A yeast model of 5-oxoproline accumulation reveals a general toleration to 5-oxoproline.
5-oxoproline (5-OP) or pyroglutamic acid is an intermediate in the degradation arc of the glutathione cycle. It is metabolized into glutamate through the action of the 5-oxoprolinase enzyme, the only enzyme known to act on this metabolite. 5-OP has long been known to be relatively inert with a proposed role as an osomoprotectant. Recent studies on the 5-oxoprolinase enzyme in mammalian cells have, however, shown that knockdown or deletion of 5-oxoprolinase makes mice (and humans) prone to heart failure, an effect ascribed to oxidative stress caused by a twofold increase in 5-OP. To examine the consequences of 5-oxoproline accumulation more rigorously, we created a yeast model for 5-oxoproline accumulation. Using this model, we observed retardation of growth only when intracellular levels of 5-OP were increased 12- to 20-fold over normal levels. Performing an analysis of transcriptomic changes under these conditions, we observed a large number of genes were differentially regulated and while there was no unifying dysregulated pathway, there was an upregulation of various efflux pumps. Ultimately, modulating the expression of these genes by knockout or overexpression highlighted that many of the upregulated genes were involved in the cellular response to 5-OP accumulation. However, our results failed to show any significant oxidative stress response. In conclusion, our study suggests a need to reevaluate previous suppositions of the 5-OP induced oxidative stress response and proposes alternate mechanisms for this effect.