The Mode of Action of Chlorsulfuron in Culture Cells of Tobacco and Hamster

Abstract

Chlorsulfuron inhibited the growth of tobacco cell cultures at 1ppb, with a half inhibition (I₅₀) at 0.5ppb. In contrast, cultured cells of hamster kidney (BHK) continued to grow even in a medium containing 100ppm chlorsulfuron. Acetolactate synthase (ALS) from tobacco cell cultures was strongly inhibited by chlorsulfuron with an I₅₀ values of 5-10ppb (14-28nM). Gas chromatography confirmed the production of acetolactate in the enzyme preparations from BHK cells and chick livers, but, no inhibition with 500ppb chlorsulfuron. The properties of acetolactate-forming enzymes from tobacco cells and BHK cells were compared in cofactor requirement, pH dependence and valine inhibition. The enzyme from BHK cells strongly required thiamine pyrophosphate (TPP), it was not inhibited by valine, and the optimum pH in the enzyme reaction was 7.4. On the other hand, the tobacco enzyme did not strongly require TPP, it was inhibited by valine, and the optimum pH was 8. That chlorsulfuron does not inhibit acetolactate-forming enzymes in mammalian cells is one likely reason for its low toxicity against mammalian cells. The binding mechanism of the herbicide with the tobacco cell enzyme was noncompetitive with pyruvate as a substrate and uncompetitive with TPP as a cofactor.