How an element gets into a protein
Zinc and selenium appear in the same annex to Regulation (EU) No 432/2012, but they reach the proteins of the human body by two fundamentally different routes. Anyone wanting to read the quoted sentences without confusing the two nutrients cannot get past this difference — it also explains why the reference amounts for the two lie so far apart.
Andalusitriff is an editorial information site. We do not sell capsules, powders or drops. What we offer is a written-out dossier as a digital file: the connection between the chemistry of the two elements, the figures from the nutrition tables and the wording the European Union has authorised for labelling.
Coordination: held, but not linked
In the organism zinc is present throughout as a doubly positively charged ion and takes no part in electron transitions. Its job is structural: side chains of a protein — usually histidine, cysteine or glutamic acid — fold around the ion and hold it at a precisely defined position. The bond is strong enough to hold the molecule in shape, and loose enough for the ion to vacate the position again.
An adult's total stock is of the order of two to three grams. There is no dedicated storage organ for it; the element is distributed across muscle, bone and the covering tissues, and it is continuously replenished and released again.
Zinc contributes to the maintenance of normal skin.
Under Regulation (EU) No 432/2012
Covalent: selenium takes the place of sulphur
Selenium takes a different route. It is not attached as an ion to a finished protein but incorporated as a building block while the chain is still being made. In the amino acid selenocysteine, selenium sits exactly where sulphur sits in ordinary cysteine. The bond is covalent: the atom only leaves the molecule when the molecule itself is broken down.
In the human genome about twenty-five genes are described whose products carry selenium in this way. All other selenium in the body is either on its way to such a protein or present as selenomethionine at positions where incorporation happened without any particular control.
Selenium contributes to the maintenance of normal hair.
Under Regulation (EU) No 432/2012
Why this makes a difference for analysis
One practical consequence follows from the two routes of incorporation. A zinc value in blood plasma reflects a point of transfer, not a store: by far the greatest part of the body's zinc sits in cells and is not captured at all by a blood draw. With selenium, by contrast, certain selenoproteins in plasma indicate how well supplied the production of these proteins currently is.
Both measurements belong in specialist medical hands and are neither offered nor evaluated on this page. We set down how they come about — interpreting an individual finding is a matter for the practice that obtained it.