Aflatoxin B-1 (AFB(1)) action on cyclic nucleotide phosphodiesterase (PDE)
activity has been tested on tissue extracts of various organs. In the prese
nce of 100 mu M AFB(1) a significant inhibition of cAMP and cGMP hydrolytic
activity is observed in all tested tissue extracts. However, cGMP hydrolyt
ic activity appears more sensitive to AFB(1) inhibition than cAMP hydrolyti
c activity and a considerably higher inhibition is observed in lung and spl
een, than in liver, brain, kidney, and heart. When cGMP is used as substrat
e, the inhibitory response reaches 72% in lung and spleen extracts. We have
also tested AFB(1) effects on lung and liver PDE activity peaks separated
by DEAE-cellulose chromatography. These data confirm the poor sensitivity t
o the toxin of all PDE activities present in liver, while the lung peak (wh
ere PDE V in present) shows a higher sensitivity to AFB(1). In order to est
ablish whether PDE V is in fact more sensitive to AFB(1), we have used mous
e neuroblastoma cells, in which cGMP hydrolytic activity has been shown to
be due to PDE V only. In this case, the calculated IC50 is 24 mu M and Dixo
n plot analysis shows a competitive inhibitory effect with a Ki of 16.7 mu
M. We have also used aflatoxin B-2 and M-2, and they proved to be much less
effective than AFB(1): AFB(2) inhibits PDE V with an IC50 of 117 mu M, whi
le AFM(2) does not show any effect. These results provide the first evidenc
e of a competitive inhibition of AFB(1) on an enzymatic activity and sugges
t that an alteration of cellular cyclic nucleotide levels may play a role i
n the mechanism of aflatoxin action. All rights reserved. (C) 1999 Elsevier
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