Catalase: an old enzyme with a new role?

Although animal catalase has been studied for decades, its physiological role has remained perplexing. It has two enzymatic functions, not only catalyzing the breakdown of H2O2 into O2 and H2O, but also in the presence of low concentrations of H2O2 catalyzing the oxidation of electron donors such as ethanol or phenols. In this article, I have summarized some well-known properties of the enzyme and have also described several recently discovered features. Of particular interest is the finding that, although catalase has been regarded as an intracellular enzyme, there is published evidence for its association with the plasma membrane of the erythrocyte. Moreover, recent work from my laboratory indicates that in vitro at alkaline pH in the presence of Mg2+, the biologically active diphenols (beta-3,4-dihydroxyphenylalanine and the beta-adrenergic agonists isoproterenol, norepinephrine, and epinephrine) appear to function as electron donor substrates for human erythrocyte catalase and inhibit the production of O2 from H2O2 at micromolar concentrations. The beta-adrenergic antagonist propranolol inhibits O2 production much less effectively and appears to competitively inhibit the reaction of catalase with epinephrine. These observations suggest an analogy between catalase and the beta-adrenergic hormone receptor and raise many questions of interest to basic science, health, and disease.