Heme-containing oxidoreductase which catalyzes the conversion of thiocyanate (SCN(-)) into antimicrobial agent hypothiocyanous acid (OSCN(-)) in the presence of hydrogen peroxide (H2O2). Also involved in the conversion of iodide (I(-)) into hypoiodite (IO(-)) in the presence of H2O2 (By similarity). Responsible for the inactivation of a wide range of micro-organisms and hence, important component of defense mechanism. May be implicated in airway host defense against infection (By similarity). May contribute to maintaining an appropriate H2O2 cellular level, therefore protecting cells from H2O2-caused injuries and inflammation (By similarity). 2 a phenolic donor + H2O2 = 2 a phenolic radical donor + 2 H2O H(+) + H2O2 + thiocyanate = H2O + hypothiocyanous acid H2O2 + iodide = H2O + hypoiodite Binds 1 Ca(2+) ion per heterodimer. Binds 1 heme b (iron(II)-protoporphyrin IX) group covalently per heterodimer. Secreted by the lacrimal gland into tears. Expressed in the lacrimal gland with higher levels and 3-fold higher activity in adult females than males and secreted into tears (at protein level). Repressed by the androgen dihydrotestosterone (DHT) and the estrogen estradiol (E2) (at protein level). Thiocyanate (SCN(-)) and hypothiocyanite (OSCN(-)) are bound in the distal heme cavity. The iodide ion (I(-)) occupies a position which is stabilized by the interactions with heme moiety, His-224, Arg-370 and Glu-373. Hydrogen peroxide is held between the heme iron and His-224. Belongs to the peroxidase family. XPO subfamily.