Micrococcus radiodurans surface exonuclease. Dimer to monomer conversion by ionizing radiation-generated aqueous free radicals.

Micrococcus radiodurans possesses an exonuclease firmly bound to a middle cell wall membrane layer. Aqueous OH. radicals generated chemically or by ionizing radiation cause the immediate release of this enzyme into the surrounding medium. The enzyme is located in a hydrophobic site and can also be released by aqueous n-butanol. When extracted by this solvent it is a non-covalently linker dimer and has a molecular weight of 260 000 as determined by gel filtration. When released by radiation generated OH. radicals, the enzyme initially appears in solution as the dimer but is rapidly split by further aqueous radical attack into two 130 000 molecular weight subunits. Hydroxyl radicals are most effective but reducing radicals are also able to monomerize the enzyme. Only the released dimer enzyme is subject to free radical monomerization. Bound dimer enzyme is not split prior to release. No detectable loss of activity or change in catalytic properties accompanies the free radical cleavage of the enzyme. Both subunits of the dimer enzyme possess a tightly bound metal ion (probably Ca2+) required for activity. The monomer but not the dimer enzyme will bind to an anion exchanger. The monomer is susceptible to loss of its metal ion, and consequent inactivation, when exposed to the exchanger in the absence of Ca2+. Besides providing information on some of the immediate non-lethal effects of ionizing radiation, the behavior of this enzyme system demonstrates a potential cellular mechanism by which internally or externally generated free radicals could be utilized by the cell to control various enzymic reactions.