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Abstract
The isolate of Bacillus pumilus associated with the marine sponge Ircinia sp. produced
the surfactin-like lipopeptides, cyclic acyldepsipeptides. The hemolytic activity
of individual cyclic acyldepsipeptides, bacircines (BI) 2, 3, 4, 5 and 5A having different
acyl side chain structures (anteiso-C13, iso-C14, normal-C14, anteiso-C15, and iso-C15,
respectively) was studied. The hemolytic power of bacircines depended on both the
structure of the side chain (n->iso->anteiso-) and pH values (5.6 and 6.5 > 7.4).
Hemolytic potency as a function of BI 5 concentration was given for pH 6.5; 7.4; 8.0;
9.0. pH dependent hemolysis induced by BI 5 was shown to be reversible. The membrane
damaging potential of bacircine 5 (5 microM) at pH 6.5 was characterized by a higher
rate of hemolysis and by a shorter time between the introduction of BI 5 solution
into the RBC samples and the onset of hemolysis. Under this condition, BI 5 decreased
abnormally the microviscosity of erythrocyte ghosts bilayer. The damaging potency
of BI 5 decreased with an increase pH from 6.5 to 7.4 or its decrease from 6.5 to
4.9. It was shown that fatty acid bacircine fragment penetrated into the lipid bilayer
to a depth of minimum 7 carbon atoms. Constants of dissociation of the Asp (pK 4.75)
and Glu (pK 6.65) residues of bacircine in the lipid bilayer were obtained. These
results showed that at pH 6.5 BI 5 possessed membranotropic activity in the monoionic
form.