Search

Regulation of acetolactate synthase (ALS, EC 4.1.3.18) in the phototrophic prokaryote Rhodospirillum rubrum was studied. In cell free extracts of 5 strains investigated, enzyme activity was very labile (about 80% loss of activity within 12h during storage at 4 °C) but was stabilized to some extent by 10 (μM FAD and 20 vol.% glycerol. By filtration of extracts through Superose 6 HR gels (FPLC technique), ALS activity of all strains was separated in two fractions of 200 and 600 kDa, respectively. The enzyme fractions had about the same affinity to pyruvate (ATm = 1.6 - 1.8 m M) , the same sensitivity to L-valine (50 and 65% inhibition by 0.1 m M valine in the standard test mixture) and the herbicide sulfometuron methyl (90 and 92% inhibition by 1 μM herbicide), but differed greatly in their sensitivity to inhibition by 0.4 m NaCl. In culture media with 2-oxobutyrate (2-OB), growth began only after a lag-phase of several days (5 days with 1 mM of the inhibitor). Cells grown in the presence of 2-OB had a reduced total ALS activity and did not contain the 200 kDa fraction. The inhibition of ALS by valine was noncompetitive in respect to pyruvate (K1= 0.l m M ) . From other branched-chain amino acids tested (L-leucine, L-isoleucine, norvaline, norleucine) only isoleucine was inhibitory (K1; = 3.1 m M )

Sulfonyl urea-herbicides (sulfometuron methyl, chlorsulfuron) strongly inhibit phototrophic growth of the bacterium Rhodospirillum rubrum in the absence of ʟ-valine as a feedback inhibitor of the enzyme acetohydroxy acid synthase. The latter enzyme was shown to be highly sensitive to the sulfonyl urea and, to a much lesser extent, to an imidazolinone herbicide (Arsenal®).

Stability constants and dissociation rate constants of thallium (I) cryptates have been measured in several solvents at 25 °C. The Tl+ cryptates are more stable and less sensitive to ligand cavity size than the corresponding complexes of the alkali-metal cations. The stability constants vary strongly with solvent, and the solvent dependence of the complex stabilities appears to reflect mainly changes in the solvation of Tl+. It is shown that free energies of transfer of the solvated Tl+ among non–aqueous solvents calculated on the assumption that the difference in the free energies of transfer of the Tl+ cryptates and the corresponding cryptands is zero are in good agreement with literature data. Changes in the stability constants with solvent and ligand are reflected in changes in both dissociation and formation rate constants, but more so in the former. Thus the solvation of the transition state, (Tl+⋯Cry)[graphic omitted], is rather closer to that of the reactants, and includes additional solvent interactions compared with the stable cryptate complex, TlCry+.