PEROXISOMAL COPPER,ZINC SUPEROXIDE-DISMUTASE - CHARACTERIZATION OF THE ISOENZYME FROM WATERMELON COTYLEDONS

The biochemical and immunochemical characterization of a superoxide di smutase (SOD, EC 1.15.1.1) from peroxisomal origin has been carried ou t. The enzyme is a Cu,Zn-containing SOD (CuZn-SOD) located in the matr ix of peroxisomes from watermelon (Citrullus vulgaris Schrad.) cotyled ons (L.M. Sandalio and L.A. del Rio [1988] Plant Physiol 88: 1215-1218 ). The amino acid composition of the enzyme was determined. Analysis b y reversed-phase high-performance liquid chromatography of the peroxis omal CuZn-SOD incubated with 6 M guanidine-HCl indicated that this enz yme contained a noncovalently bound chromophore group that was respons ible for the absorbance peak of the native enzyme at 260 nm. The amino acid sequence of the peroxisomal CuZn-SOD was determined by Edman deg radation. Comparison of its sequence with those reported for other pla nt SODs revealed homologies of about 70% with cytosolic CuZn-SODs and of 90% with chloroplastic CuZn-SODs. The peroxisomal SOD has a high th ermal stability and resistance to inactivation by hydrogen peroxide. A polyclonal antibody was raised against peroxisomal CuZn-SOD, and by w estern blotting the antibody cross-reacted with plant CuZn-SODs but di d not recognize either plant Mn-SOD or bacterial Fe-SOD. The antiSOD-i mmunoglobulin G showed a weak cross-reaction with bovine erythrocytes and liver CuZn-SODs, and also with cell-free extracts from trout liver . The possible function of this CuZn-SOD in the oxidative metabolism o f peroxisomes is discussed.