Glutathione reductase from Lactobacillus sanfranciscensis DSM20451T:: Contribution to oxygen tolerance and thiol exchange reactions in wheat sourdoughs

The effect of the glutathione reductase (GshR) activity of Lactobacillus sanfranciscensis DSM20451(T) on the thiol levels in fermented sourdoughs was determined, and the oxygen tolerance of the strain was also determined. The gshR gene coding for a putative GshR was sequenced and inactivated by single-crossover integration to yield strain L. sanfranciscensis DSM20451(T) Delta gshR. The gene disruption was verified by sequencing the truncated gshR and surrounding regions on the chromosome. The gshR activity of L. sanfranciscensis DSM20451(T) Delta gshR was strongly reduced compared to that of the wild-type strain, demonstrating that gshR indeed encodes an active GshR enzyme. The thiol levels in wheat doughs fermented with L. sanfranciscensis DSM20451 increased from 9 mu M to 10.5 mu M sulfhydryl/g of dough during a 24-h sourdough fermentation, but in sourdoughs fermented with L. sanfranciscensis DSM20451(T) Delta gshR and in chemically acidified doughs, the thiol levels decreased to 6.5 to 6.8 mu M sulfhydryl/g of dough. Remarkably, the GshR-negative strains Lactobacillus pontis LTH2587 and Lactobacillus reuteri BR11 exerted effects on thiol levels in dough comparable to those of L. sanfranciscensis. In addition to the effect on thiol levels in sourdough, the loss of GshR activity in L. sanfranciscensis DSM20451(T) Delta gshR resulted in a loss of oxygen tolerance. The gshR mutant strain exhibited a strongly decreased aerobic growth rate on modified MRS medium compared to either the growth rate under anaerobic conditions or that of the wild-type strain, and aerobic growth was restored by the addition of cysteine. Moreover, the gshR mutant strain was more sensitive to the superoxide-generating agent paraquat.