Salt responses of carboxylation enzymes from species differing in salt tolerance

This paper reports effects of salts on in vitro activity of phosphoenolpyruvate carboxylase and ribulose-1,5-diphosphate carboxylase, isolated from species differing in salt tolerance.Inhibition of phosphoenolpyruvate carboxylase by the inorganic salts KCl, NaCl, and Na(4)SO(4) depended on the source of the enzyme. Phosphoenolpyruvate carboxylase isolated from leaves of C(4) plants was extremely sensitive to inorganic salts, whereas the enzyme extracted from roots of C(4) plants or from both shoots and roots of C(3) plants was much less sensitive. Ribulose-1,5-diphosphate carboxylase was less salt-sensitive than the phosphoenolpyruvate carboxylases. Differences in salt sensitivity of carboxylases were observed over a wide pH range. The results suggest substantial physical-chemical differences between phosphoenolpyruvate carboxylases functioning in photosynthesis and in CO(2) dark fixation.Among C(4) species, phosphoenolpyruvate carboxylase from halophytic species was more salt-sensitive than that from a salt-sensitive species. This anomaly, between in vitro response of enzymes and growth response of the plants, is briefly discussed.