658
17 Ester HydroJases, ATPases and Carboanhydrases
the chromo gens 4-nitrophenylester and naphthylester.
2. The substrate preferences of the different
classes show considerable overlap.
3. Most esterases exist in multiple forms, on the
one hand because of marked genetic polymorphism, and on the other hand as the result
of variable glycosylation and other posttranslational modifications.
4. Other enzymes have esterase activity; examples include serine proteases, and even proteins which are not usually considered to be
enzymes, like serum albumins.
5. Only a few esterases have been sequenced so
far.
Thus, classification of the esterases relies not only
on substrate specificities (which overlap considerably) but also on responses to inhibitors. The
Enzyme Commission of the International Union
of Biochemistry (rUB) differentiates between
four main classes. The carboxyl esterases in a narrow sense, named also ali- or B-esterases
(EC 3.1.1.1), are inhibited by diisopropylphosphofluoridate (DIPF) and other organophosphates. They therefore belong to the serine hydrolases, which are irreversibly damaged by phosphorylation of the active serine. In contrast, the arylor A-esterases (EC 3.1.1.2) not only are insensitive to organophosphates but can even cleave
compounds such as paraoxon (diethyl-4nitrophenylphosphate); however, they are inhibited by p-hydroxy- or p-chloromercuribenzoate
(PHMB, PCMB). The acetylcholinesterases
(EC 3.1.1.7) and the so-called non-specific cholinesterases (EC 3.1.1.8, also pseudo- or butyrylcholinesterases) are inhibited not only by DIPF
but also by eserine (physostigmine). The last
group, the acetyl- or C-esterases (EC 3.1.1.6) are
insensitive to all known inhibitors and are characterized by their specificity for acetic esters.
To describe the esterase spectrum of fish
muscle, Hart and Cook in 1976 introduced three
further classes, of which type ER is just as insensitive as the acetyl esterases but shows broader
specificity, ESe are inhibited only by eserine and
not by DIPF, and finally Esdp are inhibited by
DIPF as well as by PHMB or PCMB [55]. A
strong distinction between "A-esterases", which
are active against paraoxon, and "aryl esterases",
which are active against phenylacetate, has been
suggested by investigations of the blood serum of
11 mammalian species, where both activities were
always found, and 14 bird species, where higher
aryl esterase activity was always present but little
or no A-esterase activity occurred [88]. Other
authors, however, recommend the rejection of all
such classifications and recognition only of those
esterases for which the natural substrate is
known, such as the juvenile hormone esterases of
the insects, or from Drosophila melanogaster
esterase 6 which cleaves the pheromone cisvaccenylacetate [123]. At least a dozen esterases
can be separated by, for example, electrophoresis
in practically all cells of animals down to the Protozoa; they include representatives of several of
the classes defined by the Enzyme Commission.
However, the same tissue, even in closely related
species, may have a completely different pattern.
Thus, no conclusions about the biological roles of
esterases can be arrived at from this classification.
The carboxyl esterases in the narrowest sense
(EC 3.1.1.1) have been investigated most thoroughly in mammalian liver. There they are bound
to the cytoplasmic side of the endoplasmic reticulum, probably via phospholipids. Their function is
not clear but they are at least involved in detoxification processes. Several such enzymes from
mouse and rat liver can be separated by electrophoresis; some are monomers of 57-60 kDa and
others have the otherwise rather rare trimeric
structure. In the trim eric esterase 6A of the
mouse, titration of the active centre with paraoxon gives an equivalent mass of 178 kDa; therefore, the active centres of the three subunits are
not functionally equivalent. The 14 isoenzymes
known so far from the mouse are encoded by two
gene clusters on chromosome 8. The trimeric
esterase 6A, widely found in many organs, is
immunologically very similar to esterase 1F from
the blood and esterase 9A from the liver and
kidney; the corresponding genes lie adjacent in
cluster 1. Apart from this, there are considerable
differences in immunological reactivity, substrate
specificity, inhibitor responses and quaternary
structure between the products of the two gene
families [135, 139].
The best-known aryl esterases (EC 3.1.1.2) are
those from mammalian blood plasma. The bovine
and human enzymes have molecular masses of
440 and 240 kDa, respectively; they are associated with high-density lipoproteins (HDL). These
enzymes are defined not so much by their substrate specificity, as arylesters can be cleaved by
other esterases, carboanhydrases and proteases,
but much more by their responses to inhibitors.
They are not inhibited by DIPF or eserine but by
PHMB, PCMB and other SH reagents as well as
by heavy metals. They cleave aromatic esters, in
particular acetates, but not aliphatic esters.
17 Ester HydroJases, ATPases and Carboanhydrases
the chromo gens 4-nitrophenylester and naphthylester.
2. The substrate preferences of the different
classes show considerable overlap.
3. Most esterases exist in multiple forms, on the
one hand because of marked genetic polymorphism, and on the other hand as the result
of variable glycosylation and other posttranslational modifications.
4. Other enzymes have esterase activity; examples include serine proteases, and even proteins which are not usually considered to be
enzymes, like serum albumins.
5. Only a few esterases have been sequenced so
far.
Thus, classification of the esterases relies not only
on substrate specificities (which overlap considerably) but also on responses to inhibitors. The
Enzyme Commission of the International Union
of Biochemistry (rUB) differentiates between
four main classes. The carboxyl esterases in a narrow sense, named also ali- or B-esterases
(EC 3.1.1.1), are inhibited by diisopropylphosphofluoridate (DIPF) and other organophosphates. They therefore belong to the serine hydrolases, which are irreversibly damaged by phosphorylation of the active serine. In contrast, the arylor A-esterases (EC 3.1.1.2) not only are insensitive to organophosphates but can even cleave
compounds such as paraoxon (diethyl-4nitrophenylphosphate); however, they are inhibited by p-hydroxy- or p-chloromercuribenzoate
(PHMB, PCMB). The acetylcholinesterases
(EC 3.1.1.7) and the so-called non-specific cholinesterases (EC 3.1.1.8, also pseudo- or butyrylcholinesterases) are inhibited not only by DIPF
but also by eserine (physostigmine). The last
group, the acetyl- or C-esterases (EC 3.1.1.6) are
insensitive to all known inhibitors and are characterized by their specificity for acetic esters.
To describe the esterase spectrum of fish
muscle, Hart and Cook in 1976 introduced three
further classes, of which type ER is just as insensitive as the acetyl esterases but shows broader
specificity, ESe are inhibited only by eserine and
not by DIPF, and finally Esdp are inhibited by
DIPF as well as by PHMB or PCMB [55]. A
strong distinction between "A-esterases", which
are active against paraoxon, and "aryl esterases",
which are active against phenylacetate, has been
suggested by investigations of the blood serum of
11 mammalian species, where both activities were
always found, and 14 bird species, where higher
aryl esterase activity was always present but little
or no A-esterase activity occurred [88]. Other
authors, however, recommend the rejection of all
such classifications and recognition only of those
esterases for which the natural substrate is
known, such as the juvenile hormone esterases of
the insects, or from Drosophila melanogaster
esterase 6 which cleaves the pheromone cisvaccenylacetate [123]. At least a dozen esterases
can be separated by, for example, electrophoresis
in practically all cells of animals down to the Protozoa; they include representatives of several of
the classes defined by the Enzyme Commission.
However, the same tissue, even in closely related
species, may have a completely different pattern.
Thus, no conclusions about the biological roles of
esterases can be arrived at from this classification.
The carboxyl esterases in the narrowest sense
(EC 3.1.1.1) have been investigated most thoroughly in mammalian liver. There they are bound
to the cytoplasmic side of the endoplasmic reticulum, probably via phospholipids. Their function is
not clear but they are at least involved in detoxification processes. Several such enzymes from
mouse and rat liver can be separated by electrophoresis; some are monomers of 57-60 kDa and
others have the otherwise rather rare trimeric
structure. In the trim eric esterase 6A of the
mouse, titration of the active centre with paraoxon gives an equivalent mass of 178 kDa; therefore, the active centres of the three subunits are
not functionally equivalent. The 14 isoenzymes
known so far from the mouse are encoded by two
gene clusters on chromosome 8. The trimeric
esterase 6A, widely found in many organs, is
immunologically very similar to esterase 1F from
the blood and esterase 9A from the liver and
kidney; the corresponding genes lie adjacent in
cluster 1. Apart from this, there are considerable
differences in immunological reactivity, substrate
specificity, inhibitor responses and quaternary
structure between the products of the two gene
families [135, 139].
The best-known aryl esterases (EC 3.1.1.2) are
those from mammalian blood plasma. The bovine
and human enzymes have molecular masses of
440 and 240 kDa, respectively; they are associated with high-density lipoproteins (HDL). These
enzymes are defined not so much by their substrate specificity, as arylesters can be cleaved by
other esterases, carboanhydrases and proteases,
but much more by their responses to inhibitors.
They are not inhibited by DIPF or eserine but by
PHMB, PCMB and other SH reagents as well as
by heavy metals. They cleave aromatic esters, in
particular acetates, but not aliphatic esters.