The Mode of Action of Phosphine

Abstract

The mode of action of phosphine (PH3) was investigated principally using adult maize weevil, Sitophilus zeamais. Mortality of the insects did not increase with an increase in PH3 concentration. High PH3 concentrations narcotized the insects by decreasing the respiratory rate. There was a significant relation between the oxygen uptake and the mortality. The results of in vitro experiments led to the conclusion that PH3 inhibited the cytochrome oxidase activity as in the case of HCN. However, PH3 showed a low inhibitory effect on state 4 respiration of both mitochondria of the maize weevil and the rat liver, and a weak action on the reduced form of cytochrome oxidase obtained from the beef heart. In vivo experiments demonstrated that PH3 inhibited the activity of cytochrome oxidase in mitochondria obtained from the insects treated at lower PH3 concentrations more effectively than that in mitochondria from the insects narcotized at higher PH3 concentrations. A special factor to absorb PH3 was detected in a soluble fraction obtained from the homogenate of flour beetles, Tribolium castaneum. This factor may act as a catalyst to promote a reaction between oxygen and PH3 to produce an oxygen radical. The effect of PH3 on the insects varied with the concentration. PH3 at higher concentrations exerted a narcotic effect on the insects by decreasing the oxygen uptake as the mitochondria reached state 4 respiration and cytochrome oxidase was reduced, thus making the target site insensitive to PH3. PH3 at lower concentrations did not exert a narcotic effect on the insects, leaving the target site sensitive to PH3 as the mitochondria exhibited state 3 respiration and cytochrome oxidase was oxidized. In addition, the reaction to produce an oxygen radical at the cellular level and the anoxic condition of the insects may account for the action mechanism of PH3.