Adrenalin autoxidation and Cu 2Zn 2 superoxide dismutase

Abstract

Superoxide dismutase is known to specifically inhibit adrenalin autoxidation. In contrast noradrenalin and dihydroxyphenylalanine autoxidation reactions are inhibited by 20% only. To account for this phenomenon the formation of a specific ternary complex between oxidizing adrenalin and the copperzinc protein was deduced from extensive circular dichroism measurements. Contrary to observations with the latter catecholamines, characteristic changes of the chiroptical properties of both adrenalin in the presence of oxygen and the protein moiety of superoxide dismutase were seen. The changes of optical properties could not be observed, when the enzyme devoid of its active site or the partially reconstituted apoprotein employing either zinc or copper, reacted with adrenalin. This suggested the histidine-bridged copperzinc active site to be essential for the protein's specific reactiviy. This conclusion is supported by 360 MHz proton nuclear magnetic resonance measurements, which revealed an induced chemical shift of histidine resonance by the adrenalinsuperoxide dismutase binding. X-ray photoelectron spectrometric measurements support another aspect of the specific inhibitory action of superoxide dismutase. An unusual low nitrogen 1s-electron binding energy of adrenalin compared to other catecholamines can be explanatory for the fact that borate inhibited adrenalin cannot prevent the formation of the ternary complex with the enzyme. Therefore binding of adrenalin to the protein via the ethanol-amine side chain must be made responsible for an autoxidation pathway in the presence of superoxide dismutase.