Production and purification of a low molecular weight haemolysin produced by Actinobacillus pleuropneumoniae serotype 1.

An unstable haemolytic activity produced by a strain of serotype 1 of Actinobacillus pleuropneumoniae was isolated when 1 per cent bovine serum albumin (BSA) was added to RPMI 1640 medium. BSA acts as a carrier molecule and stabilises activity. This haemolysin (BSA-haemolysin) was precipitated with ammonium sulphate, dialysed and lyophilised. Of the species tested, bovine erythrocytes were the most susceptible to the BSA-haemolysin while mouse and rabbit erythrocytes were the least susceptible. The haemolytic activity was dependent on the incubation temperature, no activity being observed at or below 24 degrees C. The haemolytic activity was also partly stabilised by 100 micrograms ml-1 dithiothreitol (DTT). The DTT-haemolysin was purified to homogeneity by ultrafiltration and high pressure liquid chromatography on a Protein Pak DEAE-5PW column. The molecular weight was estimated at 16 kDa by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and at 23 kDa by molecular gel filtration from the elution position of the haemolytic activity. The DTT-haemolysin activity was completely destroyed by pronase treatment suggesting that this substance could be a polypeptide. The addition of BSA to DTT-haemolysin increased its activity and stability to lyophilisation. The addition of 10 mM calcium chloride in the titration assay increased the activity of DTT-haemolysin from 220 to 476 haemolytic units ml-1. The BSA-haemolysin activity was only slightly affected by the addition of calcium chloride.