Phosphine toxicity: a story of disrupted mitochondrial metabolism

Abstract

Rodenticides and pesticides pose a significant threat not only to the environment but also directly to humans by way of accidental and/or intentional exposure. Metal phosphides, such as aluminum, magnesium, and zinc phosphides, have gained popularity owing to ease of manufacture and application. These agents and their hydrolysis by-product phosphine gas (PH3 ) are more than adequate for eliminating pests, primarily in the grain storage industry. In addition to the potential for accidental exposures in the manufacture and use of these agents, intentional exposures must also be considered. As examples, ingestion of metal phosphides is a well-known suicide route, especially in Asia; and intentional release of PH3 in a populated area cannot be discounted. Metal phosphides cause a wide array of effects that include cellular poisoning, oxidative stress, cholinesterase inhibition, circulatory failure, cardiotoxicity, gastrointestinal and pulmonary toxicity, hepatic damage, neurological toxicity, electrolyte imbalance, and overall metabolic disturbances. Mortality rates often exceed 70%. There are no specific antidotes against metal phosphide poisoning. Current therapeutic intervention is limited to supportive care. The development of beneficial medical countermeasures will rely on investigative mechanistic toxicology; the ultimate goal will be to identify specific treatments and therapeutic windows for intervention.

Keywords: mitochondria; phosphine; reactive oxygen species; rodenticide; systemic poison.

Figure 1

A schematic representation of the mitochondrial respiratory chain, ROS formation, and phosphine toxicity. The respiratory chain consists of specialized metal heme complexes such as complexes II, III, IV, and an ATP synthase known as complex V. Complex I, ubiquinone, cytochrome c, proton pumps, membrane potential, and proton motive force are critical parts of electron flow in the respiratory chain. Oxidative phosphorylation is carried out in the inner membrane by respiratory assemblies. From these assemblies, two electrons from NADH are transferred to O₂ by a series of electron carriers, starting with NADH dehydrogenase. Electrons are transferred to the ubiquinone (also known as coenzyme Q (CoQ)) and then to the cytochrome a group of hemeproteins. There are five cytochromes between CoQ and molecular O₂: cytochromes b, c1, c, a, and a3. Cytochromes a and a3 are also known as cytochrome oxidase. The production of ATP occurs at 3 sites: (1) between NADH and CoQ, (2) between cytochromes b and c, and (3) between cytochrome c and O₂. The inner membrane is impermeable to most molecules because of its high proportion of phospholipids and cardiolipin. Transport proteins embedded in the inner membrane selectively incorporate metabolites into the matrix and export ATP for biological processes. The matrix is the site of high energy–yielding reactions from the metabolism of pyruvate and fatty acids derived from carbohydrates and other nutrients. Pyruvate and fatty acids that are transported into the matrix help to generate a pair of electrons that have a high energy transfer potential. These are carried by NADH and FADH2 (flavin adenine dinucleotide reduced form), which are formed in glycolysis, fatty acid oxidation, and the citric acid cycle. The released high energy from this reaction is used to produce ATP through oxidative phosphorylation, which is a major metabolic reaction in aerobic organisms. Oxidative phosphorylation is strongly coupled to fatty acid oxidation, which forms acetyl CoA and the citric acid cycle to produce the energy needed for aerobic cell metabolism. Respiratory assemblies are an integral part of the inner membrane, whereas fatty acid oxidation and the citric acid cycle activity occur in the matrix. Flow of electrons causes an electrochemical proton gradient under the control of the proton motive force for the production of ATP. During active respiration, mitochondria produce a proton motive force across the inner membrane. This results in a negative charge inside, thereby producing a pH gradient. Electron leakage from the respiratory chain from enhanced ROS production causes additional oxidative insult. If active electron flow ceases through the respiratory chain, proton motive force collapses and ATP production discontinues. This causes loss of membrane potential and release of additional ROS. The dashed arrows indicate the putative inhibition of critical components of the respiratory electron transport chain by phosphine.