The Indole Chemistry of Bananas

Challenged by his daughter, Martin explores the Indole Chemistry of Bananas. He half remembered some Biochemistry he learnt as an undergraduate and later, while he was in hospital, he regurgitated this at a baffled clinician. It earned him some kind of reputation. But could he remember more; could he research this subject further on the Internet? Could he explain it simply?

In-spite of all the subsequent computery stuff, I studied Biochemistry as an undergraduate. How good I was at it was always moot, but every now and again I'd be quite engaged by something and it stuck. Time past, and several career changes later, I find that my interest in Biochemistry has been renewed by changes to my health. As a consequence, biochemical things reappear like scenes in that "Chuck" spy series, or a bad acid flashback.

Bananas

I've a bizarre little disease, and it's quite rare. It thus proved to be a diagnostic challenge. Once it was suspected, however, it was really quite easy to confirm. The disease involves the excessive production of a hormone called Serotonin, which may ring some bells in the biological orientated. In this case I produce rather a lot of it, in the wrong place to boot. The circulating Serotonin is broken down in the lungs, of all places, and turns into 5-Hydroxy Indole Acetic Acid. That's a big chemical name, so it's generally termed 5HIAA. Anyhow, probably too much detail. There's some more too-much detail, in that the 5HIAA is secreted in the urine. So, diagnosing this disease involves looking for 5IAA in my urine, which I have to collect over a day. Too much 5HIAA and it's likely you've my disease, Carcinoids. This test can be compromised by eating things that have chemicals similar to this important 5HIAA marker. Bananas, and indeed other fruit have just such a chemical in them, in abundance. I'm therefore counselled not to eat Bananas immediately before and during the test. It all seemed a little odd, that Bananas have all this chemical in them, to the person delivering these instructions. But I knew, and the patient suddenly started spewing out all this weired plant Biochemistry. I thus became The Banana Man, on that Ward at least, and I feel doomed to repeat these expansions every time I take the urine Indole test. It all reinforces my reputation for oddity, probably richly deserved.

Having a high level of Serotonin in my blood stream does not turn me into a happy-camper, inspite of what the Daily Mail thinks.

I find that I've bandied the term 'Indole' around a little freely in the last paragraph. Indole is a chemical group, that forms larger molecules. It comes from one of the basic Amino Acids, Tryptophane, which is 'essential', in that Humans and other animals cannot make it themselves, they must get it from the food they eat.

Bananas are chock full of Indole based chemicals, specifically an Indole amine, not dissimilar to Serotonin. This can be chemically altered, oxidised, and then joined into long chains, polymerised. When this occurs a large amount of blackish-brown gunge ensues. This polymer is the reason why bruised fruit, and in particular Bananas go brown. The trigger for this is molecular Oxygen, that's Oxygen in the air.

There are a whole bunch of chemistry know as 'oxidation'. We can all be familiar with oxidation, it occurs all around us. The combination of Oxygen with carbon heats our houses, and with iron rusts our nails and our cars. Oxidation, in proper chemical terms, is more complicated than that. It's all about the exchange of electrons as Chemists say. As a lesser mortal, a mere Biochemist, I have to agree, but with Oxygen not necessarily, but usually the final receiver of these electrons in biological systems. The vast majority of biological systems therefore do not involve oxidation with Oxygen, but rather some intermediate chemical, usually one called NADH.

But the Banana Indole oxidation is unusual. It actually uses Oxygen, from the air. The chain of reactions that will eventually lead to the brown sludge is all dependent upon Oxygen, which is not present unless there is tissue damage. So you get the brown sludge, but only if there's tissue damage and, indeed, Oxygen. The whole chemical pathway is poised and ready to go, but without Oxygen it doesn't.

As a biochemical reaction it was an easy one to follow, for it could be done with an Oxygen Electrode that measured that amount of Oxygen being used. Normally, Biochemists have to construct artificial chemical pathways that involve the fore-mentioned NADH, and track it by shining an Ultraviolet light through the mixture. It's an important reaction, in the food industry too, for nobody likes brown Bananas, even if the generation of the brown gunge stops invading bacteria and has defiant evolutionary advantages. These reactions are thus extensively studied by Biochemists in the food industry and we, as undergraduates, were invited to practise with what could be our future.

At one point I thought to be a Food Scientist, but I couldn't get on the MSc conversion course, it was too popular.

It turns out that it's relatively easy to stop the formation of the brown gunge, by stopping the oxidation. You can deprive the reaction of Oxygen from the air, by simply keeping the air away from the damaged Banana; in a jar, or underwater, for example. You can also stop the reaction by interfering with the stuff that facilitates the oxidation, something called mono-amine-oxidase. This chemical is called an enzyme and it serves to hold the components of a reaction together so they react. This particular enzyme has, at its heart an atom of Magnesium which can be ripped out, rendering the enzyme broken and ineffective. Chemicals which have a claw like structure are particularly good at this. For example, Citric acid, found in Lemons, works. Other chemicals can be even more effective, but they're generally poisons. This is considered an undesirable quality in the food processing industry. But Citric acid is quite effective, you can slosh lemon juice on it. You don't have to be as knowledgeable as Hester Bluemental, and most cookbooks will tell you that.

At the end of all this, I'd not discovered anything new about stopping Bananas going brown. Some of the background biochemistry had sunk in, to emerge some thirty years later.

As part of the whole learning process, I also did some reading around the subject, all those years ago. This sometimes pops out too. Serotonin is an interesting chemical. Not only is it a hormone, something I didn't know until I fell ill, and had it explained to me, but it is also a Neuro-transmitter.

A Neuro-transmitter is a chemical that connects two (or more) nerves together. They are particularly and obviously important in the Brain, where there are lots of nerves which have to be connected together. Neuro-transmitters tend to be specialised in specialised bits of the Brain and Serotonin is no exception. It is found in particular in the base of the brain where it is involved in mediating how we perceive things. We know that if we interfere with things drastically you see things, and that's how LSD works. If one interfere with it gently, by either slowing its breakdown, or its re-absorption then we feel slightly happier about our selves, and that's how antidepressants work. The breakdown is an oxidation process, once again, but this time the enzyme is rather more orthodoxed, it doesn't use molecular Oxygen, but rather one of those other odd chemicals, something called FAD.

My strange little disease produces quite fantastic amounts of Serotonin, which is destroyed in the lungs. I've often speculated, to myself, for fear of having a clever person laugh at me, on how the Serotonin is oxidised. Since it's oxidation, and in a place where there's lots of of it, might that involve molecular Oxygen?

And thus my rambling narrative comes back to Bananas, and what makes them go brown. Hopefully it's understandable. I was tempted to write some old-skool review paper, to show that I could still do it. It would have been titled "Tryptophane derived alkaloids and related metabolic pathways in Musa sp.", or something similar. But, I doubt that you'ld have read much further than that.

Cockermouth, 15th March 2011.

~Z~

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Martin is currently on a health sabbatical, but is interested in the odd bit of pro-bono work by the way of therapeutic recovery. So, if you've anything that he can tackle on a non-commercial basis from his base in Cockermouth please let him know. Unsurprisingly, he's particularly interest in biology, again, and the application of computing to it.