Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

separate concentration-effects models for each chemical are used to predict individual effects and
the sum of these predictions is then compared to the actual effect of the mixture. Deviations
from the mixture effect are then classified as less than additive (antagonistic) or more than
additive (synergistic) (see Piegorsch et al., 1988; Finney 1947 for further information).
The complexity of metal mixture interactions as different metal concentrations,
environmental conditions (e.g., temperature, pH), and other factors can cause marked changes in
the effects observed (Norwood et al., 2003). In describing Pb interactions with other elements,
the different approaches to modeling mixture toxicity are considered. Specific reference to
known Pb-metal interactions and implications on Pb uptake and toxicity will also be made in
each of the studies below.
Less than additive or antagonistic interactions can reduce metal bioavailability when
metals are present in combination, and may lead to reduced potential for toxicity (Hassler et al.,
2004). A number of elements act in an antagonistic fashion with Pb. For example, Pb is a wellknown
antagonist to Ca 2+ (Niyogi and Wood, 2004; Hassler et al., 2004), which is an essential
element, required for a number of physiological processes in most organisms. Lead ions have an
atomic structure similar to Ca 2+ and can be transported either actively or passively across cell
membranes in place of Ca 2+ . An example of this interaction was reported by Behra (1993a,b)
where Pb was shown to activate calmodulin reactions in rainbow trout (O. mykiss) and sea
mussel (Mytilus sp.) tissues in the absence of calcium. Calmodulin (CaM) is a major
intracellular calcium receptor and regulates the activities of numerous enzymes and cellular
processes. Allen (1994) reported that Pb can replace calcium in body structures (e.g., bones,
shells); replace zinc in ALAD, which is required for heme biosynthesis; and react with
sulfhydryl groups, causing conformation protein distortion and scission of nucleic acids
(Herkovits and Perez-Coll, 1991). Lead is also a known antagonist to Mg 2+ , Na + , and Cl !
regulation in fish (Ahern and Morris, 1998; Rogers and Wood, 2003, 2004; Niyogi and Wood,
2004). Li et al. (2004) reported on the interaction of Pb 2+ with Cd 2+ in the context of adsorption
from solution by Phanerochaete chrysosporium, a filamentous fungus. The authors found that
cadmium uptake decreased with increasing concentration of Pb ions with Pb 2+ outcompeting
Cd 2+ for binding sites.
Hassler et al. (2004) reported that in the presence of copper (Cu 2+ ), there was a
significantly higher rate of internalization of Pb in the green algae Chlorella kesserii. It was
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