Involved in defense against toxic arsenate. Involved in the mycothiol/myoredoxin redox pathway which uses a mycothioltransferase mechanism; functions as a monothiol mixed disulfide reductase and is recycled by a second mycothiol forming mycothione which in turn is reduced in a NADPH-dependent manner. [mycoredoxin]-L-cysteine + arseno-mycothiol + H(+) = [mycoredoxin]-S-mycothiol-L-cysteine + arsenite Belongs to the glutaredoxin family.