Photodynamic action of proflavine on coliphage T3. II. Protection by L-cysteine

Three kinetically different reactions (Rx1, Rx2, and Rx3) have been distinguished in the photoinactivation of phage T3 in the presence of the dye proflavine. The response of these reactions to the presence of the radical trap l-cysteine has been examined. At dye concentrations equal to or less than 2.2 mug/ml, Rx1 was composed of at least two parallel first-order reactions, one cysteine-insensitive (Rx1A) and one cysteine-inhibited (Rx1B). Rx2 was completely cysteine-insensitive (Rx2A). The cysteine sensitivity of these reactions changed abruptly at dye concentrations above 2.2 mug/ml. Rx1A and Rx1B now operated in tandem, rather than simultaneously, with Rx1B being confined to the first 1 min at most. Rx2, on the other hand, was completely cysteine-inhibited (Rx2B). Rx3 was inhibited roughly 75 to 80% by saturating concentrations of cysteine regardless of the time of addition of cysteine. The dark inactivation associated with Rx3 was inhibited roughly 85% whether the radical trap was added during the light or dark regimes. Changes of initial phage titer did not alter the cysteine sensitivity of a reaction.