Volume Regulation in Poterioochromonas

In Poterioochromonas malhamensis Peterfi (syn. Ochromonas malhamensis Pringsheim) osmotically induced shrinkage is reversed by an accumulation of isofloridoside. Addition of Ca(2+) 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 Ca(2+) by calmodulin, isolated from the same alga or from bovine brain, and requires the presence of membranes. The stimulation observed when Ca(2+) is added without exogenous calmodulin is inhibited by the calmodulin-binding substance R 24571. These results show that the effect of Ca(2+) is mediated by calmodulin. The Ca(2+)/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.