Biological space understanding

Confusion often occurs with the use of the terms exposure , concentration , and dose . Dose is the amount of contaminant that is deposited or absorbed in the body of an exposed individual over a specific duration. Dose occurs as a result of exposure. Concentration is that level of contaminant present in the air potentially available to be inhaled. The atmospheric concentration of a chemical by itself does not define the total dose of a chemical delivered or the specific sites of potential injury. For a substance present in inhaled air to be toxic, a significant dose must first be removed from the inhaled air and be deposited on sensitive tissue. Knowledge of the dose to initial target sites provides a critical link between exposure and the subsequent biological response. Understanding the disposition of inhaled xenobiotics is complex and, due to space limitations, cannot be described in detail here. However, certain basic concepts need to be presented to provide information on the various factors related to exposure, dose, and response that are fundamental to understanding the potential human risk from inhaled chemical agents. 

In reality, however, such interactions are parts of a complex molecular interaction networks that are highly dynamic in time and space. Understanding such complex biological systems requires the unravelling of these networks and catching them in quantitative and predictive models, based on quantitative experiments. 

The properties of individual hemoglobins are consequences of their quaternary as well as of their secondary and tertiary structures. The quaternary structure of hemoglobin confers striking additional properties, absent from monomeric myoglobin, which adapts it to its unique biologic roles. The allosteric (Gk alios other, steros space ) properties of hemoglobin provide, in addition, a model for understanding other allosteric proteins (see Chapter 11). 

The concept of property space is progressively being used to gain a deeper understanding of the dynamic behavior of a single compound in different media (as we illustrate below with acetylcholine, see Section 1.4.2) or bound to biological targets (the carnosine-carnosinase complex, see Section 1.4.3), but it can be used also with a set of compounds to derive fertile descriptors for dynamic QSAR analyses (4D QSAR, see Section 1.4.4). 

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