Acid phosphatase associated with discharging secretory vesicles (mucocysts) of Tetrahymena thermophila.

The localization of acid phosphatase in discharging mucocysts of Tetrahymena thermophila is reported. Electron dense, lead phosphate enzyme reaction product is found associated with the fibrillar meshwork of secreting mucocysts and in cytoplasmic vesicles (e.g. lysosomes). Resting mucocysts, containing highly condensed contents, exhibit no label. The specificity of the stain was controlled by reaction media without exogenous substrate and reaction media containing the inhibitor sodium fluoride. No lead phosphate deposits were found in these controls. Secreting organelle contents have no preferential affinity to lead phosphate as shown by tests with reaction medium lacking substrate and subsequent incubation in phosphate containing medium.