Catalysis of the reaction: 7alpha,12alpha-dihydroxy-3-oxochol-4-en-24-oate = 12alpha-hydroxy-3-oxochola-4,6-dien-24-oate + H2O.
7alpha,12alpha-dihydroxy-3-oxochol-4-enoate hydro-lyase (12alpha-hydroxy-3-oxochola-4,6-dienoate-forming) activity