Influence of coprecipitation and interactions with anionic surfactants on the activity of plant phospholipase D

The effects of two surfactants (sodium dodecyl: sulfate and sodium dodecyl dioxyethylene sulfate) of similar structure but differing water solubility (of their calcium salts) on the enzymatic activity of cabbage phospholipase D have been studied. The solubility difference is insignificant because th e two surfactants activate phospholipase D similarly. To elucidate the mech anism of their influence on the enzyme, the phase behavior in the:reaction media and the interactions of the surfactants with the enzyme were investig ated by potentiometry and by light scattering and UV spectroscopy. Calcium dodecyl dioxyethylene sulfate (which is more soluble in water than calcium dodecyl sulfate) precipitates in the: presence of phosphatidylcholine, the substrate of the enzymatic reaction. In the reaction media phospholipase D was involved into a precipitate consisting of calcium salts of the surfacta nts and phosphatidylcholine that might be interpreted as its immobilization . In addition, the surfactants were adsorbed on the enzyme, unfolding the g lobular enzyme molecule due to electrostatic repulsion between adsorbed sur factant anions. The observed increase in the functional activity of phospho lipase D is accounted for by transfer to an optimal tertiary structure for the enzyme molecule in the course of consecutive conformational transitions induced by the surfactants.