The surface charge of Toxoplasma gondii: a cytochemical and electrophoretic study
Cintra, W.M.; Silva-Filho, F.C.; De Souza, W.
Journal of Submicroscopic Cytology 18(4): 773-781
1986
ISSN/ISBN: 0022-4782 PMID: 2431157 Document Number: 286051
The surface charge of Toxoplasma gondiii tachyzoites was evaluated by means of binding of colloidal iron hydroxide particles at pH 1.8, cationized ferritin particles at pH 7.2 and ruthenium red to the cell surface, as visualized by electron microscopy and by direct measurements of the electrophoretic mobility (EPM) of the cells suspended in solutions of different ionic strength and pH. At pH 7.2, T. gondii has a negative surface charge with a mean EPM of -1.1272 .+-. 0.0917 .mu.m s-1 V-1 cm. No significant difference was observed between the EPM of living cells at 25.degree. C and that of glutaraldehyde-fixed cells. At lower pH, there is a decrease in the negative surface charge, with an isoelectric point at pH 3.5. At higher pH (> 10), there is an increase in the surface charge reaching an EPM of -1.5675 .+-. 0.0848 .mu.s-1 V-1 cm at pH 7.2. These results indicate that the surface of T. gondii contains both negatively and positively charged dissociating groups. Binding of cationized ferritin particles and ruthenium red throughout the cell surface of glutaraldehyde-fixed cells was observed. However, when living parasites were incubated at 4.degree. C in the presence of cationized ferritin some cells showed a uniform distribution of the label, others showed a patch-distribution and still in others no label was seen, indicating a process of mobility and shedding of surface anionic sites. Colloidal iron hydroxyde particles did not bind to the surface of T. gondii. Incubation of the parasites in the presence of neuraminidase from Clostridium perfringens or Vibrio cholerae or in the presence of proteolytic enzymes (trypsin or protease) did not interfere with the surface charge. Incubation in the presence of phospholipase C reduced by about 20% the mean EPM of the parasites. These observations indicate that tachyzoites of T. gondii possess surface anionic components and that there are no sialic acid residues exposed on the parasite's surface.