EESA10H3 Lecture Notes - Lecture 5: Organic Form, Bioavailability, Intellectual Disability
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EESA10 Lecture 5 – Feb 8, 2011
Some of the heavy metals have a tendency to accumulate in specific tissues – eg arsenic in hair, nails.
Some metals in liver, others in bone or brain. We can look for specific metals in certain tissues using this
Metals are classified in three classes:
- Class A – these metals are macronutrients eg calcium found in bones. Very low toxicity, but we
don’t say that they aren’t toxic. RULE: any chemical in the environment can be toxic, but the
question is what dose is toxic?
- Class B – not essential to physiological processes e.g. lead, mercury. Some of the most toxic
- Borderline – essential for some physiological processes, but not in large amounts
(micronutrients) such as copper, zinc, iron. In terms of toxicity, they are in between A and B.
Mechanism of Toxicity
- Mess up the function of carrier or receptor protein.
- Displace other metals that are not as toxic e.g. chromium replacing calcium. (Borderline or B
class metals do this).
- Some organic molecules have different conformations. The metals can change the conformation
of some biomolecules from the active into the inactive form (class B metals do this).
- Resistance – No uptake of the metal. Plants don’t take up lead, even if there is a high
concentration in lead. There are species that can live normally even though there is lead in the
- Tolerance – take up the metal, but can live with it inside us (can tolerate it). Able to withstand
high levels of a metal. Metal binds somewhere where it doesn’t make a problem or doesn’t
disrupt a process. Or, the metal can be converted to a non toxic or less toxic form. E.g. arsenic is
very toxic in inorganic form, not too toxic in organic form. This is important in marine
environments, where the arsenic gets converted into its organic form. Plants can tolerate many
Bioavailability of Metals:
- Not everything that we inhale, ingest, etc. will be the amounts that cause a toxic effect. We take
up much more than the receptors in our body pick up. We take up a higher amount than needed
to cause a problem. Why? Because we can excrete/metabolize some of the toxins. If we intake
100 ppm of a substance, the receptors in our body won’t pick up 100. Maybe only 80, for
- Bioavailability ranges from 0-100, where 0 means it is not bioavailable at all. If bioavailability is
low, toxicity is low. Elements with a valence of 0 are low bioavailability. Charged substances
have higher bioavailability.
- Acidic conditions, for most metals, higher bioavailability and toxicity.
- Redox: Oxidized forms of metals are usually less toxic (more oxygen). Reduced conditions (less
oxygen) where the metal is in the reduced form, is more toxic.
Routes of exposure: