Effect of certain quinone pesticides on acetate photometabolism and dark CO₂ fixation in Chlorella

The effect of dichlone (2,3-dichloro-1,4-naphthoquinone), 06K-quinone (2-amino-3-chloro-1,4-naphtoquinone), and chloranil (2,3,5,6-tetrachloro-1,4-benzoquinone) on photometabolism of acetate and on dark ¹⁴CO₂ fixation by Chlorella was studied. Treatment of the cells with 3 × 10^-6 M or higher concentrations of these pesticides reduced acetate assimilation. Lower quinone concentrations did not appreciably change the pattern of ¹⁴C distribution among the products resulting from acetate assimilation, although acetate incorporation was markedly inhibited. At higher concentrations, quinones decreased the proportion of ¹⁴C incorporated in lipids and increased the proportion of ¹⁴C incorporated in glutamic and citric acids. The data suggest that higher concentrations of quinones preferentially inhibited lipid synthesis resulting in the availability of more acetate for the intermediates of the tricarboxylic acid cycle. The above quinones also inhibited ¹⁴CO₂ fixation in the dark and significantly altered the distribution of ¹⁴C; both 06K-quinone and chloranil caused a decrease in the percent of ¹⁴C incorporated in glutamic acid and an increase in ¹⁴C-citric acid. The cells treated with chloranil also showed an increase in the proportion of ¹⁴C-malic acid accompanied by a decrease in ¹⁴C-aspartic acid. Possible explanations are given to account for the changes in the distribution of carbon in the metabolic products resulting from acetate assimilation and dark CO₂ fixation in the treated cells. Our findings indicate that the quinone pesticides act at a number of sites in the cell and their inhibitory effect is not restricted to photosynthetic reactions.