CHARACTERISTICS OF THE BINDING OF HUMAN AND BOVINE HIGH-DENSITY-LIPOPROTEINS BY BLOOD-STREAM FORMS OF THE AFRICAN TRYPANOSOME, TRYPANOSOMA-BRUCEI-BRUCEI

Bloodstream forms of Trypanosoma brucei brucei are unable to synthesize cholesterol but appear to bind and take up plasma low-density lipoproteins (LDL) from their host. Whether cholesterol homeostasis of this unicellular parasite also requires interactions with host high-density lipoprotein (HDL) particles is unknown. Equilibrium binding of radioiodinated apolipoprotein E-depleted human HDL3 (d = 1.125-1.21 g/ml) and bovine HDL (d = 1.063-1.21 g/ml) by T. b. brucei was rapid (< 30 min) at 4-degrees-C and was characterized by a saturable, specific component. There were five times the number of high-affinity binding sites for human HDL3 as for bovine HDL (64,000 vs. 11,500 per trypanosome) and their binding affinity was greater with an equilibrium dissociation constant (K(d)) of 157 nM compared to 315 nM for bovine HDL). Binding of rat and rabbit HDL3 was similar to bovine HDL. By contrast, equilibrium binding of human LDL was slower (approximately 6 h) and the number of high-affinity binding sites (K(d) = 23 nM) was much lower for this ligand (660 per trypanosome). Total binding of HDL3 was independent of divalent cations and was only slightly inhibited by heparin, but when the trypanosomes were preincubated with trypsin or pronase the binding was markedly reduced. After 30 min at 37-degrees-C, binding of bovine HDL and human HDL3 was 10-20% higher than at 4-degrees-C; after 45 min trypanolysis occurred with human HDL3 but not with bovine HDL. Chemical modification of HDL3 by treatment with cyclohexanedione, by acetylation or by reductive alkylation had little effect on its ability to compete with [I-125] labelled HDL3 for binding by the parasite. Nitrosylation of HDL3 with tetranitromethane increased its binding ability, suggesting that trypanosomes might possess scavenger receptors, and native HDL3 was less effective than nitrosylated HDL3 in displacing bound [I-125]labelled nitrosylated HDL3. These findings suggest that, in addition to a receptor for LDL, T. b. brucei has other lipoprotein binding sites which separately recognize HDL from permissive host species such as bovine, trypanolytic HDL such as human HDL3, and more negatively charged HDL particles such as nitrosylated HDL3.