Biophysical Journal 8: 109-124 (1968)
© 1968 the Biophysical Society

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Sulfur Isotope Fractionation and Kinetic Studies of Sulfite Reduction in Growing Cells of Salmonella heidelberg

ABSTRACT

A pulsed feeding technique was used during studies of sulfite reduction by Salmonella heidelberg in order to realize large percentages of SO₃⁼ conversion while simultaneously maintaining a reasonably stable cell population. As a consequence, much data for conventional kinetic and sulfur isotope fractionation computations were obtained in any one experiment. Under the conditions of supplying 150 µg glucose per ml of medium every 6 hr, anaerobiosis, and varying the SO₃⁼ concentration, the following observations were made: 1. Below 0.01% w/v Na₂SO₃, the reduction strictly followed first order kinetics with respect to SO₃⁼ concentration. At higher concentrations, the rate of SO₃⁼ reduction fell below that predicted by first order kinetics suggesting that a saturation effect was occurring. 2. At lower concentrations, the ratio of the isotopic rate constants k₁/k₂ was 1.02 whereas at higher SO₃⁼ levels, k₁/k₂ values of 1.04 were found. These latter effects are much higher than those obtained in the equivalent chemical reduction. On the basis of these observations, a model is considered which features two isotopically dependent steps and an intermediate reservoir which forms at higher SO₃⁼ concentrations. Results of an experiment under aerobic conditions and an experiment wherein the reduction rate was thermally altered, are also presented.

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