The last one really took me by surprise when I heard about it from Dr. Blakey (who, again, is a genius who knows everything about everything and who of course was able to not only identify the small fruits as loquats, not apricots, but also knew that their seeds are toxic). It turns out that the pebble-resembling seeds contain small amounts of cyanogenic glycosides that release cyanide when they are digested. In particular, the seeds contain amygdalin and the amygdalin enzyme, which upon digestion is broken down into hydrogen cyanide. Therefore, people are strongly advised not to eat the seeds, because eating too many could result in cyanide poisoning.
I became intrigued and decided to look up the mechanism of cyanide poisoning (just for kicks, don't worry I'm not planning anything). Once hydrogen cyanide is in your system, the cyanide ions will bind with iron in the enzyme cytochrome a3, which is part of the cytochrome oxidase complex in mitochondria. These cytochromes are proteins that carry out electron transport. Cyanide inhibits the cytochrome a3 enzyme, thereby it shutting down the mitochondrial electron transport chain and preventing cells from using oxygen. The cells are forced to revert to anaerobic metabolism, so they produce lactic acid and trigger lactic acidosis, when your body tissues and blood become too acidic.
The rapidity with which cyanide affects the body is actually terrifying. Cyanide affects the central nervous system and the heart before other organs, so you go into cardiac arrest. Your body experiences hyperpnea (hyperventilation) from the lack of oxygen and begins convulsing after only 30 seconds. You stop breathing after 2-4 minutes, and all cardiac activity stops after 6-8 minutes.
One of the most unique aspects of cyanide poisoning that distinguishes it from other toxic chemicals is that a lethal dose is time dependent. Normal human tissue contains traces of cyanide that is naturally metabolized, so our bodies are able to detoxify small amounts of cyanide. Therefore, a lethal dose of cyanide may not be lethal if administered over a very long period of time, since the body can metabolize small amounts. It's only when the dose of cyanide is absorbed all at once that the natural metabolic mechanism is overwhelmed and the effects are lethal.
For humans, the lethal dose of cyanide is 2500-5000 (mg)(min)/m^3. The units have to include minutes since a lethal dose depends on the duration it's administered. In the form of sodium cyanide or potassium cyanide, the lethal dose is only 100-200 mg.
I couldn't find the exact amount of cyanide in the loquat seeds, but I did see that the recommended maximum consumption was 2 full loquats per day, so apparently you can still eat two sets of loquat seeds and be okay.
Peaches, which are in the same fruit family as loquats, also contain some amygdalin, but one peach pit only amounts to 9 mg of cyanide, so you would have to eat a lot of peach pits to get cyanide poisoning.
Actually, when I read further into amygdalin, I found that it's been getting publicity as a potential cancer treatment. There have been lab studies where amygdalin was administered with the enzyme glucosidase to release cyanide in the area of injection. This cyanide kills the cancer cells. However, these studies were only conducted in vitro using cancer cells in plates, and researchers haven't figured out a way to keep the cyanide from killing normal human cells. Who knows, perhaps scientists will eventually discover a key difference between cancerous and healthy cells that will allow them to only direct the cyanide at the cancer cells.
Long story short, don't be deterred from eating those cute little loquats at breakfast! I'm pretty sure no one's about the eat one of those seeds anyways, but even if you do, it's not enough to kill you.
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