One important consequence of the hydrophobic character of lipids is the concentration of nonpolar substances along ecological food chains, a process known as bioamplification. As an example, consider organisms in an aqueous environment such as a river or lake. Any substance which is more soluble in living tissues than in the surrounding water will tend to concentrate in even the simplest plants and animals. These plants and animals are often the food supply for more complex life forms—a food supply which contains a greater concentration of the substance in question. As we proceed up the food chain to larger predatory animals, the concentrations of some substances can be increased by factors of 10 000 or more.
What kinds of substances are likely to undergo bioamplification? Clearly those which are more soluble in living systems. Since the surroundings are dilute aqueous solutions while organisms contain both aqueous and lipid phases, nonpolar substances which dissolve in lipids are most likely to be concentrated. This is a problem with DDT, polychlorinated biphenyls (PCB’s), and other long-lived synthetic organic compounds. It also applies to metal ions which can combine with organic groups to form uncharged molecules. A good example of such an organometallic compound involves mercury. Certain aquatic microorganisms can convert relatively inert mercury metal into chloromethylmercury(II) (commonly known as methylmercuric chloride):
Since the charge of Hg2+ is neutralized by Cl3– and Cl–, this organometallic compound is quite soluble in lipid tissues. Concentration of mercury along aquatic food chains has led to several episodes where hundreds of Japanese people whose diet consisted mainly of fish became ill and many died. It also accounts for the relatively high level of mercury observed in some species of fish taken from the Great Lakes of the United States and Canada.
Humans and other organisms do have mechanisms for eliminating toxic and otherwise undesirable organic substances. A wide variety of enzymes in the human liver can convert hydrophobic molecules to more polar forms which are less soluble in lipid tissues and more readily excreted. However, there is often a problem with synthetic substances which have been synthesized by chemists and whose structures are quite different from any the liver enzymes are equipped to handle. As in the case of the ABS detergents, the reactions which decompose such substances may be relatively slow. If they are not converted into structures which can be eliminated easily, even small quantities may remain in the body for long periods, sometimes causing chronic illness. It is for this reason that toxic effects of synthetic organic or organometallic chemicals should be thoroughly tested before large quantities are released to the environment.
From ChemPRIME:20.3: Fats and Lipids