Sc. Huber et al., DIFFERENTIAL RESPONSE OF NITRATE REDUCTASE AND SUCROSE-PHOSPHATE SYNTHASE-ACTIVATION TO INORGANIC AND ORGANIC SALTS, IN-VITRO AND IN-SITU, Physiologia Plantarum, 92(2), 1994, pp. 302-310
Studies were conducted to compare the modulation of beta-nicotinamide
adenine dinucleotide (NADH):nitrate reductase (NR; EC 1.6.6.1) and suc
rose-phosphate synthase (SPS; EC 2.4.1.14) with respect to regulation
by the inorganic anions, phosphate (P-i), sulfate and tungstate. Follo
wing inactivation of both enzymes in vivo by transferring spinach plan
ts (Spinacia oleracea L. cv. Bloomsdale) to a darkened growth chamber,
spontaneous reactivation occurred in vitro when desalted leaf extract
s were preincubated at 25 degrees C prior to assay. All three inorgani
c anions inhibited SPS activation in vitro and also reduced the light
activation of SPS in situ when they were fed to excised leaves via the
transpiration stream. As expected, feeding tungstate to excised leave
s prevented the light-dependent increase in extractable NR activity. H
owever, in contrast to SPS, the light activation of NR in situ was rel
atively unaffected by P-i and sulfate, and in vitro, both anions stimu
lated (rather than inhibited) the reactivation of NR. Part of the stim
ulation by P-i and sulfate was the result of increased ionic strength,
and stimulation could also be demonstrated with other inorganic and o
rganic salts. In the presence of high ionic strength (0.1 to 0.2 M KCl
), the rate of NR activation in vitro was relatively constant when the
pH of the preincubation medium was varied from pH 6.5 to 8.0, whereas
in the absence of added salt the rate of activation was nearly zero a
t pH 6.5 but increased progressively as pH was raised. The stimulation
by salts could be reversed, in part, by glycerol and ethylene glycol
suggesting that hydrophobic interactions might play some role in the a
ctivation of NR.