 Chemical reduction of PDE5 relieves autoinhibition of enzyme functions; allosteric cGMP-binding activity is increased 10-fold, and catalytic activity is increased ~3-fold.The redox effect on allosteric cGMP-binding occurs in the isolated regulatory domain.A change in the state of reduction of PDE5 or the isolated regulatory domain is associated with an apparent conformational change similar to that caused by phosphorylation.