Peptidylarginine deiminase, which catalyzes the deimination of arginyl residues in protein, required Ca²⁺ as an essential cofactor and the half-maximal activity was attained at 40 - 60 μM Ca²⁺. Other divalent cations were practically inactive except for Sr²⁺, which was about 50% as active as Ca²⁺ when tested at lOmM. However, Sr²⁺ at less than the concentration of 100 μM had little or no activity. The direct Ca²⁺ -binding for the enzyme showed a sigmoidal curve with a transition midpoint of about 110μM, indicating that the binding is cooperative. Analysis of Hill plots of the data revealed that the enzyme binds 3mol of Ca²⁺ /mol of protein with an apparent dissociation constant of 110μM. A conformational change upon Ca²⁺ -binding was also described for the enzyme using UV-diiference spectra. The alteration could be attributed to an increased exposure of the aromatic residues to a more aqueous environment, as has been described for Ca²⁺- bindingproteins such as calmodulin. Phosphatidylserine enhanced the reaction velocity and concomitantly reduced the Ca²⁺ -requirement for the enzyme. These effects were stimulated by the addition of diacylglycerol. Diacylglycerol alone had little or no effect. On the other hand, calmodulin had no effect on the enzymatic activity over a wide range of Ca²⁺ concentrations. These suggest that the activity and Ca²⁺ -sensitivity of Peptidylarginine deiminase is increased at the cell membrane.

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