Article

The effect of Ca²⁺ and some divalent metal ions related to it (Mg, Sr, Ba, Cd) on the hydrolysis of natural and artificial substrates catalyzed by vipoxin secreted phospholipase A₂ subunit (sPLA₂) was evaluated. The results showed that the hydrolysis of natural glycerophospholipids proceeds at a highest rate when the enzyme is activated by calcium ions. The catalytic activity of sPLA₂ decreased in the presence of other metal ions possibly due to their lower coordination ability to the head group of lipids. Oppositely, the hydrolysis of artificial substrates was facilitated by metal ions with ionic radii larger than Ca²⁺, such as Sr²⁺ and Ba²⁺,suggesting that not only the interaction with metal cations, but also the origin of the head group should be considered. The fluorescence assay revealed that conformational changes occur during the coordination of metal ions into the catalytic site of sPLA₂ prior to subsequent hydrolysis. It could be concluded that the rate-limiting step in the catalytic cycle (e.g. the chemical interaction) is not completely adequate to evaluate all factors affecting the hydrolytic activity of vipoxin sPLA₂.