16 April 2018

Convallatoxin – the toxin of wild garlic’s deadly doppelganger

Natural toxin

When picking wild garlic (Allium ursinum), you better take care not to mistake the characteristically smelling green leaves with those of Convallaria majalis L., better known as lily of the valley. The latter contains up to 40 different cardenolide glycosides of which convallatoxin is the major toxin causing cardiac arrhythmia.

Convallaria majalis L. (C. majalis or lily of the valley) is one of only three species of the genus Convallaria and the only one naturally found in central Europe. The wild growing form is found in open forests while in many gardens lily of the valley is planted for decorative purposes.

All parts of the plant contain the toxic cardenolide glycosides which are only formed in the above surface organs of the plant. Highest toxin concentrations are found in very young leaves, flowers and seeds (~0.45%) while ripe berries of lily of the valley hardly contain any toxins at all.

Up until now, more than 40 different toxins were isolated from C. majalis. In addition to the major toxin convallatoxin (CAS 508-75-8), other glycosides such as convallosid, convallatoxol or strophanthidin can be found but the spectrum generally depends on the age of the plant, its habitat and chemotype. The western and northern European type is dominated by convallatoxin and lower amounts of convallatoxol, while the one found in central Europe contains convallatoxin, convallatoxol and convallosid in similar proportions. Relatively often, the glycosides are accompanied by steroid saponins such as convallamarosid.

Convallatoxin structure. Click image for interactive 3D model

Formerly, Convallaria extracts have been used in therapeutic applications against dropsy with convallatoxin being one of the most active natural substances affecting the heart. Due to its side effects it is no longer allowed to administer these extracts as in the worst case intoxication due to wrong administration or also when individuals mistake Convallaria leaves for wild garlic possibly leads to heart failure (LD 0.07-0.08 mg/kg bodyweight, intravenous, cat). Other symptoms of convallatoxin poisoning are nausea, vomiting, severe headaches and a slowed, irregular heartbeat. However, it is not clear yet whether some of those symptoms are actually caused by the saponins found in the plants.

In 2011, lily of the valley became a prominent fatal plant used as a poison in the US show Breaking Bad. Nevertheless, convallatoxin and other cardenolide glycosides only show very small resorption rates in the body and therefore death is a very unlikely result of convallatoxin poisoning. According to predictions (EPISuite, US EPA), both the glycoside convallatoxin and aglycone strophanthidin persist for almost half a year in the environment and show high mobility with a log Koc of -0.64 and 0.16, respectively. Thus, it is possible that these toxins reach drinking water resources. Nonetheless, estimates of aquatic toxicity (ECOSAR, US EPA) show that in the long-term both glycoside and aglycone may only pose a risk to aquatic organisms, when exceeding concentrations in the low mg/L range. Due to the only locally occurring large stocks of lily of the valley, the rarity of the species in Europe and the generally low amount of toxins in the plant, Convallaria toxins will most likely not affect drinking water quality.

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