Post-Mortem Changes of Methomyl in Blood with Hemoglobin.

Methomyl, (E,Z)-methyl N-{[(methylamino)carbonyl]oxy}ethanimidothioate, is a widely used pesticide that has been detected in many fatal cases of accidental exposure or suicide. Forensic toxicologists have been baffled that the blood methomyl concentration in persons who have died of methomyl poisoning is much lower than the expected concentration in blood. In this study, we speculated two mechanisms underlying the insufficient recovery of methomyl in blood. First, methomyl is decomposed by serum albumin as esterase. Second, methomyl is bound to a specific blood protein, resulting in insufficient recovery in the free form. However, human serum albumin does not show esterase activity for the decomposition of methomyl. On the contrary, specific methomyl hemoglobin adducts have been detected by liquid chromatography quadrupole time-of-flight mass spectrometry (LC-Q/TOF-MS). The mass spectra indicated that methomyl was specifically bound to tryptophan (W), tyrosine (Y), and valine (V) residues in hemoglobin. The amounts of W- and V-adducts dose-dependently increased in vitro when the methomyl concentration was lower than the lethal concentration. In addition, the W-adduct was detected in blood sampled from an autopsied subject who died of intentional methomyl ingestion, suggesting that the W-adduct could be used as a biomarker of methomyl poisoning. We were able to estimate the amount of methomyl ingested on the basis of the amount of the W-adduct.

Repetitive nerve stimulation and stimulation single fiber electromyography studies in rats intoxicated with single or mixed insecticides.

The function of the neuromuscular transmission in rats dosed with phoxim (P), methomyl (M), fenvalerate (F), and mixtures of P+M and P+F was studied by using both the stimulation single fiber electromyography (SSFEMG) and repetitive nerve stimulations (RNS) to determine the single muscle fiber action potential and compound muscle action potential (CMAP) respectively. The results showed that the mean consecutive difference (MCD) in SSFEMG was significantly prolonged in P, P+M and P+F intoxicated rats during the presence of myasthenia, but not in rats dosed with F and M when stimuli were given at 10 Hz or 20 Hz, thus indicating a transmission blocking at the neuromuscular junction (NMJ) induced by P. The frequency of neuromuscular transmission abnormalities detected by SSFEMG was significantly higher than those detected by RNS. This study demonstrated that the neuromuscular junction (NMJ) blocking is more frequently seen in P, P+M and P+F poisoning than in M and F poisoning, and that SSFEMG is a more sensitive electrophysiological method than RNS for detecting neuromuscular transmission blockage in myasthenia rats with acute insecticides poisoning.

Fatal and non-fatal methomyl intoxication in an attempted double suicide.

A 79-year-old man and his 73-year-old wife attempted double suicide by ingesting methomyl powder. The woman died 19 h after ingestion in spite of intensive care. At autopsy a large number of miliary hemorrhages were found in both thalami of the brain. Her husband, however, recovered after 10 days of treatment. Methomyl (CAS No. 16752-77-5, Lannate) in the biological materials was analyzed by gas chromatography-mass spectrometry. The methomyl concentration was 44 micrograms/g in the wife's serum sample collected 1 h after ingestion, and 0.2 microgram/g in the blood sample collected at autopsy. The methomyl concentration in the husband's blood sample collected 28 h after ingestion was from 0.01 to 0.1 microgram/g. It is suggested that prompt and adequate intensive care including a direct hemoperfusion is necessary to effect the recovery of patients with lethal blood levels of methomyl. The miliary hemorrhages found in the thalami of the brain are suspected to have been caused by asphyxia induced by methomyl intoxication.

Methomyl in the blood of a pilot who crashed during aerial spraying.

An aerial-spray pilot died when his aircraft crashed while he was spraying methomyl. We measured the pesticide in his blood by gas chromatography with flame photometric detection and confirmed the results by mass spectrometry with direct liquid injection through a liquid chromatography interface. The whole blood methomyl concentration was 570 ng/mL.

Application of energy dispersive X-ray fluorescence spectrometry (EDX) in a case of methomyl ingestion.

We applied energy dispersive X-ray fluorescence spectrometry (EDX) in a case of poisoning by methomyl, a carbamate pesticide. Quantitative GC/MS analysis showed that the concentration of methomyl-oxime in the femoral blood was 4.0 µg/ml. The elemental analysis by EDX identified the high peak of silicon and sulfur in the stomach contents. We concluded that the cause of his death was methomyl poisoning. This indicates that screening of stomach contents by EDX provides useful information for the forensic diagnosis.

Sensitive determination of methomyl in blood using gas chromatography-mass spectrometry as its oxime tert.-butyldimethylsilyl derivative.

A sensitive, selective and reliable method was developed to determine methomyl ¿methyl-N-[(methylcarbamoyl)oxy]-thioacetimidate¿, a carbamate insecticide in human blood, using gas chromatography-mass spectrometry. Dimethylglyoxime served as an internal standard (I.S.). Methomyl in the blood was converted to its oxime form by sodium hydroxide. The solution made acidic with hydrochloric acid was poured into a column packed with Extrelut. Methomyloxime and I.S. were eluted from the column with a mixture of dichloromethane-ethyl acetate-chloroform (65:25:10), transformed to tert.-butyldimethylsilyl derivatives, and analyzed by gas chromatography-mass spectrometry in the electron impact mode. The calibration curves were linear in the concentration range from 1 ng/g to 100 ng/g and 100 ng/g to at least 5000 ng/g. The lower limit of detection was 0.5 ng/g. The absolute recoveries were 72-93% and within-day coefficients of variation were 3.1-5.6% at blood concentrations of 10 and 1000 ng/g. Two practical forensic applications are described.