The reality of oxidative stress is demonstrated by the existence, in all aerobic organisms, of several antioxidant enzymes devoted to ROS detoxification, such as peroxidase, catalase, superoxide dismutase, and peroxiredoxins. A number of small molecules also act as ROS scavengers. Unlike antioxidant enzymes, which catalyze the transformation of ROS into less toxic molecules, the concept of scavenger is that it must be an easily oxidizable target for ROS, and it must be present at concentrations high enough that the probability that a ROS reacts with the scavenger is higher than that of reacting with another target. When dealing with lipid oxidation (such as in rancidification of butter), the most important scavenger is probably vitamin E. As proteins with a sulfydryl group are often target of oxidation (-SH groups can be easily oxidized), small-molecular-weight thiols are potent scavengers.