7.2 Invertebrate Haemoglobins and Chlorocruorins
265
predominates and constitutes the native form of
the pigment. In the sperm whale, the deviant
form makes up less than 10 % of the myoglobin at
equilibrium; in contrast, in the tuna fish the two
myoglobin forms are equally represented [10].
7.1.7 Methaemoglobin Formation
and Reduction
Although the Fe (II) in the haem pocket is relatively well protected from oxidation, both erythrocyte haemoglobin and muscle myoglobin of the
vertebrates is continuously converted to methaemoglobin (or -myoglobin) with Fe (III) , which can
bind no more O2• The auto-oxidation rate is temperature dependent; the haemoglobins of the
marine teleosts show some adaptation to the average temperatures of their habitats, such that at
any given temperature cold-water fish have a tenfold higher auto-oxidation rate than warm-water
fish. On the other hand, hydrostatic pressure has
no influence upon auto-oxidation [187]. The
reduction of methaemoglobin to functional haemoglobin in vertebrates of all classes is achieved
mainly by NADH- and NADPH-specific MetHb
reductases, all of which are partly membrane
bound [71, 103, 145]. Vertebrate blood at equilibrium contains 1-3 % methaemoglobin. The much
higher values given in the literature for the reptiles are probably artefacts.
7.2 Invertebrate Haemoglobins
and Chlorocruorins
The haemoglobins of invertebrates are remarkably sporadic in their distribution. Whereas the
vertebrates always have myoglobins in their heart
and skeletal muscle and, with few exceptions,
possess haemoglobin-containing blood cells, haemoglobins are found in many phyla of invertebrates (Table 7.2) but only in individual species or
groups. Therefore, the question of the biological
importance of the haemoglobins to the invertebrates is not so clearly answerable as it is for the
vertebrates. Haemoglobins are found either
freely dissolved or in haemoglobin-containing
cells (erythrocytes) in the blood, haemolymph or
body-cavity fluid (Table 7.2). Intracellular haemoglobins are also present, often simultaneously,
in invertebrate muscle, nerve cells and other cell
types. For example, in the echiurid Thalassema
haemoglobins are present in cells of the coelom
fluid, coelom epithelium, body wall muscles, gut
wall, fat cells, nervous system, anal vesicles, and
eggs. Haemoglobins are even found in several
protozoans. Some animal species have several
different types of respiratory pigment, e.g. haemoglobin in the muscle or body cells together
with chlorocruorin dissolved in the body fluid
(e.g. the polychaete Potamilla), or haemocyanin
(e.g. Polyplacophora, Gastropoda Busycon and
Aplysia). Other species possess both dissolved
haemoglobin and an intracellular form in the cells
of the coelom fluid (certain polychaetes), or they
have both extracellular haemoglobin and chlorocruorin in the blood (the polychaete Serpula).
The universally used term "haemoglobin"
emphasizes the presently accepted homology of
all these proteins, regardless of whether they
occur in plants or animals [55, 174]. Before this
homology was defined, other terms were applied
and these are still partly in use today. The name
"erythrocruorin" has a complicated history. It
was coined by Lankester in 1868 to distinguish the
haemoglobins of certain invertebrates from green
chlorocruorin; it was adopted in 1933 by Svedberg to differentiate between the intracellular,
low molecular weight vertebrate haemoglobins
and the then known extracellular, high molecular
weight invertebrate haemoglobins. Today, when
many low molecular weight invertebrate haemoglobins are known, the name erythrocruorin may
no longer be used for all invertebrate haemoglobins, but should be reserved for the highly polymeric, extracellular haemoglobins of the molluscs
and annelids in order to emphasize their unique
character. Myoglobin refers only to the muscle
haemoglobins and not to those of other body
cells.
The intracellular haemoglobins of the invertebrates are, as a rule, mono-, di- or tetramers,
and only seldom higher polymers, with subunits
of 12-16 kDa and one haem group. There are,
however, intracellular haemoglobins with subunits of twice this size, and these include two
domains each with a haem group, e.g. in the larvae of the bot fly Gasterophilus and in the mussel
Barbatia reeveana. The extracellular haemoglobins are divided into four types according to their
structure [174]. The first type are mono-, di- or
tetramers with subunits of 16 ± 2 kDa and with
one haem group; this is the structure of the haemoglobins in the haemolymph of the larvae of
non-biting midges (Chironomidae). Haemoglobins of the second type have subunits of
30-40 kDa with two haem-bearing domains; the
haemoglobin molecule of several lower crusta-
265
predominates and constitutes the native form of
the pigment. In the sperm whale, the deviant
form makes up less than 10 % of the myoglobin at
equilibrium; in contrast, in the tuna fish the two
myoglobin forms are equally represented [10].
7.1.7 Methaemoglobin Formation
and Reduction
Although the Fe (II) in the haem pocket is relatively well protected from oxidation, both erythrocyte haemoglobin and muscle myoglobin of the
vertebrates is continuously converted to methaemoglobin (or -myoglobin) with Fe (III) , which can
bind no more O2• The auto-oxidation rate is temperature dependent; the haemoglobins of the
marine teleosts show some adaptation to the average temperatures of their habitats, such that at
any given temperature cold-water fish have a tenfold higher auto-oxidation rate than warm-water
fish. On the other hand, hydrostatic pressure has
no influence upon auto-oxidation [187]. The
reduction of methaemoglobin to functional haemoglobin in vertebrates of all classes is achieved
mainly by NADH- and NADPH-specific MetHb
reductases, all of which are partly membrane
bound [71, 103, 145]. Vertebrate blood at equilibrium contains 1-3 % methaemoglobin. The much
higher values given in the literature for the reptiles are probably artefacts.
7.2 Invertebrate Haemoglobins
and Chlorocruorins
The haemoglobins of invertebrates are remarkably sporadic in their distribution. Whereas the
vertebrates always have myoglobins in their heart
and skeletal muscle and, with few exceptions,
possess haemoglobin-containing blood cells, haemoglobins are found in many phyla of invertebrates (Table 7.2) but only in individual species or
groups. Therefore, the question of the biological
importance of the haemoglobins to the invertebrates is not so clearly answerable as it is for the
vertebrates. Haemoglobins are found either
freely dissolved or in haemoglobin-containing
cells (erythrocytes) in the blood, haemolymph or
body-cavity fluid (Table 7.2). Intracellular haemoglobins are also present, often simultaneously,
in invertebrate muscle, nerve cells and other cell
types. For example, in the echiurid Thalassema
haemoglobins are present in cells of the coelom
fluid, coelom epithelium, body wall muscles, gut
wall, fat cells, nervous system, anal vesicles, and
eggs. Haemoglobins are even found in several
protozoans. Some animal species have several
different types of respiratory pigment, e.g. haemoglobin in the muscle or body cells together
with chlorocruorin dissolved in the body fluid
(e.g. the polychaete Potamilla), or haemocyanin
(e.g. Polyplacophora, Gastropoda Busycon and
Aplysia). Other species possess both dissolved
haemoglobin and an intracellular form in the cells
of the coelom fluid (certain polychaetes), or they
have both extracellular haemoglobin and chlorocruorin in the blood (the polychaete Serpula).
The universally used term "haemoglobin"
emphasizes the presently accepted homology of
all these proteins, regardless of whether they
occur in plants or animals [55, 174]. Before this
homology was defined, other terms were applied
and these are still partly in use today. The name
"erythrocruorin" has a complicated history. It
was coined by Lankester in 1868 to distinguish the
haemoglobins of certain invertebrates from green
chlorocruorin; it was adopted in 1933 by Svedberg to differentiate between the intracellular,
low molecular weight vertebrate haemoglobins
and the then known extracellular, high molecular
weight invertebrate haemoglobins. Today, when
many low molecular weight invertebrate haemoglobins are known, the name erythrocruorin may
no longer be used for all invertebrate haemoglobins, but should be reserved for the highly polymeric, extracellular haemoglobins of the molluscs
and annelids in order to emphasize their unique
character. Myoglobin refers only to the muscle
haemoglobins and not to those of other body
cells.
The intracellular haemoglobins of the invertebrates are, as a rule, mono-, di- or tetramers,
and only seldom higher polymers, with subunits
of 12-16 kDa and one haem group. There are,
however, intracellular haemoglobins with subunits of twice this size, and these include two
domains each with a haem group, e.g. in the larvae of the bot fly Gasterophilus and in the mussel
Barbatia reeveana. The extracellular haemoglobins are divided into four types according to their
structure [174]. The first type are mono-, di- or
tetramers with subunits of 16 ± 2 kDa and with
one haem group; this is the structure of the haemoglobins in the haemolymph of the larvae of
non-biting midges (Chironomidae). Haemoglobins of the second type have subunits of
30-40 kDa with two haem-bearing domains; the
haemoglobin molecule of several lower crusta-