Chemicals used as plasticizers are released into the environment in large quantities. They are hydrophobic and are able to both bioaccumulate (Van der Oost et al. 2003
) and cross the blood–brain barrier (Waterhouse 2003
). The results from the present study clearly show that some plasticizers can inhibit SULT 2A1 at micromolar concentrations. Concentrations of the alkylphenols in polluted waters can reach 10–70 nM, and these lipophilic compounds have been found to bioaccumulate 400-fold in estuarine fish (see Kirk et al. 2003
). Nevertheless, even in the case of 4-tert
-octylphenol—the most potent of the inhibitors in this study and one of the most commonly used plasticizing agents—it is unlikely that it reaches micromolar concentrations in the human CNS. However, because the physiologic concentration of DHEA is very much lower than the Km
of the enzyme for this substrate, the Michaelis-Menten equation (v
], where v
is the rate of reaction, V
is the maximum rate of reaction, S
is the substrate concentration, and Km
is the Michaelis constant) simplifies to v
. In the presence of a competitive inhibitor, the rate is given by v
(1 + i
), where i
is the inhibitor concentration. Hence, even relatively low concentrations of inhibitor will interfere with DHEA sulfation. Long-term accumulation of plasticizers in brain tissue may therefore reduce the efficiency of memory processes as pregnenolone sulfate and DHEAS are required for acetyl choline release and receptor modulation.
From these results, it appears that the presence of a single benzyl or phenyl group and another compact hydrophobic side-chain are both important for tight binding to the active site. Benzyl butyl phthalate is three times more potent than the aliphatic dibutyl phthalate, which is many times more potent than its longer-chained counterparts. Similarly, 4-tert-octylphenol, which has a compact, hydrophobic side-chain, binds nearly five times more tightly than its straight-chain isomer. However, although a single aromatic group appears to aid binding, bisphenol A, which has two such moieties, is a less potent inhibitor.
The real-time RT-PCR results also showed significant dose-dependent decreases in CDO1, SUOX, and PAPSS1 mRNA expression levels in TE 671 cells treated for 24 hr with 0.005–0.5 μM 4-n-octylphenol, diisodecyl phthalate, or bis(2-ethylhexyl)phthalate. Endocrine-disrupting effects of some plasticizers may therefore also be a consequence of modulation of expression of enzymes supplying PAPS for hormone sulfation. Our results indicate that exposure to 4-n-octylphenol, diisodecyl phthalate, or bis(2-ethylhexyl)phthalate may negatively affect the sulfate supply pathway. This could result in reducing PAPS production, giving increased levels of free hormones and decreased capacity for detoxification via sulfate conjugation
Although in vitro
studies with tissue homogenates and human cell lines cannot necessarily be correlated with the experiences in vivo
, it is increasingly evident from recent work (Chen et al. 2006
) that DHEAS plays an important role in memory function. In addition. animal experiments have shown that chemicals of the type used in the present study inhibit cognitive function (MacLusky et al. 2005
), alter synaptic plasticity (Kawato 2004
), and affect the acquisition of memory (Carr et al. 2003
). Similarly, exposure to polychlorinated biphenyls and brominated fire retardants has also been linked to adverse effects on both memory and learning (Fonnum et al. 2006
). Although the genetic mechanisms were not explored in detail, the authors of this article have analyzed the first 4,000 bases of the upstream flanking region of the PAPSS1
gene using the MatInspector program (Genomatix Software GmbH, Munich, Germany). This revealed two potential estrogen response elements 483–501 and 2916–2934 bases upstream of the transcription start site, and two aryl hydrocarbon receptor response elements 39–61 and 346–368 bases upstream of the transcription start site. In addition, there were other potential nuclear hormone receptor response elements and numerous cAMP response elements. Endocrine disruptors, including these plasticizers, often have weak estrogenic properties, interact with the aryl hydrocarbon receptor (Bonefeld-Jørgensen et al. 2007
), and influence a range of cell signaling pathways. The reduction in PAPSS1
expression could occur via any of these routes.
In conclusion, the results from these experiments suggest that plasticizers may have deleterious effects on physiologic function including neuronal pathways and that, where possible, alternatives should be sought that do not interact with critical metabolic pathways. The information provided by the present study creates novel avenues for hazard identification and risk assessments and has shown that environmental contaminants may interact with nonreproductive steroidal function.