Enzymatic reduction of disulfide bonds in lysosomes: Characterization of aGamma-interferon-inducible lysosomal thiol reductase (GILT)

Proteins internalized into the endocytic pathway are usually degraded. Effi cient proteolysis requires denaturation, induced by acidic conditions withi n lysosomes, and reduction of inter- and intrachain disulfide bonds. Cytoso lic reduction is mediated enzymatically by thioredoxin, but the mechanism o f lysosomal reduction is unknown, We describe here a lysosomal thiol reduct ase optimally active at low pH and capable of catalyzing disulfide bond red uction both in vivo and in vitro. The active site, determined by mutagenesi s, consists of a pair of cysteine residues separated by two amino acids, si milar to other enzymes of the thioredoxin family. The enzyme is a soluble g lycoprotein that is synthesized as a precursor. After delivery into the end osomal/lysosomal system by the mannose 6-phosphate receptor, N- and C-termi nal prosequences are removed. The enzyme is expressed constitutively in ant igen-presenting cells and induced by IFN-gamma in other cell types, suggest ing a potentially important role in antigen processing.