This enzyme catalyzes the hydrogen sulfide production from sulfite. It is strictly anaerobic. It is regulated by electron acceptors rather than by cysteine. 3 H2O + hydrogen sulfide + 3 NAD(+) = 4 H(+) + 3 NADH + sulfite Binds 3 [4Fe-4S] clusters per subunit. Binds 1 siroheme per subunit. Sulfur metabolism; sulfite reduction. The anaerobic sulfite reductase seems to consist of three subunits. By sulfite. Belongs to the nitrite and sulfite reductase 4Fe-4S domain family.