Volume regulation in poterioochromonas: involvement of calmodulin in the ca-stimulated activation of isofloridoside-phosphate synthase

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.