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

Asp-hemolysin is a cytolytic toxin that is produced by Aspergillus fumigatu s. This toxin is lytic for erythrocytes of humans, rabbits and sheep. Howev er, Asp-hemolysin is inactived by the addition of serum or blood plasma. Th is study was undertaken to identify plasma components inhibitory to the hem olytic activity of Asp-hemolysin. alpha (2)-Macroglobulin (alpha M-2) was i solated from the human blood plasma by affinity chromatography on a column containing Asp-hemolysin coupled to Sepharose. However, the hemolytic activ ity was only partially inhibited by alpha M-2. Apolipoprotein B (apoB) cont aining lipoproteins, such as low density lipoprotein (LDL), inhibit the act ivity of this hemolytic toxin. When 20 mug apoB was added, the hemolytic ac tivity was almost completely inhibited. Furthermore, similar inhibition was observed in the filtrates separated from the incubation mixture of Asp-hem olysin with LDL or apoB following ultrafiltration through a membrane with a molecular mass cutoff of 100000. These results suggest that the inhibition by LDL is due to apoB binding to Asp-hemolysin. The binding activity of LD L to Asp-hemolysin was measured. LDL binds to Asphemolysin with an affinity as high as the LDL receptor. The apparent Kd, determined by Scatchard plot analysis, was 8.9 X 10(-9) M I-125-LDL. Oxidized LDL (Ox-LDL), but not ace tylated LDL, inhibited the hemolytic activity of this toxin. The inhibitory effects of Ox-LDL increased with the time of Cu2+-induced LDL oxidation. S imilar inhibition was observed in the filtrate separated from the incubatio n mixture of Asp-hemolysin with Ox-LDL (for 2 h of oxidation) following ult rafiltration through a membrane with a molecular mass cutoff of 100000. How ever, at longer LDL oxidation times, the inhibition by the filtrates was le ss than the control mixture without ultrafiltration. These results suggest that the inhibition of the hemolytic activity by Ox-LDL was due to the bind ing of Ox-LDL to Asp-hemolysin at short LDL oxidation times.