Star Fruit’s Chemical Curse

Most folks sampling a tropical treat called star fruit can expect a tart, citrusy burst in their mouths. But people with kidney disease can also expect hiccups, confusion, a feeling of intoxication, seizures, vomiting, and sometimes even death. That’s because star fruit contains a toxin called caramboxin that healthy kidneys can easily metabolize but diseased ones cannot, reports a team of researchers led by Norberto Garcia-Cairasco and Norberto P. Lopes of the University of São Paulo, in Brazil (Angew. Chem. Int. Ed. 2013, DOI: 10.1002/anie.201305382). Researchers have tried previously to identify the toxic component of this fruit, initially focusing erroneously on oxalate. Lopes and Garcia-Cairasco’s team isolated and solved the structure of caramboxin, showing that the phenylalanine-like molecule can reproduce the toxic symptoms in animal models by acting on important ion channel receptors for the neurotransmitter glutamate. They also discovered that over time, isolated caramboxin stored at room temperature forms a closed ring structure that is no longer toxic. The team proposes that caramboxin and its inactive analog could be useful tools to study neurotransmitter receptors.