Denaturation of proteins by ascorbic acid: Effects on dopamine-?-hydroxylase

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Abstract
Under conditions that are optimum for DβH, ascorbic acid denatures serum albumin, γ-globulin, catalase, and D\H. With ascorbate plus Cu2+, the proteins arc almost completely destroyed. Pyrazole protects DβH and albumin, but not catalase. Superoxide dismutase (SOD) is not denatured by ascorbate, with or without Cu2+, and in combination with catalytic amounts of catalase or Fe2+ it stimulates maximum DβH activity. In other words, a combination of catalase and SOD, or Fe2+ and SOD, will protect DβH. Excessive amounts of catalase and/or other protein, either native or denatured will prevent the effects of superoxide and/or ascorbate, but cannot replace the requirements for catalytic quantities of catalase or Fe2+. The results suggest that the rate of hydroxylation of tyramine may be limited by superoxide, but that the latter per se does not denature DβH as does hydrogen peroxide. The in vitro activation of oxygen by DβH is a toxic process, involving the production of both hydrogen peroxide and superoxide and possibly other free radicals. In the absence of precise regulation of the production and concentrations of these compounds, the enzyme is denatured.