Biotransformation invertebrates

Aquatic organisms, such as fish and invertebrates, can excrete compounds via passive diffusion across membranes into the surrounding medium and so have a much reduced need for specialised pathways for steroid excretion. It may be that this lack of selective pressure, together with prey-predator co-evolution, has resulted in restricted biotransformation ability within these animals and their associated predators. The resultant limitations in metabolic and excretory competence makes it more likely that they will bioacciimiilate EDs, and hence they may be at greater risk of adverse effects following exposure to such chemicals. 

PAHs can be bioconcentrated or bioaccumulated by certain aquatic invertebrates low in the food chain that lack the capacity for effective biotransformation (Walker and Livingstone 1992). Mollusks and Daphnia spp. are examples of organisms that readily bioconcentrate PAH. On the other hand, fish and other aquatic vertebrates readily biotransform PAH so, biomagnification does not extend up the food chain as it does in the case of persistent polychlorinated compounds. As noted earlier, P450-based monooxygenases are not well represented in mollusks and many other aquatic invertebrates (see Chapter 4, Section 4.2) so, this observation is not surprising. Oxidation catalyzed by P450 is the principal (perhaps the only) effective mechanism of primary metabolism of PAH. 

The elimination rate of a compound (directly or by biotransformation) from an organism determines the extent of the bioconcentration and depends both on the chemical and the organism. Direct elimination includes transport across the skin or respiratory surfaces, secretion in gall bladder bile, and excretion from the kidney in urine. Other processes are moulting (for arthropods), egg deposition (fish, invertebrates) and transfer to offspring or via lactation (in mammals), which are more specific and not usually contemplated in bioconcentration determination. 

Invertebrates had been thought to have poor capability to biotransform many POPs, as shown by experiment [186-188]. This lack is likely from low CYP abundance and activity. Chirahty has shown that while it is likely that most aquatic invertebrates do indeed lack the capacity to biotransform POPs, some species are capable of metabolizing some POPs stereoselectively. This finding is significant, as invertebrates are a major component of lower food webs, and bioaccumulation of nonracemic POPs results in more significant enantiomer-specific exposure and toxicity to predator organisms, including humans. 

There are consistent attempts in the literature to interpret specific biotransformation of PCBs that occurred in an organism from the relative ratio of X-CB to that of a reference CB-Y. The later could be CB-153, CB-138 (2,2, 3,4,4, 5-hexa CB), and CB-180 (2,2, 3,4,4, 5,5 -hepta CB) [52]. Such structure-dependent metabolism studies predicted induction of Cytochrome P450 isozymes in the marine food chain [59], small cetaceans [52], beluga and narwhal [60], river dolphins [61], pinnipeds [52], aquatic fauna including invertebrates [62], and in humans [63]. Comprehensive characterization of xenobiotic metabolizing enzymes in whales and pinnipeds has been carried out to show the utility of this approach [51,64]. 

James MO (1989) Biotransformation and disposition of PAH in aquatic invertebrates. In Varanasi U (ed.), Metabolism of PAH in the Aquatic Environment. CRC Press, Boca Raton, Florida, pp. 69-91. 

Efficient biotransformation of TNT has been reported for fish [3,7,9] and an aquatic invertebrate [6], most of which accumulated biotransformed products including tissue-bound molecules, at higher concentrations relative to the parent compound at steady state [6,7,9], The ability of aquatic animals to biotransform other explosives and related compounds is unknown. Recent studies examining the uptake from water and elimination kinetics of nitroaromatic and cyclic nitroamine explosives in aquatic invertebrates and fish [2,3,5-9] revealed that elimination of those compounds is very efficient, leading to steady-state levels within hours. 

The nitro groups of TNT characteristically undergo biochemical reduction in living systems (see Chapter 2). Biotransformation of TNT to ADNTs during aqueous exposures occurred very quickly in T. tubifex (0.212 and 0.187 h 1 for 2-ADNT and 4-ADNT, respectively) [6], but was markedly slower (0.06 h 1 for ADNTs) in juvenile sheepshead minnows [3], Substantial biotransformation of TNT to ADNTs in fathead minnows exposed to TNT for periods as short as 10 min was observed [2], Efficient biotransformation of TNT to ADNTs was also reported for soil invertebrates... 

Nonextractable radioactivity in invertebrate and fish tissues likely represents covalent conjugates (i.e., macromolecular bound residues) [6,7], However, neither the biological half-life nor the chemical nature of nonextractable transformation products in invertebrates and fish has been investigated to date. Formation of covalent bonds with proteins in mammalian systems has been reported [28-30], Investigations on the biotransformation of other explosives or related products in aquatic animals were not found in the available literature. 

Terrestrial and aquatic vascular plants promote the biological reduction of nitro groups on TNT to amine groups, yielding 2-ADNT and 4-ADNT, similar to invertebrates and vertebrates [13]. In vascular plants, the putative reduction enzyme is a nitroreductase [44], During reductive TNT biotransformation, ADNT products are accumulated within the plant tissue or excreted to the surrounding culture medium. For Myriophyllum species, the excreted ADNT products accounted for less than 20% of the initial TNT, and only trace levels of free ADNT remained in the biomass [37,38,42], One study [40] also showed that M. aquaticum was capable of oxidative transformation of TNT via methyl oxidation or aromatic hydroxylation, with oxidation products accounting for nearly 36% of the TNT initially added. 

Finally, data indicate that di- -butyl phthalate can partition from food and water into a variety of organisms. Studies using radioactively labeled di- -butyl phthalate have shown accumulation of radioactivity in aquatic invertebrates (Sanders et al. 1973) and fish (Wofford et al. 1981). Most of the accumulated radioactivity is apparently in the form of the primary metabolite, mono- -butyl phthalate (Howard 1989). Numerous experiments have shown that the accumulation of di- -butyl phthalate in the aquatic and terrestrial food chain is limited by biotransformation (i.e., transformation of chemical compounds within a living system), which progressively increases with trophic level (Staples et al. 1997). 

Half-Life of PAHs, Because some invertebrates are capable of rapidly metabolizing PAHs and others have limited ability to biotransform these compounds, a large range in half-lives of PAHs in marine organisms has been reported (see Table 2). Some studies report rapid half-lives on the order of hours, but most report half-lives on the order of a few days to a week or more. Some studies report a classic fast and slow (biphasic) component to elimination. The fast component of elimination may indicate a half-life of a few days to a week however, the slow component is an asymptotic elimination of some PAHs that may indicate long-term retention of a significant proportion of accumulated xenobiotics. 

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