The Effect Of Mercury On Nitrate Reductase Activity In Bean Leaf Segments (Phaseolus vulgaris) And Its Chelation By Phytochelatin Synthesis

Nitrate reductase (NR) is the key enzyme of nitrate assimilation subjected to inhibition by heavy metal treatment .In general, the inhibitory effect of heavy metals may be attributed to either blocking the supply of reducing equivalents of nitrate reduction, formation of mercurial derivatives of thiol(–SH) groups of nitrate reductase or synthesis of phytochelatins (PC).The effect of mercury(Hg) on nitrate assimilation has rarely been reported. Hence, the present study may help to explore the possible mechanism(s) of toxicity and detoxification of heavy metal pollutants on nitrate assimilation in Phaseolus vulgaris, an important legume crop and a great source of nutrition to millions of people. In the present study application of mercury showed concentration dependent response on in vivo NR activity in all three treatment schedules. Supply of metabolites, glutathione (GSH) and cysteine (Cys) alone as well as with mercury, increased NR activity significantly in comparison to mercury treatment alone Addition of buthionine sulfoxamine (BSO) along with Hg and GSH/ Cys reversed the potentiating effect of glutathione or cysteine against toxicity of Hg. However, the protective effect of glutathione was more pronounced as compared to cysteine. On the basis of this study it is suggested that Hg inactivates the enzyme by binding to thiol (-SH) groups of protein. In addition, it also provides evidence that glutathione serve as precursor fo r phytochelatins and this was confirmed by using BSO, an inhibitor of Y-glutamyl cysteine synthetase, a key enzyme of PC synthesis pathway.