Interactions between Asp-hemolysin from Aspergillus fumigatus and blood plasma components

Abstract

Asp-hemolysin is a cytolytic toxin that is produced by Aspergillus fumigatus. This toxin is lytic for erythrocytes of humans, rabbits and sheep. However, Asp-hemolysin is inactivated by the addition of serum or blood plasma. This study was undertaken to identify plasma components inhibitory to the hemolytic activity of Asp-hemolysin. alpha 2-Macroglobulin (alpha 2M) was isolated from the human blood plasma by affinity chromatography on a column containing Asp-hemolysin coupled to Sepharose. However, the hemolytic activity was only partially inhibited by alpha 2M. Apolipoprotein B (apoB)-containing lipoproteins, such as low density lipoprotein (LDL), inhibit the activity of this hemolytic toxin. When 20 micrograms apoB was added, the hemolytic activity was almost completely inhibited. Furthermore, similar inhibition was observed in the filtrates separated from the incubation mixture of Asp-hemolysin with LDL or apoB following ultrafiltration through a membrane with a molecular mass cutoff of 100,000. These results suggest that the inhibition by LDL is due to apoB binding to Asp-hemolysin. The binding activity of LDL to Asp-hemolysin was measured. LDL binds to Asp-hemolysin with an affinity as high as the LDL receptor. The apparent Kd, determined by Scatchard plot analysis, was 8.9 x 10(-9) M 125I-LDL. Oxidized LDL (Ox-LDL), but not acetylated LDL, inhibited the hemolytic activity of this toxin. The inhibitory effects of Ox-LDL increased with the time of Cu(2+)-induced LDL oxidation. Similar inhibition was observed in the filtrate separated from the incubation mixture of Asp-hemolysin with Ox-LDL (for 2 h of oxidation) following ultrafiltration through a membrane with a molecular mass cutoff of 100,000. However, at longer LDL oxidation times, the inhibition by the filtrates was less than the control mixture without ultrafiltration. These results suggest that the inhibition of the hemolytic activity by Ox-LDL was due to the binding of Ox-LDL to Asp-hemolysin at short LDL oxidation times.