Apo-Bd1971 was supplemented with 10 mM MgCl2 or CaCl2 in conjunction with saturating concentrations (2.5 mM) of cAMP, cGMP or no nucleotide and phosphodiesterase activity with respect to the generic substrate PNPP (5 mM) was measured spectrophotometrically. Values are the average of three repeats, error bars represent standard deviation. 