Fig. 1. Pathways in the univalent reduction of oxygen to water leading to the formation of various intermediate reactive oxygen species (ROS). in the aging process. The evidence implicating ROS as key factors in determining longevity is accumulating. Much of the early data were correlative, however recent evidence has identified longevity-influencing genes responsive to ROS [13]. Aging is a multifactorial process; whether ROS are peripheral targets that correlate with longevity, or central regulators of aging remains to be resolved.

Another enzyme associated with oxygen-related free-radicals is catalase. Catalase action was first observed in plant and animal tissues by Thenard in 1818. He noted that such tissues readily degraded H2O2. Loew established that the degradation of H2O2 in tissues was caused by an individual enzyme, which he named ‘catalase’. Chance demonstrated that H2O2 is produced by respiring mitochondria while studying the reduction of H2O2 by this enzyme [14]. Warburg suggested that because it is inhibited by cyanide, catalase is an iron-containing enzyme and Zeile and Hellstrom later provided evidence for its hematin prosthetic group. Catalase was first purified and crystallized by Sumner and Dounce from beef liver [15].