Effect of oxygen free radicals on myosin in muscle fibres

Experiments were performed on glycerol-extracted muscle fibres prepared from psoas muscle of rabbit in the presence of hydroxyl free radical generating system. Short irradiation of spin-labelled muscle fibres by UV light showed the interaction of probe molecules with oxygen free radicals. The intensity of the EPR signal from maleimide or isothiocyanate spin labels attached to the essential thiol groups decreased following irradiation. Oxygen free radicals affected the rate constant of the transition AM.ADP.Vi-->AM.ADP in the ATP hydrolysis cycle. It was found that the essential -SH groups of myosin were involved in the oxidation of sulphydryls by Ce(IV). Ce(IV) complexed to nitrilotriacetic acid in the presence of spin trap produced long-lived free radicals located partly on SH-1 sulphydryls.

Spin labeling of protein sulfhydryl groups by spin trapping a sulfur radical: application to bovine serum albumin and myosin

Reaction of sulfhydryl-containing compounds, RSH, with Ce4+ in the presence of the spin trap phenyl-N-t-butylnitrone results in the appearance of a nitroxide ESR spectrum, which is greatly diminished if the sulfhydryl group is blocked prior to reaction. The spectra have short lifetimes which can be increased two- to fivefold to half-lives of 5-60 min by prior flushing of the solutions with nitrogen. For small molecules, such as cysteine, N-acetylcysteine, glutathione, and 2-mercaptoethanol, the spectrum is that of a freely rotating nitroxide while for the proteins, bovine serum albumin and myosin, the spectrum is characteristic of a strongly immobilized nitroxide spin label rigidly attached to the protein. Since Ce4+ is reported to oxidize the sulfhydryl group via the thiyl radical, RS, the following reactions are proposed to account for the formation of the nitroxide: (formula; see text) These reactions permit the spin labeling of sulfhydryl proteins such that the nitroxide is much closer to the point of attachment than when using conventional spin-labeling methods.