A Bloodstream Trypanosoma congolense Sialidase Could Be Involved in Anemia during Experimental Trypanosomiasis

The release of Sialic acid (SA) into the serum by Trypanosoma congolense infected BalbC mice was investigated. A progressive increase in the level of serum SA corresponding to anemia and parasitemia was observed. At maximum parasitemia, the level of total SA from the red blood cells (RBC) dropped by about 45%. Solved polynomials revealed an association between free serum SA and RBC-SA. Positive roots of quadratics were used to predict complete cleavage of RBC-SA on day 7.01 and maximum accumulation of free serum SA on day 6.6. A steady rise in the level of serum sialidase (SD) activity and a low packed cell volume (PCV) with an increase in parasitemia were observed. Mice infused with galactose, methyl-β-gal, lactose, mannose, or l-arabinose and challenged by intraperitoneal inoculation with Trypanosoma congolense neither developed anemia nor secreted free SA above the control level even though there was detectable SD activity. Bloodstream Trypanosoma congolense parasites were isolated using DEAE cellulose from heparinized blood of experimentally infected BalbC mice. The parasites were lysed with 0.2% Triton-CF 54 to release membrane bound SD. The activity of the SD was proportional to the number of parasites. The enzyme was partially purified on Q-Sepharose and Fetuin agarose columns successively. The final active fraction from the latter column was used as the partially purified SD. The enzyme had an optimum pH of 6 and was maximally active at 37°C with a requirement for the divalent ions Ca2+ and Mg2+. The enzyme was highly specific for NeuAc5α2,3 lac and Methylumbelliferyl-Neu5Ac (4-MU-Neu5Ac) with KM values of 0.34 and 0.025 mM, respectively. It was inhibited competitively by 2,3-didehydroneuraminic acid (Neu5Ac2en) and para-nitro-phenyloxamic acid (pNPO) with inhibition binding constants Ki of 65 and 215 µM, respectively. In deviation from the procyclic trypanosomal SD, it lacked trans-sialidase (TS) activity. The possible role of a secreted bloodstream Trypanosoma congolense SD and the development of anemia in the pathogensesis of trypanosomiasis are discussed.