FACTORS INFLUENCING THE OXIDATION OF CYSTEAMINE AND OTHER THIOLS - IMPLICATIONS FOR HYPERTHERMIC SENSITIZATION AND RADIATION PROTECTION

Some of the factors influencing the O2 uptake and peroxide formation for cysteamine (MEA) and other thiols in serum-supplemented modified McCoy''s 5A, a well-known medium used to cultivate a variety of cells in vitro, were studied. The oxidation of MEA and cysteine in modified McCoy''s 5A has been compared with that in Ham''s F-12, MeM,m and phosphate-buffered saline. All of the growth media were supplemented with 10% calf serum and 5% fetal calf serum. The rate of O2 uptake for all of the studied thiols was greatest in McCoy''s 5A. This medium may contain more Cu than the other preparations. MEA and cysteine were more effective at 0.4 mM at producing peroxide than dithiothreitol (DTT). N-acetylcysteine was the least reactive. The ability to produce peroxide is dependent upon the temperature, the concentration of thiol, the presence of Cu ions, and pH of the medium. MEA and other thiol oxidation is inhibited by the Cu chelator diethyldithiocarbamate. Catalase also reduces the O2 uptake for all thiols. This inhibition involves the recycling of peroxide to O2. Superoxide dismutase (SOD) was found to stimulate the O2 uptake in the case of MEA and cysteine, but had little or no effect with DTT And glutathione. The combined presence of SOD and catalase resulted in less inhibition of O2 uptake than that obtained by catalase alone. Alkaline pH enhanced the oxidation of cysteine and MEA. An important observation was the inhibition of MEA oxidation at 0.degree. C and the stimulation at 42.degree. C. Many problems may arise when thiols are added to various media. A major consideration is concerned with the production of peroxide, superoxide, and reduced trace metal intermediates. The presence of these intermediates may result in the production of hydroxyl radical intermediates as well as the eventual O2 depletion from the medium. O2 depletion may alter the results of radiation sterilization and carcinogen activation. Radical production will cause cell damage that is temperature dependent. Therefore, careful consideration must be given to changes in O2 tension when thiols are added to cells growing in complicated growth medium to protect against either chemical or radiation damage.