 Bay 60-7550 and oxindole lack the purine core but do possess a related ring system.Hydrogen bond acceptors, mostly nitrogen but also oxygen, reside in the ring system of the inhibitors.These atoms might interact with hydrogen bond donators, which are part of amino acids in the active site of the enzyme and thereby contribute to the inhibition of the enzyme from hydrolyzing cAMP and cGMP similar to how the natural substrates bind to the active site.