SUMMARY: Extracts made by crushing infected tobacco leaves in buffer solution containing sodium diethyldithiocarbamate (DIECA), centrifuging at 8000 and dialysing the supernatant fluids against dilute buffer, were used to study the inactivation of cucumber mosaic virus (CMV). Incubating such extracts with chlorogenic acid and copper inactivated them; inactivation was usually much less when they were incubated with chlorogenic acid alone. Inactivation did not occur or when DIECA was added. DIECA did not reactivate inactivated virus. CMV was inactivated rapidly by incubating with caffeic acid and copper, and slowly with catechol and copper, but not with five other phenols. The end-products of oxidation formed when chlorogenic acid was incubated with extracts of uninfected leaved did not inactivate CMV. Some features of the CMV-inactivating system are explained by the properties of tobacco leaf polyphenoloxidase. The compounds most effective in preserving the infectivity of CMV during extraction from leaves, DIECA and potassium ethylxanthate, are those which most strongly inhibit the enzyme. Conversely, the polyphenols that inactivate CMV are those oxidized most rapidly. Inconsistent effects of copper on the inactivating system can be partly explained by the different copper requirement of the polyphenoloxidase in extracts from plants infected for different times. Iron salts also accelerate the inactivation of CMV by chlorogenic acid. The process requires air and is prevented by DIECA; iron did not reactivate the DIECA-inhibited polyphenoloxidase. This system seems different from the one stimulated by copper. The concentration of iron in leaf extracts is usually less than that needed by the inactivating system.


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