Arch. Gerontol. Geriatr., 3 (1984) 287-295 287 Elsevier AGG 00082 Effects of ionic strength on the activity of superoxide dismutase in vitro I. Semsei and I. Zs.-Nagy F. Verzitr International Laboratory for Experimental Gerontology (V1LEG), Hungarian Section, University Medical School, H-4012 Debrecen, Hungary (Received 15 May 1984; accepted 3 September 1984) Summary Changes in superoxide dismutase (SOD) activity were studied in vitro at increasing NaC1 or KC1 concentrations. SOD activity was measured using two different systems of superoxide radical generation: pyrogallol autoxidation, and xanthine-xanthine oxidase reaction. Pyrogallol autoxidation was directly measured by spectrophotometry, whereas in the second case cytochrome c reduction was followed at 550 nm. The inhibition of SOD on those parameters was taken as measure of SOD activity. Increasing concentrations of NaCl and KC1 significantly increased the rate of pyrogallol autoxidation. The inhibitory effect of SOD significantly decreased under the influence of these salts and followed an exponential curve. The two salts studied resulted in essentially identical changes in SOD activity. Increasing concentrations of NaC1 and KCI decreased the rate of cytochrome c reduction in the xanthine-xanthine oxidase system. When correcting the results for these primary effects, SOD activity also displayed in this system an exponential decay with increasing salt concentrations. The results are interpreted in terms of the known charge distribution pattern on the surface of the SOD molecule, and of the age-dependent increase of the intracellular potassium and sodium concentrations in the postmitotic cells. superoxide dismutase; ionic strength dependence of SOD activity; intracellular ionic composition during aging; pyrogallol autoxidation for measuring SOD activity; xanthine-xanthine oxidase reaction; cyto- chrome c reduction Introduction Oxygen free radicals have been implicated as causal factors in many biological processes, among others in aging (Harman, 1960; Pryor, 1971). It has been shown that all oxygen metabolizing organisms generate free radicals (McCord et al., 1971), damaging various cell components such as proteins (Demopoulos, 1973), lipids (Tappel, 1973), nucleic acids (Hallett et al., 1970; Rosenthal and Pitts, 1971), etc. The living systems contain several self-protecting factors against the oxygen free radicals such as vitamins E and C (Packer et al., 1979), polyamines (Vanella et al., 1979), as well as several enzymes. Superoxide dismutase (SOD) (McCord and 0167-4943/84/$03.00 © 1984 Elsevier Science Publishers B.V.