher order has been established. We shall become acquainted with examples of the latter type later. 

Thus, they concluded that ATP-sulfurylase was subject to feed-forward stimulation by a reaction product of nitrate reduction and that nitrate reductase was subject to an analogous feed forward by a product of sulfate, thus coordinating the first step of each of the two pathways. Some support for this mechanism has come fi om experiments with cell cultures of Ipomea (Zink, 1984) and from measurements of ATP-sulfurylase and nitrate reductase in the leaves of whole tobacco plants (Barney and Bush, 1985), though in the latter study the response of ATP-sulfurylase to nitrogen stress was far less pronounced than the response of nitrate reductase to sulfur stress. However, as discussed below, APS sulfo-transferase rather than ATP-sulfurylase appears to be the principal determining site for the feed-forward control of sulfate assimilation by nitrogen assimilation products in Lemna. 

---