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The interaction of chloride, fluoride and phosphate ions with the molybdenum centre of sulphite oxidase in the pH range 6.2 to 9.6 has been studied by e.p.r. of Mo(V) in the enzyme reduced by sulphite. Detailed studies were made from e.p.r. spectra recorded at about 120K and more limited studies from spectra of liquid samples at about 295K and also from enzyme activity measurements. Interconversion between low-pH and high-pH Mo(V) e.p.r. signal-giving species [described by Lamy, Gutteridge &amp; Bray (1980) Biochem. J. 185, 397-403] is influenced by chloride concentration, a 10-fold increase in concentration (in the range of about 1 mM to 100 mM) causing an increase of about 1 pH unit in the apparent pK for the conversion. This suggests that chloride is a constituent of the low-pH species. In support of this, high concentrations of fluoride modified the e.p.r. spectrum. Partial conversion to a Mo(V) species, in which F- has presumably replaced Cl- and showing hyperfine coupling of A(19F)av. 0.5mT, is indicated. It is proposed that interconversion between high-pH and low-pH species is of the form: (formula; see text) No evidence that Cl- is essential for enzymic activity was found. Data relating to equilibria amongst low-pH, high-pH and also the phosphate species are presented. Depending on pH and on concentrations of Cl- and H2PO4-, one, two, or all three species may be present. Qualitatively, under appropriate conditions, the phosphate species tends to replace some or all of the low-pH species. Quantitative analysis by a computer procedure permitted an appropriate scheme to be deduced and equilibrium constants to be evaluated. Studies on the e.p.r. signals at 295K indicated that similar equilibria applied in liquid solution, but with some changes in the values of the constants. The structure of the molybdenum centre in its various states and the nature of the enzymic reaction are discussed.

Equilibria amongst different molybdenum (V)-containing species from sulphite oxidase. Evidence for a halide ligand of molybdenum in the low-pH species