The kinetics and other characteristics of nitrate reductase (NR, EC 1.6.6.1) in cowpea [Vigna unguiculata (L.) Walp.] seedlings irradiated with biologically effective UV-B radiation (280-320 nm, 3.2 W m-2 s-1) were recorded. The in vivo and in vitro NR activities were inhibited by 34 and 41 % under UV-B treatment, respectively. Both Vmax and Km for the substrate were enhanced by UV-B radiation. The Km for nitrate increased from 1.2 to 1.7 mM after the UV-B irradiation. The change in Km for NADH was from 0.12 to 0.17 mM. The increases in Km indicate that UV-B radiation seriously changes the topology of NR, particularly with respect to the nitrate and NADH binding sites. The rate of NR turnover indicates the extent of damage inflicted by UV-B radiation on the nitrate metabolism. The half-life (t1/2) of NR was reduced from 7 to 4 h in the UV-B treated seedlings. UV-B also inhibited the kinetics of nitrate uptake by plants: its Km increased from 0.08 to 0.12 mM. and T. Balakumar ... [et al.].
Cowpea [Vigna unguiculata (L.) Walp. cv. Co 4] seedlings were subjected to a weighted irradiance of 3.2 W m-2 s-1 of biologically effective ultraviolet-B radiation (UV-B, 280-320 nm) and the changes in the kinetic and other characteristics of nitrite reductase (NiR) were recorded. The activity of NiR was hampered by 19 % under UV-B irradiation compared to the control. The UV-B treated plants required higher concentrations of nitrate for the induction of NiR synthesis than the controls. The NiR activity decay kinetics showed that the UV-B treatment significantly lowers the t1/2 of the enzyme, thereby indicating a reduced rate of enzyme turnover. The comparison of kinetic characteristics of nitrate reductase (NR) and NiR under UV-B treatment showed that NiR was not so sensitive to UV-B radiation as NR. As shown by enzyme turnover rates, NiR extracted from plants irradiated by UV-B in situ was less sensitive to UV-B radiation than the enzyme extract subjected to in vitro UV-B irradiation. Though NiR was less damaged by UV-B treatment than NR, subtle changes occurred in its kinetic characteristics. and T. Balakumar ... [et al.].