Lipid effects, chemically specific

There are several such toxic agents that cause considerable medical, public and political concern. Two examples are discussed here the heavy metal ions (e.g. lead, mercury, copper, cadmium) and the fluorophosphonates. Heavy metal ions readily form complexes with organic compounds which are lipid soluble so that they readily enter cells, where the ions bind to amino acid groups in the active site of enzymes. These two types of inhibitors are discussed in Boxes 3.5 and 3.6. There is also concern that some chemicals in the environment, (e.g. those found in industrial effluents, rubbish tips and agricultural sprays), although present at very low levels, can react with enhanced reactivity groups in enzymes. Consequently, only minute amounts concentrations are effective inhibitors and therefore can be toxic. It is suggested that they are responsible for some non-specific or even specific diseases (e.g. breast tumours). 

There are two kinds of effects of the membrane on the enzyme behavior a specific interaction between the enzyme and the lipid membrane and a nonspecific interaction of the membrane structure by itself on the enzyme kinetics. In the case of ATPase, the enzyme in solution is working in homogeneous and isotropical conditions. At the opposite extreme, in the membrane the enzyme is working under asymmetrical boundary conditions. In the last case there is a coupling between a scalar process and the vectorial transport effect. In conclusion, the effect of the membrane on the enzyme behavior is not only a chemical effect, but also a geometrical one. 

The chemical compositions and isomeric structures of the fatty acid chains of phospholipids is well known to have large effects on the physical properties of lipid bilayers, such as the temperatures of endothermic chain melting phase transitions. Lipid vesicles sensitized with lipid haptens can be agglutinated with specific antibodies directed against the haptens (see Fig. 1). 

---