Chapter 5
Factors Affecting Xenobiotic Action
5.1 INTRODUCTION
Many factors can affect toxicity of xenobiotics. This chapter examines some of
these factors, including physicochemical properties of toxicants, dose or
concentration, mode and duration of exposure, environmental factors,
interaction, and biological and nutritional factors.
5.2 PHYSICOCHEMICAL PROPERTIES
Physical and chemical characteristics – such as whether a pollutant is solid,
liquid, or gas, whether it is soluble in water or in lipid, organic or inorganic
material, ionized or non-ionized, etc. – can affect the ultimate toxicity of a
pollutant. For instance, a non-ionized substance may be more toxic than an
ionized or charged counterpart because the non-ionized species can pass
through the membrane more easily than the ionized species and, therefore, is
more readily absorbed and able to elicit its toxic action.
5.3 DOSE OR CONCENTRATION
Dose or concentration of any pollutant to which an organism is exposed is
often the most important factor affecting its toxicity. Once a pollutant gains
entry into a living organism and reaches its target site, it may exhibit an
injurious action. For this reason, any factors capable of modifying internal
concentrations of the pollutant can alter its toxicity. The effect of the pollutant
is therefore a function of its concentration at the locus of its action. A pollutant
may either depress or stimulate normal metabolic function. In general, minute
amounts of a pollutant may stimulate metabolic function, whereas large doses
may impede or destroy its activity. For example, a recent epidemiological study
showed that in the area of Kuitan, a city situated in the western part of China,
many residents suffer from arsenism, a disease caused by arsenic (As)
poisoning, after consuming well water containing high levels of the mineral.
Residents who had consumed well water containing 0.12 mg As/l for 10 years
manifested arsenism with a prevalence rate of 1.4% of the city population.
However, in residents who had consumed water containing 0.6 mg As/l for
only 6 months, the prevalence rate increased to 47%, and the patients showed
more severe symptoms.
1
[16:54 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-005.3d]
Ref: 4365 MING-HO YU Chap-005 Page: 65 65-84
65
Factors Affecting Xenobiotic Action
5.1 INTRODUCTION
Many factors can affect toxicity of xenobiotics. This chapter examines some of
these factors, including physicochemical properties of toxicants, dose or
concentration, mode and duration of exposure, environmental factors,
interaction, and biological and nutritional factors.
5.2 PHYSICOCHEMICAL PROPERTIES
Physical and chemical characteristics – such as whether a pollutant is solid,
liquid, or gas, whether it is soluble in water or in lipid, organic or inorganic
material, ionized or non-ionized, etc. – can affect the ultimate toxicity of a
pollutant. For instance, a non-ionized substance may be more toxic than an
ionized or charged counterpart because the non-ionized species can pass
through the membrane more easily than the ionized species and, therefore, is
more readily absorbed and able to elicit its toxic action.
5.3 DOSE OR CONCENTRATION
Dose or concentration of any pollutant to which an organism is exposed is
often the most important factor affecting its toxicity. Once a pollutant gains
entry into a living organism and reaches its target site, it may exhibit an
injurious action. For this reason, any factors capable of modifying internal
concentrations of the pollutant can alter its toxicity. The effect of the pollutant
is therefore a function of its concentration at the locus of its action. A pollutant
may either depress or stimulate normal metabolic function. In general, minute
amounts of a pollutant may stimulate metabolic function, whereas large doses
may impede or destroy its activity. For example, a recent epidemiological study
showed that in the area of Kuitan, a city situated in the western part of China,
many residents suffer from arsenism, a disease caused by arsenic (As)
poisoning, after consuming well water containing high levels of the mineral.
Residents who had consumed well water containing 0.12 mg As/l for 10 years
manifested arsenism with a prevalence rate of 1.4% of the city population.
However, in residents who had consumed water containing 0.6 mg As/l for
only 6 months, the prevalence rate increased to 47%, and the patients showed
more severe symptoms.
1
[16:54 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-005.3d]
Ref: 4365 MING-HO YU Chap-005 Page: 65 65-84
65