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Extracellular haemolytie activities of Aeromonas salmonicida ssp. salmonicida to salmon red blood cells were shown to be due to different forms of the mcmbranc-activc enzyme glyccrophospholipid:choicstcrol acyltransferase {GCAT). About 10% of the total haemolytie activity was due to a high molecular mass complex of LPS and GCAT (mol. mass > 1000kDa), containing 35-50% neutral sugars and 1-5-2-0% protein. Some haemolytie activity (30-40% of total), corresponding to 5O-70kDa by gel filtration, also contained GCAT-aetivity and may represent aggregated forms of GCAT. However, about 50% or more of the haemol>tic activity was due to a protein of 26kDa free GCAT. Rabbit antibodies to GCAT neutralized the haemolytie activity of both GCAT and GCAT-LPS. A transposon-produced serine protease negative mutant of the same A. salmonicida strain showed reduced haemolytie -activity. The mutant produced a 38-kDa GCAT preform of low haemolytie activity. The proform was processed by autogenous serine protease to a highly haemolytie 26-kDa molecule with pi 6-3. similar to GCAT of the parent strain. The weakly haemolytie GCAT-LPS analogue of the mutant strain did not contain detectable amounts of the 26-kDa molecule and was not activated by proteases.

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