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1. Some effects of anions on the rates of phosphoarginine synthesis by monomeric (lobster) and by dimeric (Holothuria forskali) arginine kinases are reported. 2. As with creatine kinase, acetate ions activate both enzymes: Cl- was also found to activate both although this was an inhibitor of creatine kinase. 3. NO3- inhibits the lobster enzyme. Inhibition is of the mixed type with respect to MgATP. Ki greater than Ki' and Ks greater than Ks' indicating that the presence of NO3- promotes the binding of substrate and vice versa. 4. NO3- alone has no effect on the difference spectrum of the lobster enzyme but in the presence of arginine, MgATP, MgADP, MgAMP or MgIDP the difference spectrum is greatly enhanced. A profound effect on the ionization state of tyrosine residues is inferred. 5. With the Holothuria enzyme low concentrations of NO3- activate in a manner that is competitive with arginine. Higher concentrations cause inhibition of the mixed type with respect to arginine in a similar manner to that found with MgATP for the lobster kinase. 6. Of a range of anions tested only NO3- and NO2- enhanced the inhibition of enzyme activity by MgADP, indicating the formation of a pseudo-transition-state dead-end complex, enzyme-arginine-NO3--MgADP. The effect was essentially independent of temperature with the Holothuria enzyme, but with the lobster enzyme was much less marked and temperature dependent. The difference may reflect the different stabilities of the monomer and dimer enzymes, although with neither arginine kinase is the stabilization of the dead-end complex as marked as is found with creatinine kinase.

Effects of anions on a monomeric and a dimeric arginine kinase