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When N,N-dimethyl p-phenylenediamine is incubated with fresh frozen sections of rat heart muscle, there is a rapid, enzymatic, single electron oxidation of the diamine to form the pink Wurster's free radical with absorption maxima at 550 and 510 nm. This is followed by radical dimerization and other interactions to form a purple color accompanied by a new peak at 660-670 nm and finally a substantive blue dye. In contrast, with frozen sections of gingiva, instead of the pink color of the Wurster's free radical, one first sees a green color which slowly changes to blue. If heart muscle is kept at room temperature for 30-60 min or in a deep freeze for several days prior to use, it also exhibits formation of the green color. Spectrophotometric studies of the above tissue systems reveal that the new green color does not represent formation of a new compound with new absorption maxima. The same absorption maxima are present in each of the above systems; however, there are differences among the systems with respect to the rates of formation of the absorption maxima in different regions of the spectrum. The latter phenomena produce the color changes seen by the eye. It is suggested that the kinetics of Wurster's Red free radical interactions in tissue may be dependent upon the metabolic (redox) status of the tissue.

THE FATE OF WURSTER'S RED FREE RADICAL IN TISSUES WITH HIGH AND LOW OXYGEN UTILIZATIONS (HEART MUSCLE AND GINGIVA)