Pyrularia
thionin (P.
thionin) is a strongly basic
peptide of 47
amino acids which is hemolytic, cytotoxic and neurotoxic. It shows the greatest hemolytic activity toward human erythrocytes. Rabbit, guinea pig and pig erythrocytes show decreasing activity in that order, and little or no activity is shown with sheep, horse, cow or mouse erythrocytes. Crotalus
venoms are inactive, but the
venoms from Naja naja atra, Naja naja ceylonicus and Naja naja melanoleuca and, more specifically,
cardiotoxin from Naja naja kaouthia have significant hemolytic activities toward human erythrocytes. The
cardiotoxin preparation used had no
phospholipase activity, and was less active than P.
thionin (23% compared to 35%
hemolysis by P.
thionin in 60 min
at 10 micrograms/ml toxin). Since iodinated P.
thionin is inactive, it was used as an inhibitor of
hemolysis catalyzed by native P.
thionin, N. ceylonicus
venom and by
cardiotoxin. Examination of the kinetics of the reactions catalyzed by N. ceylonicus
venom and
cardiotoxin in the absence and presence of iodinated P.
thionin shows that both N. ceylonicus
venom and
cardiotoxin exhibit Michaelis-Menten kinetics, yielding apparent Km values of 7.4 micrograms/ml and 0.69 microM, respectively. These values compare to an apparent Km for P.
thionin of 1.6 microM for erythrocyte
hemolysis and a binding constant of 2.1 microM (Osorio e Castro, V. R. Van Kuiken, B. A. and Vernon, L. P. (1989) Action of a
thionin isolated from nuts of Pyrularia pubera on human erythrocytes. Toxicon 27, 501). The inhibition constants Ki for iodinated P.
thionin in the reactions with N. ceylonicus
venom and
cardiotoxin are 3.8 and 5.3 microM, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)