Molecular and Histocytological Biomarkers - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 84

AChE forms only in the gills of the two bivalves could be the consequence of particular
adaptive features in these filter feeding organisms (Romani et al. 2006). The interpretation
proposed by these authors is that AChEs located in the gills must react first with toxic
compounds as a protection for other AChEs involved in neurotransmission. The resis-
tance of AChE forms to modern pesticides could be considered a preadaptation of a com-
mon origin resulting from the development of resistance to natural marine neurotoxins.
In vertebrates, two isoforms occur—AChE, the main function of which is the rapid
hydrolysis of the neurotransmitter acetylcholine, and butyrylcholinesterase (BChE; or
pseudocholinesterase), which has no known specific natural substrate, although it is able
to hydrolyze acetylcholine. The sensitivity of different ChEs differs greatly, as shown in
the three-spined stickleback ( Gasterosteus aculeatus ) after exposure to the OP insecticide
parathion-ethyl (Wogram et al. 2001). After exposure to 1 mg L -1 parathion, BChE activity
was significantly decreased in liver (~60%) and axial muscle (~30%), whereas its decrease
in gills (~30%) was not significant. No effects on BChE activity were observed with 0.1 and
0.01 mg L -1 parathion. AChE activity remained unaffected at all parathion concentrations
used. Similarly, Monteiro et al. (2005) highlight the fact that different forms of ChE existing
in fish have different sensitivities to cholinesterase-inhibiting compounds. Thus, with ChE
properties differing between species, several authors are happy to characterize the type of
enzyme present in the species studied in order to interpret this biomarker correctly (Scaps
et al. 1996; Kristoff et al. 2006; Gagnaire et al. 2008; Jebali et al. 2011).
Oliveira et al. (2007) have examined brain AChE in 20 fish species from the coast of Rio de
Janeiro state, Brazil, as a possible pesticide biomarker in marine environmental monitor-
ing. The enzyme sensitivity to methyl paraoxon, shows that Paralonchurus brasiliensis and
Genidens genidens —belonging to the super-order Acanthopterygii, which includes more
recently evolved species—are more sensitive than Merluccius hubbsi and Percophis brasil-
iensis —belonging to the super-order Paracanthopterygii, which includes the more ancient
bony fish species. These authors suggest a possible evolutionary linkage for AChE sensi-
tivity to methyl paraoxon. Interspecific differences in the responses of ChEs to environ-
mental pressure are well illustrated by the studies of Solé et al. (2009) and Fossi Tankoua
et al. (2010), who have determined biomarkers including AChE in the bivalve Scrobicularia
plana and the polychaete Nereis diversicolor collected from the same sites at the same dates.
Both studies carried out independently in Spain and France concluded that the polychaete
was highly responsive, whereas the bivalve was of no help in distinguishing sites accord-
ing to different degrees of contamination by cholinesterase-inhibiting compounds.
In addition to being inhibited by different xenobiotics, AChE activity may also be influ-
enced by natural factors. In a recent review, Burgeot et al. (2010) explain that an increase in
water temperature significantly affects the expression of AChE activity, because tempera-
ture can change the activity of the enzymes by changing the protein conformation and the
catalytic efficiency or binding capacity. The literature provides numerous examples of the
influence of temperature on AChE activity and as a corollary, temporal variations have been
observed in different species (Kopecka and Pempkowiak 2008; Burgeot et al. 2010). Seasonal
variations can also result from physiological changes as exemplified by Xuereb et al. (2009b),
who report that significant differences in AChE activity were observed between female
amphipod crustaceans depending on gonadal and embryonic development. In estuarine
species, salinity is an important factor influencing AChE expression, for instance, in poly-
chaetes (Scaps and Borot 2000), copepods (Cailleaud et al. 2007), and bivalves (Fossi Tankoua
et al. 2011). In addition to salinity effects, changes in AChE levels were observed during the
tidal cycle and between surface and bottom-living copepods related to variations in hydro-
phobic organic contaminant concentrations (Cailleaud et al. 2009). Body size (or weight and