Glutathione Reductase from Lactobacillus sanfranciscensis DSM20451 T : Contribution to Oxygen Tolerance and Thiol Exchange Reactions in Wheat Sourdoughs

Abstract

ABSTRACT 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 Δ 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 Δ 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 μM to 10.5 μM sulfhydryl/g of dough during a 24-h sourdough fermentation, but in sourdoughs fermented with L. sanfranciscensis DSM20451 T Δ gshR and in chemically acidified doughs, the thiol levels decreased to 6.5 to 6.8 μM 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 Δ 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.