Printed in Great Britain The Catalase-Hydrogen Peroxide System A THEORETICAL APPRAISAL OF THE MECHANISM OF CATALASE ACTION

and by Nicholls (1964) are compared in terms oftheir relative plausibilities and their utility for extension to accommodate more recent experimental information. 2. A revised formal mechanism is advanced that avoids the less satisfactory features of these mechanisms and attempts to account for the roles of catalase sub-units in both reversible and irreversible deactivation phenomena. 3. Theoretical studies of the redox chemistry ofperoxides are used to provide the basis for a discussion ofthe mechanism ofthe redox act in catalatic action at the molecular level. It is suggested that an important feature of catalase action may be a mediation of the formation of a reactive intermediate by stereospecifically located acid-base functions in the active site. 4. A more detailed statement of this concept is attempted in terms of a hypothetical partial molecular model for the composition and stereochemistry of the active site of catalase. The utility of this model in describing the catalatic and peroxidatic actions of catalase is assessed. Recent experimental studies of the catalase-hydrogen peroxide system (Jones & Suggett, 1968a,b; Jones, Pain & Suggett, 1968) have revealed new phenomena for which existing theories of catalase action do not account. In particular, this work has shown that reversibly formed catalase sub-units play important roles in catalase action at physiological pH. Jones & Wynne-Jones (1962) proposed a mech-anism for catalase action that attempted to account for the data available at that time. It was shown that the mechanism could also comprehend the available information on the catalytic action ofFe3+ ion, both in the decomposition ofhydrogen peroxide and as a mediator of coupled 'peroxidatic ' oxida-tions. Nicholls (1964) rejected the Jones & Wynne-Jones (1962) scheme for catalase action and put forward an alternative model. Nicholls's (1964) model implies that the mechanisms by which cata-lase and Fe3+ ion bring about the mutual oxidation and reduction of two molecules of hydrogen per-oxide differ qualitatively