Rapid and sensitive assays for phagosomal acidification in Paramecium and Tetrahymena

Biochemical and cytochemical procedures were developed to measure the rate of phagosomal acidification for phagosomal pH ranging from 5 to 2.5. These assays were based on the pH-dependent inactivation with time of horseradish peroxidase (HRP) activity, a result attributable to the dissociation of this enzyme to a colorless protein and ferriprotoporphyrin in acidic solutions. When preincubated in buffers of varying pH, the rate of HRP inactivation followed a sigmoid curve, with the highest rate of inactivation between 4.3 and 3.5 when using citrate-phosphate buffer and between pH 3.4 and 2.8 when using the universal ABC buffer. This inactivation was temperature but not concentration dependent. When Paramecium caudatum, members of the P. aurelia complex or Tetrahymena thermophila was pulsed briefly with HRP and small fluorescent beads, the loss of HRP activity, measured biochemically in cell homogenates and/or cytochemically in phagosomes, was rapid and followed the kinetics of a first-order rate reaction. Both assays gave similar values for the rate constant for acidification and similar rates of inhibition when P. caudatum was exposed to a proton ionophore, carbonyl cyanide p-trifluoromethoxyphenyl hydrazone. These assays can readily be adapted to other phagocytic cells as long as a rapid procedure is available for removing all unphagocytosed HRP and latex beads. These procedures are sensitive and rapid thus allowing many samples to be quickly prepared and analyzed.