IDENTIFICATION OF HAPTOGLOBIN AS A NATURAL INHIBITOR OF TRYPANOCIDAL ACTIVITY IN HUMAN SERUM

Trypanosomes are protozoan parasites of medical and veterinary importa nce. Trypanosoma brucei rhodesiense and Trypanosoma brucei gambiense i nfect humans, causing African sleeping sickness. However, Trypanosoma brucei brucei can only infect animals, causing the disease Nagana in c attle, Man is protected from this subspecies of trypanosomes by a toxi c subtype of high density lipoproteins (HDLs) called the trypanosome l ytic factor (TLF). The toxic molecule in TLF is believed to be the hap toglobin-related protein that when bound to hemoglobin kills the trypa nosome via oxidative damage initiated by its peroxidase activity, The amount of lytic activity in serum varies widely between different indi viduals with up to a 60-fold difference in activity, In addition, an i ncrease in the total amount of lytic activity occurs during the purifi cation of TLF, suggesting that an inhibitor of TLF (I-TLF) exists in h uman serum, We now show that the individual variation in trypanosome l ytic activity in serum correlates to variations in the amount of I-TLF . Immunoblots of I-TLF probed with antiserum against haptoglobin recog nize a 120-kDa protein, indicating that haptoglobin is present in part ially purified I-TLF. Haptoglobin involvement is further shown in that it inhibits TLF in a manner similar to I-TLF. Using an anti-haptoglob in column to remove haptoglobin from I-TLF, we show that the loss of h aptoglobin coincides with the loss of inhibitor activity. Addition of purified haptoglobin restores inhibitor activity. This indicates that haptoglobin is the molecule responsible for inhibition and therefore c ausing the individual variation in serum lytic activity.