Metabolism of toxic heavy metals in growing organisms: A review

Abstract

The metabolism of nonessential heavy metals in the growing organism is disscussed. The existing experimental data can be summarized as follows. 1. 1. The absorption of heavy metals is greatly enhanced in the suckling animal until weaning. The combined action of a milk diet and a greater nonselective permeability of undeveloped intestinal barrier has been proposed as a possible explanation of this phenomenon. 2. 2. Transportability and distribution between organs of several heavy metals in the growing animal are different from those in adults. Most important is the fact that toxic heavy metals such as lead and inorganic mercury are accumulated in the brain of the immature rat in much greater amounts than in adults, suggesting that at the same uptake level the immature organism could be much more severely injured. 3. 3. Spontaneous and provoked (as induced by chelating agents) excretion of heavy metals is lower in the young organism. Greater stability and inertness of heavy metal linkages with body ligands in the growing organism with a consequently lower level of “free metal” could serve as explanation of this fact. 4. 4. The toxicity of heavy metals is lower for the young animal than for adults. This supports the hypothesis about a lower level of “free metal” and a lower level of active fraction of metal body burden in undeveloped organism. However, the lower toxicity of heavy metals in the young animal is greatly overwhelmed by much higher absorption rate, lower excretion, and special unfavorable distribution in the immature organism. Therefore, it is recommended that current standards on permissive daily exposure to toxic heavy metals should be carefully reconsidered, taking into account the specificities of behavior of heavy metals in growing organism.