444
12 Small Nitrogenous Compounds
type, B. Types Band M can form heterodimers
but cannot associate with the mitochondrial subunit. In addition to its occurrence in the brain, the
homodimer BB is found in kidney, lung and
smooth muscle; MM is present in fully differentiated skeletal muscle and, together with MB, in
the heart. Embryonal skeletal muscle has exclusively type B, which is later exchanged for type M
during differentiation. Chicken heart and brain
also contain different types of mitochondrial subunits; these differ in 10 of their 30 N-terminal
amino acids but can form heterodimers and -octomers. As in all higher vertebrates, the chicken
also has cytosolic creatine kinases of type M and
B. However, in this case, the two isoenzymes with
differing N-terminal sequences arise by alternative splicing of the B-CK gene [265, 274]. Polyploid vertebrate species, e.g. Xenopus laevis and
some teleosts, have even more complicated patterns of creatine kinases [206]. The gene duplication leading to the separation of the Band M
types took place quite early in vertebrate evolution. This is seen from the fact that the sequence
agreement between enzymes of the same type in
different vertebrate classes is higher than that
between isoenzymes from the same individual.
For example, human creatine kinase M agrees
89-91 % with the M chain of other mammals,
81 % with that of the chicken and 80 % with that
of the rays Torpedo marmorata and T. cali/ornica,
but only 76-79 % with the B chain of various
mammals. Canine B agrees 96 % with rat Band
90 % with chicken B, but only 81 % with canine
M [25, 181]. From these sequence comparisons, a
"unitary evolutionary period" (UEP) can be calculated as 30 million years for the formation of a
1 % sequence difference, i.e. a relatively slow
rate of evolution. It may be assumed that a stabilizing selection pressure arose both from the prerequisites of the enzyme activity and from structural relationships, e.g. to the M lines of skeletal
muscle and to membrane-bound ATPases.
According to the UEP value, the Band M subunits separated about 600 million years ago [33].
The gene duplication of the cytoplasmic creatine kinases took place relatively early in the
evolution of the chordates. While the tunicates
have only one locus, the lamprey Petromyzon
marin us already has two. More than two loci for
cytoplasmic creatine kinase are found only in the
teleosts and some anurans [92, 267). All teleosts
possess four loci, which show increasing tissue
specificity during evolution. Isoenzyme A is predominantly or exclusively expressed in skeletal
muscle, B in the brain and eye, C in the stomach
musculature, and D only in the mature testis.
Quite complicated electrophoretic patterns of
creatine kinases show up to six bands in the skeletal muscle of various salmonids; these variants
arise by post-translational modifications of the
enzyme. There are also four isoenzyme loci in
Xenopus laevis. All four are expressed in the eye,
and several in the stomach (I, II, IV) and heart
muscle (II, III, IV), whilst the skeletal muscle
switches from III in the tadpole to II in the adult;
the testis and kidney express only IV. All Xenopus
species, including the diploid X. tropicalis,
express the same four loci but with speciesspecific tissue distributions; thus, in the tetraploid species diploidization appears to be complete. Hymenochirus boettgeri and Pipa cavalhoi,
which also belong to the family Pipidae, express
only three loci and type III is absent [267].
12.10 Oligopeptides
and Their Metabolism
Oligopeptides, i.e. peptides with at most a dozen
amino acids, can arise by the hydrolytic cleavage
of proteins: However, such peptides can also arise
by de novo synthesis, in which case they often
show structural characteristics that are not usually
encountered in proteins, e.g. peptide bonds with
the y-carboxyl group of glutamic acid, or ~-amino
acid residues. Some insects have been found to
contain large amounts of short-chain peptides
which apparently function as a store of amino
acids. The respective roles of the dipeptides ~alanyltyrosine and y-glutamylphenylalanine in
the sclerotization of insect cuticulae have already
been discussed. A general storage function, especially of essential amino acids, can also be
assumed for the numerous small peptides found
in the tissues and haemolymph of Drosophila larvae and adult Calliphora erythrocephala. In both
cases these consist of several hundred chromatographically distinct compounds which probably
arise by intracellular proteolysis before being
released into the haemolymph. The haemolymph
of Calliphora contains at least five different peptidases which can release amino acids from these
peptides [27, 46, 56]. The basic di- and tripeptides detected in the haemolymph of larvae of
Phormia regina, Sarcophaga bullata and Musca
domestica probably also have a storage function
[27, 46]. Glutathione and several histidine peptides, on the other hand, have other specific functions.
12 Small Nitrogenous Compounds
type, B. Types Band M can form heterodimers
but cannot associate with the mitochondrial subunit. In addition to its occurrence in the brain, the
homodimer BB is found in kidney, lung and
smooth muscle; MM is present in fully differentiated skeletal muscle and, together with MB, in
the heart. Embryonal skeletal muscle has exclusively type B, which is later exchanged for type M
during differentiation. Chicken heart and brain
also contain different types of mitochondrial subunits; these differ in 10 of their 30 N-terminal
amino acids but can form heterodimers and -octomers. As in all higher vertebrates, the chicken
also has cytosolic creatine kinases of type M and
B. However, in this case, the two isoenzymes with
differing N-terminal sequences arise by alternative splicing of the B-CK gene [265, 274]. Polyploid vertebrate species, e.g. Xenopus laevis and
some teleosts, have even more complicated patterns of creatine kinases [206]. The gene duplication leading to the separation of the Band M
types took place quite early in vertebrate evolution. This is seen from the fact that the sequence
agreement between enzymes of the same type in
different vertebrate classes is higher than that
between isoenzymes from the same individual.
For example, human creatine kinase M agrees
89-91 % with the M chain of other mammals,
81 % with that of the chicken and 80 % with that
of the rays Torpedo marmorata and T. cali/ornica,
but only 76-79 % with the B chain of various
mammals. Canine B agrees 96 % with rat Band
90 % with chicken B, but only 81 % with canine
M [25, 181]. From these sequence comparisons, a
"unitary evolutionary period" (UEP) can be calculated as 30 million years for the formation of a
1 % sequence difference, i.e. a relatively slow
rate of evolution. It may be assumed that a stabilizing selection pressure arose both from the prerequisites of the enzyme activity and from structural relationships, e.g. to the M lines of skeletal
muscle and to membrane-bound ATPases.
According to the UEP value, the Band M subunits separated about 600 million years ago [33].
The gene duplication of the cytoplasmic creatine kinases took place relatively early in the
evolution of the chordates. While the tunicates
have only one locus, the lamprey Petromyzon
marin us already has two. More than two loci for
cytoplasmic creatine kinase are found only in the
teleosts and some anurans [92, 267). All teleosts
possess four loci, which show increasing tissue
specificity during evolution. Isoenzyme A is predominantly or exclusively expressed in skeletal
muscle, B in the brain and eye, C in the stomach
musculature, and D only in the mature testis.
Quite complicated electrophoretic patterns of
creatine kinases show up to six bands in the skeletal muscle of various salmonids; these variants
arise by post-translational modifications of the
enzyme. There are also four isoenzyme loci in
Xenopus laevis. All four are expressed in the eye,
and several in the stomach (I, II, IV) and heart
muscle (II, III, IV), whilst the skeletal muscle
switches from III in the tadpole to II in the adult;
the testis and kidney express only IV. All Xenopus
species, including the diploid X. tropicalis,
express the same four loci but with speciesspecific tissue distributions; thus, in the tetraploid species diploidization appears to be complete. Hymenochirus boettgeri and Pipa cavalhoi,
which also belong to the family Pipidae, express
only three loci and type III is absent [267].
12.10 Oligopeptides
and Their Metabolism
Oligopeptides, i.e. peptides with at most a dozen
amino acids, can arise by the hydrolytic cleavage
of proteins: However, such peptides can also arise
by de novo synthesis, in which case they often
show structural characteristics that are not usually
encountered in proteins, e.g. peptide bonds with
the y-carboxyl group of glutamic acid, or ~-amino
acid residues. Some insects have been found to
contain large amounts of short-chain peptides
which apparently function as a store of amino
acids. The respective roles of the dipeptides ~alanyltyrosine and y-glutamylphenylalanine in
the sclerotization of insect cuticulae have already
been discussed. A general storage function, especially of essential amino acids, can also be
assumed for the numerous small peptides found
in the tissues and haemolymph of Drosophila larvae and adult Calliphora erythrocephala. In both
cases these consist of several hundred chromatographically distinct compounds which probably
arise by intracellular proteolysis before being
released into the haemolymph. The haemolymph
of Calliphora contains at least five different peptidases which can release amino acids from these
peptides [27, 46, 56]. The basic di- and tripeptides detected in the haemolymph of larvae of
Phormia regina, Sarcophaga bullata and Musca
domestica probably also have a storage function
[27, 46]. Glutathione and several histidine peptides, on the other hand, have other specific functions.