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We identified and functionally characterized yeast fRMsr homolog which showed specificity for reduction of free Met-R-SO and contributed to oxidative stress resistance of yeast cells. We further identified three conserved Cys which participate in catalysis through disulfide exchange. Evolutionary analyses revealed that the occurrence of fRMSr is restricted to unicellular organisms. We further found that mammalian MsrA can reduce free Met-S-SO, whereas MsrBs do not reduce free Met-R-SO. Consistent with these findings and the lack of fRMsr, mammalian cells could not grow in media that replaced Met with Met-R-SO. However, they grew in the presence of free Met-S-SO, which was reduced by MsrA. Expression of yeast fRMsr in mammalian cells supported the growth of mammalian cells on Met-R-SO, increased resistamce to oxidative stress, and affected expression levels of proteins that are regulated by Met availability.
We extended the finding of deficiency in free Met-R-SO reduction in mammals to examine the ability of Msrs to reduce compounds containing methylsulfinyls. Methylsulfide group has a prochiral sulfur, resulting in two diastereomers upon sulfoxidation. We found that methylsulfinyls in sulmazole sulfoxide, triclabendazole sulfoxide, and mesoridazine could be stereospecifically reduced by MsrA, but MsrB and fRMsr were not active. We also found that dimethylsulfoxide (DMSO), which has a methylsulfinyl group, can only be reduced by MsrA.
To examine the role of Met in aging, we performed lifespan analyses in fruit flies. There was no difference in lifespan of flies maintained on restricted (0.1 mM Met) and regular (1 mM) Met diets. Instead, we observed uncoupling between reproduction and longevity. A popular theory of aging suggests that dietary restriction is inversely proportional to reproduction due to nutritional redistribution of resources from egg generation to somatic maintenance, but we found that this theory did not follow our data that are based on variation in Met concentration.
Overall, our studies characterized all three known types of methionine sulfoxide reductase and uncovered biological significance of free methionine sulfoxide reduction and its deficiency in mammals.