EPR SPIN-TRAPPING OF FREE-RADICALS PRODUCED BY BLEOMYCIN AND ASCORBATE

In the presence of ascorbate, bleomycin (BLM) is converted to a redox- inactive form that is incapable of inducing DNA strand scission. We ha ve employed EPR spin trapping with 5,5-dimethylpyrroline-1-oxide (DMPO ) to examine free radical production during this process. The introduc tion of ascorbate to an Fe(III)BLM-DMPO system results in the formatio n of three EPR observable free radicals. One of these radicals is the resonance-stabilized ascorbate free radical (a(H) = 1.8 G) that is not spin trapped by DMPO; the other two are the result of DMPO spin trapp ing. These radicals appear to be two carbon-centered radicals, DMPO/.C R1, (a1N = 15.75 G, a1H = 22.30G, a(N)/a(H) = 0.706) and DMPO/.CR2 (a2 N = 15.20 G, a2H = 19.20 G, a(N)/a(H) = 0.79). Although it is not poss ible to identify the exact structures of the carbon-centered radicals that are spin trapped, the hyperfine splittings, as well as the a(N)/a (H) values, are characteristic of the presence of electron-withdrawing groups, such as the oxygen atom when attached to the carbon atom. In fact, these parameters are characteristic of DMPO spin trapping result s obtained when sugars are subjected to oxidative insult from HO.. Thu s, these BLM-ascorbate produced radicals may well be derived from the sugar moiety of BLM.