Gastronomy Meets Biochemistry Quiz

Answer: NMDA and D-Asp had been linked to the production of sex hormones

The original "scientific" association between oysters and libido came from the observation that males release 1-3 mg of zinc per ejaculate. With the conclusion that zinc was somehow important to the sperm, the observation that oysters were high in zinc confirmed in many people's minds that there was truth in the oyster's aphrodisiac powers. However, many remained unconvinced.

A more direct biochemical association came from the observation that bivalves were abundant in NMDA and D-Asp, as determined by an analytical technique known as chromatography. NMDA and D-Asp are both amino acids, and play a role in the release of sex hormones such as testosterone and progesterone. Moreover, rats injected with NMDA and D-Asp displayed increased testosterone production. Despite this, a definitive conclusion that eating an oyster increases sexual desire remains to be biochemically proven.

Answer: Caffeine mimics adenosine and so competes to bind these receptors in the brain

Caffeine is an analogue of adenosine, meaning they are structurally similar. By competing with adenosine to bind excitatory receptors in the brain (such as the glutamate receptors), caffeine prevents adenosine from exhibiting its inhibitory effects, therefore increasing neurotransmission.

Another thing you may notice with caffeine is that it makes you visit the toilet a lot more. This is because of its effects on a hormone called ADH. ADH is released from the pituitary gland and travels to the kidney, where it induces the activity of aquaporins and so allows greater reabsorption of water into the bloodstream and less is released in the urine. Caffeine inhibits ADH release and so increases the amount of water released in urine.

Answer: The body metabolises phenethylamine in chocolate before it can exert its effect on the brain

Phenethylamine is rapidly metabolised by enzymes called MAOA and MAOB. These are monoamine oxidases, and so catalyse the oxidative removal of amine groups from molecules. This means that phenethylamine no longer has the "amine" and can no longer exert feelings of love on the brain. MAOA and MAOB really are the party-poopers of the body, also metabolising dopamine and other such chemicals which are linked to the feeling of excitement and reward.

There may still be truth in finding happiness in a bar of chocolate, however. Chocolate contains tryptophan - an amino acid which our body cannot produce and so which must be required from our diets. Tryptophan is metabolised in our bodies into different chemicals, one of which is serotonin, also known as the "happiness molecule". However, like with phenethylamine, there is disagreement between scientists over whether the relatively low amount of tryptophan obtained from eating chocolate is sufficient to induce any significant physiological effects.

Answer: Retinal

Beta-carotene is metabolised in our bodies to vitamin A (or retinol), which can then be converted into retinal. Retinal contains a long hydrocarbon chain, containing many carbon-carbon double bonds. These bonds can exist in one of two so called stereoisomers - cis or trans. All double bonds in retinal are trans, apart from that at carbon 11, which is cis. In the retina, retinal combines with opsin to form rhodopsin - a photosensitive pigment. When light reaches rhodopsin, the cis bond becomes a trans bond and so there is a conformational change. This conformational change is coupled to a nerve impulse which travels down the optic nerve to the brain, which processes the information that light has been detected. In the meantime, enzymes convert the so-called "bleached" rhodopsin back into its original form (i.e. with a cis double bond at carbon 11).

The urban myth that carrots make you see in the dark arose from the RAF, where pilots would claim that successful night bombings were due to carrot consumption, thus covering up the development and use of radar.

Answer: Glucose and Galactose

Lactose intolerance is due to a lack of the enzyme lactase, which is responsible for braking lactose down into glucose and galactose (galactose gets its name from the Greek word for "milk", which is also where we get the name "galaxy"). The default setting for most mammals is for this enzyme to be lost after the baby stops breast-feeding.

However, domestication of cattle led to those who maintained the lactase enzyme having a selective advantage, since they could properly metabolise milk. It is unsurprising, therefore, that lactose-intolerance usually persists in ethnic groups which were not among the first to domesticate cattle.

In normal circumstances, the glucose obtained from lactose breakdown is used for cellular respiration (or other cellular processes) whereas galactose is taken to the liver to be converted to glucose. Lactose, however, remains in the colon of those who lack the enzyme lactase and acts as a food source for bacteria, which increase in number and contribute to "gassiness".

It also acts as a laxative and so may cause diarrhoea.

Answer: An ion channel

Specifically, VR1 is a ligand-gated ion channel, since its gating is dependent on the binding of capsaicin. It is a non-specific cation channel, meaning that, when it is open, positively charged ions such as sodium flood into the cell and cause depolarisation of the cell membrane, eventually forming an action potential.

This action potential is transmitted to the brain via neurons. The nerve impulse here is usually associated with pain and abrasion, and so the brain reacts to eating chilies in the same way as it would if something physically hot was placed on the tongue.

Answer: Sulphuric acid

Enzymes released when onions are cut break amino acid sulphoxides down into sulphenic acids. Different enzymes then metabolise these acids to form propanethiol S-oxide (also known as the lachrymatory factor), a volatile compound that diffuses through the air, inevitably into our eyes.

This gas reacts with the water in your tears to form sulphuric acid. The stinging induces more tears to be released, eventually flushing the irritating chemicals out of our eyes.

Answer: Heterocyclic amines are carcinogens

Scaremongering newspaper articles often claim that such-and-such "causes cancer". Many things do contribute to cancer, but often in very small ways which cannot be directly linked to the disease since it is, by nature, a disease caused by accumulation of mutations.

It is rare that cancers can be pinned down to one specific gene, or, for that matter, to a glass of red wine. However, heterocyclic amines have been conclusively shown to be carcinogens. They are oxidised by what are called P450s in the body, leading them to become more reactive and capable of reacting with DNA.

Heterocyclic amines are bulky molecules and by forming adducts with DNA, they can interfere with DNA replication and so increase the chance that there will be a mistake (a mutation).

These molecules are actually produced when any muscle meat is cooked, resulting from amino acids reacting with creatine, and they become more abundant the more the meat is cooked.

Answer: That they work in combination with our environment to determine our characteristics

It is a common myth that genes are the sole determinants of phenotypes. "Show me the 'Gay Gene'", people say. "This gene makes you fat", they exclaim. No it doesn't. Genes work in combination with our environment to determine what we are like. A gene may make it more likely that you will be overweight, but so will eating lots and lots of fatty foods. Experience and age can change the extent to which genes are expressed. Genes can be partially silenced by modifying proteins in chromosomes so that, although the genes are still there, they are less active. This is called epigenetics.

A gene which the Guardian called the "Brussels sprout gene" encodes a G protein-coupled receptor that detects compounds called glucosinolates, which are found in plants of the genus Brassica. Most people carry at least one copy of this gene per cell, making them more sensitive to the bitter taste of broccoli than people who contain no such gene. As mentioned above, genes work in combination with the environment to make us the way we are. It has been suggested that the aversion to bitter tastes is more strongly determined by genotype in our younger years than it is in our later years, where experience plays a greater role.

Answer: It binds to excitatory receptors and inhibits them, thus slowing nerve impulses

Thankfully, it is a myth that alcohol kills brain cells. Of course, pure ethanol will kill a brain cell just like it will kill any cell by dissolving the lipid membrane and causing the innards of the cell spill out like a gutted fish. However, a beer in the garden will not reduce your brain to mush. Now let's go back to that building...thingy, where our beds and stuff are. :)

The main receptor affected by alcohol is the NMDA receptor, which is a member of the glutamate receptors, the largest group of excitatory receptors in the brain. They normally bind neurotransmitters and allow an influx of cations, thus propagating an action potential, but alcohol inhibits this activity. This explains why words are often on the "tip of your tongue" after you've had a few, or when you've had even more, why you are less sensitive to physical pain (alongside many far less pleasant effects).