Chemistry Reference
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2.3.2.3 esterases and other Hydrolases
Many xenobiotics, both synthetic and naturally occuring, are lipophilic esters. They
can be degraded to water-soluble acids and bases by hydrolytic attack. Two impor-
tant examples of esteratic hydrolysis in ecotoxicology now follow:
O
O
--
+
--
+
RCOX
H
2
O
RCOH
XOH
Carboxyl ester
Carboxylic acid
Alcohol
RO
RO
O
O
P - OX + H
2
O
P - OH + XOH
RO
RO
Organophosphate triester
Organophosphate diester
Enzymes catalyzing the hydrolysis of esters are termed
esterases
. They belong to a
larger group of enzymes termed
hydrolases
, which can cleave a variety of chemical
bonds by hydrolytic attack. In the classification of hydrolases of the International
Union of Biochemistry (IUB), the following categories are recognized:
3.1
Acting on ester bonds (esterases)
3.2
Acting on glyoacyl compounds
3.3
Acting on ether bonds
3.4
Acting on peptide bonds (peptidases)
3.5
Acting on C-N bonds other than peptide bonds
3.6
Acting on acid anhydrides (acid anhydrolases)
3.7
Acting on C-C bonds
3.8
Acting on halide bonds
3.9
Acting on P-N bonds
3.10 Acting on S-N bonds
3.11 Acting on C-P bonds
Although it is convenient to define hydrolases according to their enzymatic function,
there is one serious underlying problem. Some hydrolases are capable of performing
two or more of the preceding kinds of hydrolytic attack, and so do not fall simply into
just one category. There are esterases, for example, that can also hydrolyze peptides,
amides, and halide bonds. The shortcomings of the early IUB classification, which
was originally based on the measurement of activities in crude tissue preparations,
have become apparent with the purification and characterization of hydrolases. As
yet, however, only limited progress has been made, and a comprehensive classifica-
tion is still some distance away. In what follows, a simple and pragmatic classification
will be described for esterases that hydrolyze xenobiotic esters (Figure 2.9). It should
be emphasized that this is a classification seen from a toxicological point of view.
Esterases are important both for their detoxifying function and as sites of action for
toxic molecules. Thus, in Figure 2.9, esterases that degrade organophosphates serve
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