662
17 Ester Hydrolases, ATPases and Carboanhydrases
acquisition in the evolution of the higher primates. More recently, it has been found in the milk
of dogs at about the same level as in humans, and
in the cat at somewhat lower levels. The activity
in these two animals is, however, more dependent
upon the lactation phase than in humans, which
explains previous negative results with the dog.
The human, dog and cat enzymes have corresponding pH optima of 7.3-8.5, and are also all
strongly inhibited by eserine; in addition, they all
have an absolute requirement for glycocholic or
taurocholic acid and are inhibited by secondary
bile acids such as taurodeoxycholic and glycodeoxycholic acids [46].
Lipases are found in the spectrum of digestive
enzymes of all animals examined but have rarely
been examined in detail. The gut lipases of the
insects are similar to the pancreas lipases in that
they are stimulated by calcium ions, preferentially
cleave primary ester bonds of TAG, and produce
sn-2-monoacylglycerol (MAG) [36]. In contrast,
the TAG/DAG lipase secreted together with other
hydro lases into the medium by the ciliate Tetrahymena is not stimulated by Ca2+, and also has a
much smaller size of 30.5 kDa [42]. Intracellular
lipolytic activity is found in almost all animal
cells. However, the assay methods used did not
distinguish clearly between lipases and esterases
in many cases, and only a few such enzymes have
been examined further. The hormonally regulated
lipase from mammalian and avian fat tissue is
especially interesting; adrenalin, noradrenalin,
glucagon and ACTH bring about the phosphorylation of the enzyme via a cAMP-dependent
protein kinase and, thereby, its activation. The
fat-tissue cells (adipocytes) ofthe rat and chicken
contain a large lipid-rich complex with the ability
to cleave acyl residues from TAG, DAG, MAG
and cholesterol esters. A 42-kDa TAG/MAG
lipase can be separated from a 45-kDa MAG
lipase. The former is also phosphorylated by a
cAMP-dependent protein kinase but is not activated; in this case activation requires interaction
with other polypeptides [16].
Lipoprotein lipase and hepatic lipase are
involved in the metabolism of plasma lipoproteins. The lipoprotein lipase supplies the ca. 100 g
plasma fatty acids that are metabolized daily in
humans [183]. This enzyme is located on the
lumen side of the endothelial cells and produces
TAG chylomicron residues and IDL (intermediate lipoproteins) by hydrolysis of chylomicrons
and VLDL (very low density lipoproteins). Its
main function is apparently to supply fatty acids
to various tissues. Lipoprotein lipase is found in
several different tissues, e.g. fat tissue, heart,
skeletal muscle, lung, milk glands, brain and
kidney; it is also detectable in milk itself. The
enzyme is a glycoprotein whose sequence of
448-450 amino acids is very similar in humans,
mice and cattle [183]. It requires apolipoprotein
C-II as the cofactor for maximal activity and is
inhibited by 1 molll NaCl. The hepatic lipase
requries no cofactor and is active in 1 molll NaCI;
however, the two enzymes are very similar and
in vitro can cleave phospholipids, TAG, DAG
and MAG. The hepatic lipase is located mainly
on the sinusoidal surface of the liver, and its
apparent function is to hydrolyse TAG in IDL and
phospholipids in HDL. The enzyme in the rat is a
glycoprotein with 8 % carbohydrate and a polypeptide chain of 472 amino acids that carries two
N-glycosylation sites [79]. Lipoprotein lipase and
hepatic lipase belong to the super-family of the
serine esterases. A hypothetical genealogical tree
can be constructed to show how gene duplication,
exon shuffling and loss of introns combined to
convert an ancestral gene with 14 introns into the
gene of the colipase-dependent pancreas lipase
with 12 introns, and the genes of the lipoprotein
lipase and hepatic lipase with 9 introns. The tree
also includes vitellogenins YPl to YP3 of Drosophila; these show 35-40 % sequence agreement
with lipoprotein lipase in part of their sequences
[77, 124].
Like the adipocyte lipase of the vertebrates,
the lipolytic activity in insect fat bodies is also
hormonally regulated; the adipokinetic hormone
(AKH) from the corpora cardiaca phosphorylates, and thereby activates, the lipase via a cAMPdependent protein kinase; this stimulates the
release of sn-l,2-DAG into the haemolymph
where it is bound to the transport protein lipophorin. It is not clear whether formation of the
DAG involves the cleavage only of the sn-3 bond
or whether sn-2-MAG is produced initially and
then converted to sn-l,2-DAG by a specific acyl
transfer. The flight muscles of the Lepidoptera
contain high lipolytic activity that is mainly
involved in the energy-yielding metabolism of
lipids; MAG is cleaved with high efficiency. The
TAG- and DAGIMAG-specific activities can be
separated electrophoretically, but it is not known
whether an enzyme complex, analogous to that in
the fat tissue of the chicken, is present [36]. The
enzyme in the flight muscle of the migratory
locust Locusta migratoria is inhibited by the CL
protein of the haemolymph, Conversion of lipoprotein N ellow to A + in the haemolymph as a result
of AKH activity occupies the majority of the CL
17 Ester Hydrolases, ATPases and Carboanhydrases
acquisition in the evolution of the higher primates. More recently, it has been found in the milk
of dogs at about the same level as in humans, and
in the cat at somewhat lower levels. The activity
in these two animals is, however, more dependent
upon the lactation phase than in humans, which
explains previous negative results with the dog.
The human, dog and cat enzymes have corresponding pH optima of 7.3-8.5, and are also all
strongly inhibited by eserine; in addition, they all
have an absolute requirement for glycocholic or
taurocholic acid and are inhibited by secondary
bile acids such as taurodeoxycholic and glycodeoxycholic acids [46].
Lipases are found in the spectrum of digestive
enzymes of all animals examined but have rarely
been examined in detail. The gut lipases of the
insects are similar to the pancreas lipases in that
they are stimulated by calcium ions, preferentially
cleave primary ester bonds of TAG, and produce
sn-2-monoacylglycerol (MAG) [36]. In contrast,
the TAG/DAG lipase secreted together with other
hydro lases into the medium by the ciliate Tetrahymena is not stimulated by Ca2+, and also has a
much smaller size of 30.5 kDa [42]. Intracellular
lipolytic activity is found in almost all animal
cells. However, the assay methods used did not
distinguish clearly between lipases and esterases
in many cases, and only a few such enzymes have
been examined further. The hormonally regulated
lipase from mammalian and avian fat tissue is
especially interesting; adrenalin, noradrenalin,
glucagon and ACTH bring about the phosphorylation of the enzyme via a cAMP-dependent
protein kinase and, thereby, its activation. The
fat-tissue cells (adipocytes) ofthe rat and chicken
contain a large lipid-rich complex with the ability
to cleave acyl residues from TAG, DAG, MAG
and cholesterol esters. A 42-kDa TAG/MAG
lipase can be separated from a 45-kDa MAG
lipase. The former is also phosphorylated by a
cAMP-dependent protein kinase but is not activated; in this case activation requires interaction
with other polypeptides [16].
Lipoprotein lipase and hepatic lipase are
involved in the metabolism of plasma lipoproteins. The lipoprotein lipase supplies the ca. 100 g
plasma fatty acids that are metabolized daily in
humans [183]. This enzyme is located on the
lumen side of the endothelial cells and produces
TAG chylomicron residues and IDL (intermediate lipoproteins) by hydrolysis of chylomicrons
and VLDL (very low density lipoproteins). Its
main function is apparently to supply fatty acids
to various tissues. Lipoprotein lipase is found in
several different tissues, e.g. fat tissue, heart,
skeletal muscle, lung, milk glands, brain and
kidney; it is also detectable in milk itself. The
enzyme is a glycoprotein whose sequence of
448-450 amino acids is very similar in humans,
mice and cattle [183]. It requires apolipoprotein
C-II as the cofactor for maximal activity and is
inhibited by 1 molll NaCl. The hepatic lipase
requries no cofactor and is active in 1 molll NaCI;
however, the two enzymes are very similar and
in vitro can cleave phospholipids, TAG, DAG
and MAG. The hepatic lipase is located mainly
on the sinusoidal surface of the liver, and its
apparent function is to hydrolyse TAG in IDL and
phospholipids in HDL. The enzyme in the rat is a
glycoprotein with 8 % carbohydrate and a polypeptide chain of 472 amino acids that carries two
N-glycosylation sites [79]. Lipoprotein lipase and
hepatic lipase belong to the super-family of the
serine esterases. A hypothetical genealogical tree
can be constructed to show how gene duplication,
exon shuffling and loss of introns combined to
convert an ancestral gene with 14 introns into the
gene of the colipase-dependent pancreas lipase
with 12 introns, and the genes of the lipoprotein
lipase and hepatic lipase with 9 introns. The tree
also includes vitellogenins YPl to YP3 of Drosophila; these show 35-40 % sequence agreement
with lipoprotein lipase in part of their sequences
[77, 124].
Like the adipocyte lipase of the vertebrates,
the lipolytic activity in insect fat bodies is also
hormonally regulated; the adipokinetic hormone
(AKH) from the corpora cardiaca phosphorylates, and thereby activates, the lipase via a cAMPdependent protein kinase; this stimulates the
release of sn-l,2-DAG into the haemolymph
where it is bound to the transport protein lipophorin. It is not clear whether formation of the
DAG involves the cleavage only of the sn-3 bond
or whether sn-2-MAG is produced initially and
then converted to sn-l,2-DAG by a specific acyl
transfer. The flight muscles of the Lepidoptera
contain high lipolytic activity that is mainly
involved in the energy-yielding metabolism of
lipids; MAG is cleaved with high efficiency. The
TAG- and DAGIMAG-specific activities can be
separated electrophoretically, but it is not known
whether an enzyme complex, analogous to that in
the fat tissue of the chicken, is present [36]. The
enzyme in the flight muscle of the migratory
locust Locusta migratoria is inhibited by the CL
protein of the haemolymph, Conversion of lipoprotein N ellow to A + in the haemolymph as a result
of AKH activity occupies the majority of the CL