p1. Citrate synthase [citrate oxaloacetate-lyase (CoA-acetylating), EC 4.1.3.7] was purified about 400-fold from the extreme halophile, Halobacterium cutirubrum, by a method involving (NH4)2SO4 fractionation, chromatography on DEAE-cellulose and hydroxyapatite and gel filtration on Sephadex G-200. 2. The purified enzyme was best activated by high concentrations of KCl (3M); the chlorides of other cations and K+ salts of other anions (Br-, NO3-, SCN-) were less effective than KCl as activators. The enzyme was best stabilized by high concentrations of NaCl or KCl. Cold-lability was found in the presence of 3M-KCl, but not in the presence of NaCl at concentrations up to 5M. The results suggest that both the shielding of negative charges on the enzyme molecule and the stabilization of hydrophobic bonds by high KCl concentrations were required for maximum activity of the enzyme. 3. The double-reciprocal plots for acetyl-CoA or oxaloacetate at several concentrations of the co-substrate intersected at the abscissa in the presence of either KCl or NaCl, at either 1 or 3M. The Km for oxaloacetate increased about fivefold with the salt concentration, from 1 to 3M./p
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