SUPEROXIDE PRODUCED IN THE HEME POCKET OF THE BETA-CHAIN OF HEMOGLOBIN REACTS WITH THE BETA-93 CYSTEINE TO PRODUCE A THIYL RADICAL

The role of the beta-93 cysteine residue in the hemoglobin autoxidatio n process has been delineated by electron paramagnetic resonance. At l ow temperatures (8 K) after incubation at 235 K, free radical signals were detected. An analysis of the free radical spectrum produced impli es that, besides the superoxide radical expected to be formed during a utoxidation, an isotropic free radical is produced with a g(iso) of 2. 0133. This g value is consistent with that expected for a sulfur radic al. Blocking the beta-93 sulfhydryl group with N-ethylmaleimide was fo und to eliminate the formation of the isotropic radical, but not the s uperoxide. This finding confirms the assignment of the isotropic radic al as a thiyl radical originating from the oxidation of the cysteine S H group. A kinetic analysis of the time course for the formation of bo th the superoxide and thiyl radicals is consistent with a reversible e lectron transfer process between superoxide in the heme pocket of the beta-chains and the cysteine residue. This reaction is expected to pro duce both a thiyl radical and a peroxide. Direct evidence for peroxide production comes from the detection of a transient Fe(III) heme perox ide complex. The significance of the electron transfer process produci ng a thiyl radical is discussed. It is shown that the formation of the thiyl radical decreases the rate of autoxidation for the beta-chain a nd reduces heme degradation attributed to the reaction of superoxide w ith the heme. The insights gained from these low-temperature studies a re believed to be relevant to room-temperature autoxidation.