Catalyzes the reduction of nitrite to ammonia, consuming six electrons in the process (PubMed:16500161, PubMed:22281743). Has very low activity toward hydroxylamine (PubMed:16500161). Has even lower activity toward sulfite (PubMed:16500161, PubMed:22281743). Sulfite reductase activity is maximal at neutral pH (PubMed:20944237). 6 Fe(III)-[cytochrome c] + 2 H2O + NH4(+) = 6 Fe(II)-[cytochrome c] + 8 H(+) + nitrite Binds 1 Ca(2+) ion per monomer. Binds 8 heme groups covalently per monomer. Inhibited by azide and cyanide. Subject to competitive inhibition by sulfite. Optimum pH is 7-10 for nitrite reduction, and the optimum pH is 7 for sulfite reduction with only 20% residual activity at pH 7.8. Optimum temperature is 80 degrees Celsius. Nitrogen metabolism; nitrate reduction (assimilation). Homohexamer. Dimer of trimers. The thioether cross-link between Tyr-331 and Cys-333 may play a structural role in the active site cavity (PubMed:19393666). Besides, it may lower the pKa of the Tyr hydroxyl group (PubMed:19393666). An additional covalent bond between Tyr-331 and Gln-388 has been observed in some protein crystals, but this may be an artifact that is due to the formation of tyrosyl radicals when the protein is exposed to oxygen (PubMed:22281743). Belongs to the cytochrome c-552 family.