ARISTOLOCHIC ACID

 

Aristolochia ringens

Leaving behind the alkaloids that affect our nervous system, we are going to examine three plant toxins that poison human DNA, causing multiple organ damage, and cancer in the long term. Let’s start with aristolochic acid, which is perhaps the most insidious of plant poisons. Owing to its peculiar flower and the doctrine of signature, the birthworts (Aristolochia) could have been responsible for hundreds of thousands of human deaths due to cancer of the urinary tract, or chronic kidney failure. Epidemics of Aristolochia poisoning were known throughout history, from the Balkan endemic nephropathy to the chronic kidney diseases induced by unregulated traditional Chinese medicine, Aristolochia continues to cause harm today. 

 

Figure 1: Chemical structure of aristolochic acid.

Aristolochic acid is an unusual alkaloid because it defies the very definition of an alkaloid to begin with. Remember I said that alkaloids must contain at least one nitrogen atom that confers it alkaline property? Here’s an exception because aristolochic acid contains a carboxylic acid functional group, which legitimately makes it acidic. Besides, the nitrogen atom of aristolochic acid is incorporated as a nitro group, the same type we find in the explosive TNT, but thankfully aristolochic acid is not explosive. However, its nitro group will prove to be just as deadly. The core skeleton of aristolochic acid is called aporphine, which is basically three hexagonal carbon rings merged back-to-back, with another hexagonal carbon-nitrogen ring at one end. I have shown the structure of an isolated aporphine moiety in Figure 1, alongside the chemical structure of aristolochic acid. Take note that although aristolochic acid contains only 3 rings, it is made by plants via intermediate alkaloids containing aporphine skeleton. That is why we consider aristolochic acid as an aporphine (derived) alkaloid. In 3D space, the aporphine ring of aristolochic acid is planar, and that means a big part of this molecule lays flat like a piece of paper (Figure 1). Besides, the aporphine ring of aristolochic acid is aromatic, so electrons can travel freely within the 3 hexagonal rings. This phenomenon is called conjugation, and it causes aristolochic acid to interact with visible light and appear as a bright yellow dye.

Aristolochic acid is present in almost all species of Aristolochia, which utilise the amino acid tyrosine as an ultimate starting material. Examine Figure 2 carefully. Tyrosine can be transformed by plant enzymes into two important precursors, namely dopamine and 4-hydroxyphenylacetyldehyde. The two precursors will merge via a biochemical step called the Pictet-Spengler reaction to produce an intermediate alkaloid called norcoclaurine, which is further transformed by oxidation and methylation into orientaline. Orientaline was first isolated from an unrelated plant, the oriental poppy (Papaver orientale), but such simple intermediate alkaloid often occurs in many plant species. If you have a sharp eye, you can see that we are already one bond away from making an aporphine skeleton. 

Figure 2: Biosynthesis of aristolochic acid, part I.

Thus, orientaline is oxidised (with removal of two electrons) at the two rings (I labelled as A and D, Figure 3) into a diradical. The two radicals being extremely reactive instantly form a bond between rings A and D to make a non-aromatic proto-aporphine intermediate called orientalinol. The orientalinol then rearranges itself into the more stable aporphine skeleton because aromaticity confers great chemical stability. The hydroxy and methoxy groups in ring A are also modified into a methylenedioxy ring, and that gives a true aporphine alkaloid called stephanine. Finally, ring B of stephanine is cleaved to produce the carboxylic acid and nitro group of aristolochic acid, respectively. Nature is scarily smart isn’t it, aeons of evolution to perfect a plant toxin! 

 

Figure 3: Biosynthesis of aristolochic acid, part II.

Once ingested, the liver transforms aristolochic acid by modifying its nitro and carboxylic acid groups into a cyclic amide called aristolactam (Figure 4). In fact, the liver is our body’s way of detoxifying poisons, but now it backfires. Aristolactam spontaneously loses a molecule of water from its nitrogen atom and becomes a highly reactive species called aristolactam nitreneum ion. Essentially, its nitrogen atom now bears a positive charged. Remember that the aporphine system is conjugated? This positive charge can even travel to the adjacent carbon atom via an effect called resonance. Here is where it gets scary because this positive charge will go heaven and earth to become neutralised, and our DNA is by nature negatively charged. DNA contains many nitrogen atoms (base pairs) that carry extra electrons. The DNA base pairs will perform a nucleophilic attack on the aristolactam nitreneum ion, forming a permanent covalent bond with the resonance contributed carbocation. In other words, aristolochic acid is irreversibly bonded (attached) to the DNA, we call it an adduct. The aristolactam-DNA adduct then rearranges itself to resume aromaticity for extra stability. This is a very serious type of DNA damage because the chemical bond between aristolochic acid and DNA is permanent. Every time the damaged DNA needs to be read or replicated, errors will occur, leading to harmful mutations, and cancer. Perhaps because the body tries to excrete aristolochic acid via the kidney, it and the urinary tract appear most affected. Besides, since aristolochic acid-DNA adduct is permanent and distinctive, it can even be traced as a biomarker from victims of poisoning.

Figure 4: Aristolochic acid toxicity, mechanism of action.

Aristolochic acid is certainly one of the most harmful plant toxins known to humans because its toxic effects are chronic, cumulative, and irreversible. It is a poison that takes time to do damage, but when the damage is done, it is often too late. In my next post, we will discover another group of alkaloids that are equally insidious. Unlike aristolochic acid, which is exclusive to the birthwort family, the next group of plant toxins is very diverse, and collectively they are responsible for massive loss of grazing livestock, as well as epidemics of human poisoning.