Kinetics and activation energetics of death in Saccharomyces cerevisiae induced by sulfur dioxide.
Death of Saccharomyces cerevisiae induced by sulfur dioxide (K(2)S(2)O(2) was used as the SO(2) source) followed saturation kinetics. The enthalpy of activation of death was not affected by concentration over the range tested (5-150) mg/L of (K(2)S(2)O(2) at pH 3.4) and averaged 3.6 x 10(4) cal/mol as compared with 8.5 x 10(4) cal/mol for DeltaH of thermal death. The entropy of activation of death was hyperbolic function of the sulfur dioxide concentration, extrapolated at zero concentration to DeltaS(0) = 36.8 cal mol(-1) K(-1) and tended to DeltaDeltaS(max) = 13.2 cal mol(-1) K(-1) at saturating concentration, yielding a dissociation constant of 5.8 x 10(-1) M sulfur dioxide. As was predicted from these results, In K(d) (the specific rate of death induced by sulfur dioxide) was hyperbolic function of concentration under isothermic conditions and extrapolated to a finite value at zero concentration. The Arrhenuis plots and the DeltaS(+/-) plot versus concentration revealed the occurrence of substrate inhabitation of the death effect at high concentrations (above 60 mg/L K(2)S(2)O(2) at pH 3.4). A model is presented involving two types of receptor sites for sulfur dioxide on the cell surface, on directly connected with the death process, the other modulating its entropy of activation.
Laboratory of Microbiology, Gulbenkian Institute of Science, 2781 Oeiras codex, Portugal.
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J
Anacleto
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N
van Uden