Catalyses the isomerisation of D-galactaro-1,5-lactone to D-galactaro-1,4-lactone. This is a step in the oxidative degradation pathway of D-galacturonate, which allows Agrobacterium tumefaciens to utilise D-galacturonate as a sole carbon source. This enzyme belongs to the amidohydrolase superfamily. Unlike many members of the amidohydrolase superfamily, this protein does not require a divalent metal for its lactonase activity