Catalyzes the hydrolysis of methenyl-H(4)MPT(+) to 5-formyl-H(4)MPT. 5,10-methenyl-5,6,7,8-tetrahydromethanopterin + H2O = H(+) + N(5)-formyl-5,6,7,8-tetrahydromethanopterin Optimum temperature is 85 degrees Celsius. Requires a high salt concentration for optimal thermostability at 90 degrees Celsius. Metabolic intermediate metabolism; lactate oxidation. Homodimer. Belongs to the MCH family.