Furin proteolytically processes the heparin-binding region of extracellular superoxide dismutase

Extracellular superoxide dismutase (EC-SOD) is an antioxidant enzyme that attenuates brain and lung injury from oxidative stress. A polybasic region in the carboxyl terminus distinguishes EC-SOD from other superoxide dismutases and determines EC-SOD's tissue half-life and affinity for heparin. There are two types of EC-SOD that differ based on the presence or absence of this heparin-binding region. It has recently been shown that proteolytic removal of the heparin-binding region is an intracellular event (Enghild, J. J., Thogersen, I. B., Oury, T. D., Valnickova, Z., Hojrup, P., and Crapo, J. D. (1999) J. Biol. Chem. 274, 14818-14822). By using mammalian cell lines, we have now determined that removal of the heparin-binding region occurs after passage through the Golgi network but before being secreted into the extracellular space. Specific protease inhibitors and overexpression of intracellular proteases implicate furin as a processing protease. In vitro experiments using furin and purified EC-SOD suggest that furin proteolytically cleaves EC-SOD in the middle of the polybasic region and then requires an additional carboxypeptidase to remove the remaining lysines and arginines. A mutation in Arg(213) renders EC-SOD resistant to furin processing. These results indicate that furin-dependent processing of EC-SOD is important for determining the tissue distribution and half-life of EC-SOD.