Oxidation of thiols in the Ca2+-ATPase of sarcoplasmic reticulum microsomes.

We recently showed that oxidative stress impairs the function of the sarcoplasmic reticulum to transport and retain calcium. Inhibition results primarily from oxidation of one or more thiol groups in the Ca2+-ATPase. We now report that thiol oxidation does not result in disulfide formation. Oxidative inhibition of Ca2+-ATPase activity was not reversed by dithiothreitol. Also, arsenite, which crosslinks dithiols, only mildly inhibited Ca2+-ATPase activity and protected against inhibition by peroxydisulfate. These data suggest the thiols susceptible to oxidation are not spatially close enough to form a disulfide. Furthermore, these thiols appear to be involved in some aspect of phosphoenzyme formation. ATP, in the presence of calcium and magnesium, protected against inhibition of Ca2+-ATPase activity by both oxidants and thiol-binding agents. Both inhibitors also decreased binding of the nucleotide analogue TNP-AMP after phosphorylation by Pi. Dithiothreitol and arsenite were protective. In conclusion, reversible redox regulation of the Ca2+-ATPase of sarcoplasmic reticulum by thiol-disulfide exchange does not occur. However, some other mechanism of redox regulation may operate because the enzyme is sensitive to oxidants, thiol-binding agents and activity can be enhanced by prolonged exposure to dithiothreitol.