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5'-Adenylic acid can inhibit both lactic dehydrogenase and glutamic dehydro-genase; since both the phosphate and adenine moieties are necessary for good in-ibition, the 5'-adenylic acid presumably competes with DPN. This inhibition by 5'-adenylic acid can be duplicated by 9- (4'-carboxybutyl)-adenine (XVIII) on both enzymes, indicating that the ω-valeric acid side chain can simulate the binding of the 5-phosphoribosyl group when the latter is attached to adenine. 9-(para-Carboxy-benzyl)-adenine (XVI), a compound with a conformation-fixed side chain, can simulate the binding of 5'-adenylic acid to glutamic dehydrogenase but not lactic dehydrogenase; this difference has been attributed to the difference in conformation of 5'-adenylic acid (and DPN) when bound to the two enzymes.