Superoxide dismutase is a naturally occurring enzyme found mainly in liver as a protein chelate of copper and zinc. Its mode of action is to scavenge free radicals such as peroxides from the body. Normal metabolic reactions, exposure to x-rays, chemicals, and UV light from the sun; and the inhalation or ingestion of pollutants results in the formation of these highly reactive molecules. These molecules can and do react with tissues to bring about cross-linking of the DNA as well as of the structural and enzyme proteins. Since these reactions cause such non-specific damage, the entire process is generally categorized as "aging".