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Volume Regulation in Poterioochromonas¹

Involvement of Calmodulin in the Ca²⁺-Stimulated Activation of Isofloridoside-Phosphate Synthase

Fachbereich Biologie der Universität, Postfach 3049, 6750 Kaiserslautern, Federal Republic of Germany

In Poterioochromonas malhamensis Peterfi (syn. Ochromonas malhamensis Pringsheim) osmotically induced shrinkage is reversed by an accumulation of isofloridoside. Addition of Ca²⁺ ions to homogenates from standard volume cells initiates an enzyme system for the activation of isofloridoside-phosphate synthase. This process is stimulated in the presence of Ca²⁺ by calmodulin, isolated from the same alga or from bovine brain, and requires the presence of membranes. The stimulation observed when Ca²⁺ is added without exogenous calmodulin is inhibited by the calmodulin-binding substance R 24571. These results show that the effect of Ca²⁺ is mediated by calmodulin. The Ca²⁺/calmodulin-dependent activation is enhanced when fluoride or molybdate ions are present in the homogenization buffer. This might indicate the involvement of a phosphorylated compound in the activation mechanism.

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