One of the things that my two year old daughter loves about grocery shopping is looking at fruits and vegetables in the produce section. The other day, it was the star fruit (Averrhoa carambola, also known simply as carambola) that caught her attention. With its bright color and distinctive shape, clearly this fruit was designed to appeal to toddlers.
But scientists are curious too, and the star fruit has been the subject of recent research. Apparently, people with some types of kidney disease are susceptible to adverse neurological effects including intoxication, seizures and even death after eating star fruit. For many years, it was thought that this was due to oxalic acid. Oxalic acid is common in many plants that we eat, such as spinach. But in sufficient quantities, oxalic acid can make plants too toxic to consume. And as a major constituent of kidney stones, its connection to kidney disease seems plausible.
But recent research from the University of Sao Paulo in Brazil has shown that the toxin responsible for the neurological problems is actually a newly described chemical compound, caramboxin. In animal models, the purified compound mimicked the adverse effects of star fruit. In vitro experiments demonstrated that it has glutamatergic effects, which means that it affects the function of the neurotransmitter glutamate, thus explaining the neurological effects.
The chemical structure of caramboxin is based on a phenylalanine core with hydroxy, methoxy, and carboxy additions. This makes me curious about the biosynthesis of this compound. Common biosynthetic steps such as oxidation and methylation could introduce the hydroxy and methoxy groups, but how might the carboxyl group be added?