Methylmercury looks like methionine, so the blood–brain barrier and the placenta open a door.
Full mechanism, in text
Mercury & the food chain · which tuna is safe
1 · Microbes recast it into a form that only goes in
Marine microbes recast the trace of mercury in seawater as methylmercury.That step is the switch: easy to absorb, hard to dump, so every fish only takes it in.
2 · The denominator is burned; the numerator is kept
Each time a fish is eaten by the layer above, its protein and fat are burned as fuel but the methylmercury stays, so concentration climbs layer by layer.After a smaller fish is eaten, most of its protein and fat is burned as fuel. Methylmercury is not fuel. It will not burn, and it almost will not leave. Each layer up, concentration is pushed higher. Time is the second amplifier: large and old means high.
3 · It disguises itself as an amino acid, and the barrier opens
Methylmercury grabs the sulfur on cysteine, and the complex is close in shape to methionine, so the blood–brain barrier and placenta let it in like an amino acid.The moment it enters it grabs the sulfur on cysteine. The complex is close in shape to methionine. Transporters recognize cargo by shape, so the blood–brain barrier and the placenta open a door that was left for amino acids. A nervous system still being built is a site under construction — the same exposure is not the same as landing on a finished building.
4 · Selenium cuffs some of it — not a free pass
Selenium in fish flesh can cuff some mercury into a hard-to-reuse complex, but that is only a partial offset, not yet measured directly in people.Mercury's affinity for selenium is much stronger than for sulfur. When the flesh has enough selenium, some mercury is cuffed into a complex that is hard to use again — before it can grab an enzyme. That is partial offset, and this step comes mainly from animal and mechanism studies; it has not been measured directly in people. The selenium that cuffs it is also spent; take a selenium supplement and eat high-mercury fish freely does not hold. The species order still stands.