Involved in an anaerobic respiration pathway that converts the sulfonate isethionate (2-hydroxyethanesulfonate) to ammonia, acetate and sulfide. Catalyzes the radical-mediated C-S bond cleavage of isethionate (2-hydroxyethanesulfonate) to form sulfite and acetaldehyde. 2-hydroxyethane-1-sulfonate = acetaldehyde + H(+) + sulfite kcat is 12 sec(-1). Organosulfur degradation; alkanesulfonate degradation. Homodimer. Highly up-regulated in the presence of isethionate. Requires the activating protein IslB to generate the key active site glycyl radical on Gly-806 that is involved in catalysis. Belongs to the glycyl radical enzyme (GRE) family.