LIFE LIVES WITH HELP OF OXYGEN AND DIES BECAUSE OF OXYGEN

The earth was born 4.6 billion years ago. Immediately after the earth was born, no life existed on the planet, as it was fireball of several thousand degrees. Now an abundance of life overflows on this planet. Innumerable animals, plants, birds, fish, insects, and microorganisms exist, and the population of Homo sapiens is six billion. After one billion years had passed since the birth of the earth, its surface temperature had cooled, and prokaryotic organisms with the ability to self-replicate emerged. Later, eukaryotes, which have mitochondria-specialized structures for utilizing oxygen in the atmosphere efficiently-emerged, and so life began to evolve, prosper, and become predominant on the earth as a result of high aerobic-energy production. Therefore, oxygen is indispensable for life.

However, one electron of oxygen is easily reduced in the cell, and approximately two percent of the utilized oxygen may become reactive oxygen species (ROS). ROS include superoxide, hydrogen peroxide (H₂O₂), and the OH radical, which oxidize and damage DNA. This oxidative damage can cause various injuries, including diabetes, heart disease, cataracts, emphysema, cancers, and other conditions in humans and can also lead to premature aging and death. Life is bound to death, because life requires oxygen. Living organisms have acquired enzymes such as superoxide dismutase (SOD), catalase, and others that can scavenge ROS.

           ROS in the form of the OH radical are not easily eliminated in the living body. Today we live in an environment in which OH radicals are generated easily. This is because a great number of synthetic chemicals that humankind developed in the latter half of the twentieth century are often sources of ROS. Environmental carcinogens, dioxins, and endocrine-disrupters, are examples of these manmade chemicals.

           My laboratory has developed a screening method, called the kat-sod assay, which detects ROS generated in the cell (Nishioka and Hayashi 1995). With this method, we have found that a number of synthetic chemicals in our environment are toxic because they initiate the generation of ROS in living cells (Kimura and Nishioka 1996; Yonezawa, Yamamoto, and Nishioka 1997; Yonezawa, Miyakoshi, Nishioka, et al. 1999; Yonezawa and Nishioka 1999a, 1999b).

           The life lives with the help of oxygen, and dies because of oxygen.