EFFECT OF HYDROXAMIC ACIDS FROM CEREALS ON APHID CHOLINESTERASES

2,4-Dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), a hydroxamic ac id involved in the resistance of wheat to the aphid Rhopalosiphum padi , and known to react with serine and cysteine residues, inactivated ac ylcholinesterases (acylcholine hydrolases, EC 3.1.1.7) from homogenate s of whole bodies of adult R. padi. The enzyme preparation was shown t o consist mainly of acetylcholinesterase (AChE) containing an essentia l cysteine residue outside the substrate binding site. AChE from R. pa di, and also ChE from other insects and from human erythrocytes which do not possess such a cysteine residue, were inhibited (I50 in the mM range) by DIMBOA. 4-Hydroxy-7-methoxy-1,4-benzoxazin-3-one, a compound known to react with cysteine residues but not with serine residues, d id not inactivate AChE from R. padi under similar conditions. The resu lts suggest that inactivation of AChE from R. padi by DIMBOA does not occur through reaction with its characteristic cysteine residue.