297
Using controlled pepsin digestion, a new type of collagen was isolated from
elastoidin of great blue shark ( Prionace glauca ). According Kimura and co- workers,
“the collagen alpha chain of elastoidin, designated alpha 1(E), was very similar in
electrophoretic and chromatographic behavior and amino acid composition to shark
skin alpha 1(I) chain, but they were genetically-distinct on the basis of CNBrpeptide maps. The collagen molecule of elastoidin was shown to be an [alpha 1(E)]3
homotrimer,” (Kimura et al. 1986 ).
The elastoidin fi bres are responsible for some stiffness, probably, because they
are packed tightly along their length on either side of the cartilaginous radials of the
shark fi n and they extend to the edge of the fi n (Alexander 1974 ). The mechanical
properties and fracture behaviour of dry and native elastoidin have been studied
as a function of strain rate, and the plastic set behaviour of the dry elastoidin is
found to be sensitive to strain rate (Arumugam and Sanjeevi 1987 ). The results are
correlated with the scanning electron microscopy done on the fractured ends of dry
and native elastoidin. Broken ends of dry elastoidin, fractured at a strain rate of
10.0 min
−1 , appear blunt. Under the same conditions, the native specimen’s ends
appear sharp. The tensile properties and mode of fracture of shark elastoidin has
been studied by Rajaram et al. ( 1981 ):
“Elastoidin fi bres were stronger than tendon in the dry state, whereas the opposite was observed for fi bres tested in the wet state. However, elastoidin was stiffer
than tendon whether dry or wet. Scanning electron micrographs of the cross- sections
and fractured surfaces revealed that elastoidin fi bres consisted of fi brils with
varying diameter arranged in a lamellar fashion. From the nature of the fractured
surfaces, it could be deduced that the primary failure mechanism for elastoidin was
probably through the structure fi ssuring,” (Rajaram et al. 1981 ).
It is probable that elastoidin plays an important role in fi sh fi n regeneration.
As suggested by Mari-Beffa et al. ( 1989 ), “during teleostean fi n regeneration the
actinotrichia are immersed in the blastema, maintaining their apical position. In
this epimorphic event the latter fact might be achieved by either a cellular carriage
or a continuous turn-over of these hyperpolimerized fi brils. A 3H-proline pulse and
radioautographic chase experiment of the isolated actinotrichia found a turn-over of
collagen within the structure,” (Mari-Beffa et al. 1989 ).
Recently, it was shown that “two zebrafi sh proteins actinodin 1 and 2 (And1 and
And2), are essential structural components of elastoidin. The presence of actinodin
sequences in several teleost fi shes and in the elephant shark ( Callorhinchus milii ,
which occupies a basal phylogenetic position), but not in tetrapods, suggests that
these genes were lost during tetrapod species evolution. Double gene knockdown of
And1 and And2 in zebrafi sh embryos results in the absence of actinotrichia and
impaired fi n folds. Gene expression profi les in embryos lacking and1 and and2 function are consistent with pectoral fi n truncation, and may offer a potential explanation
for the polydactyly observed in early tetrapod fossils. It was proposed that the loss of
both actinodins and actinotrichia during evolution may have led to the loss of lepidotrichia, and may have contributed to the fi n-to-limb transition,” (Zhang et al. 2010 ).
7.3 Chemistry of Fish Fin: Elastoidin
Using controlled pepsin digestion, a new type of collagen was isolated from
elastoidin of great blue shark ( Prionace glauca ). According Kimura and co- workers,
“the collagen alpha chain of elastoidin, designated alpha 1(E), was very similar in
electrophoretic and chromatographic behavior and amino acid composition to shark
skin alpha 1(I) chain, but they were genetically-distinct on the basis of CNBrpeptide maps. The collagen molecule of elastoidin was shown to be an [alpha 1(E)]3
homotrimer,” (Kimura et al. 1986 ).
The elastoidin fi bres are responsible for some stiffness, probably, because they
are packed tightly along their length on either side of the cartilaginous radials of the
shark fi n and they extend to the edge of the fi n (Alexander 1974 ). The mechanical
properties and fracture behaviour of dry and native elastoidin have been studied
as a function of strain rate, and the plastic set behaviour of the dry elastoidin is
found to be sensitive to strain rate (Arumugam and Sanjeevi 1987 ). The results are
correlated with the scanning electron microscopy done on the fractured ends of dry
and native elastoidin. Broken ends of dry elastoidin, fractured at a strain rate of
10.0 min
−1 , appear blunt. Under the same conditions, the native specimen’s ends
appear sharp. The tensile properties and mode of fracture of shark elastoidin has
been studied by Rajaram et al. ( 1981 ):
“Elastoidin fi bres were stronger than tendon in the dry state, whereas the opposite was observed for fi bres tested in the wet state. However, elastoidin was stiffer
than tendon whether dry or wet. Scanning electron micrographs of the cross- sections
and fractured surfaces revealed that elastoidin fi bres consisted of fi brils with
varying diameter arranged in a lamellar fashion. From the nature of the fractured
surfaces, it could be deduced that the primary failure mechanism for elastoidin was
probably through the structure fi ssuring,” (Rajaram et al. 1981 ).
It is probable that elastoidin plays an important role in fi sh fi n regeneration.
As suggested by Mari-Beffa et al. ( 1989 ), “during teleostean fi n regeneration the
actinotrichia are immersed in the blastema, maintaining their apical position. In
this epimorphic event the latter fact might be achieved by either a cellular carriage
or a continuous turn-over of these hyperpolimerized fi brils. A 3H-proline pulse and
radioautographic chase experiment of the isolated actinotrichia found a turn-over of
collagen within the structure,” (Mari-Beffa et al. 1989 ).
Recently, it was shown that “two zebrafi sh proteins actinodin 1 and 2 (And1 and
And2), are essential structural components of elastoidin. The presence of actinodin
sequences in several teleost fi shes and in the elephant shark ( Callorhinchus milii ,
which occupies a basal phylogenetic position), but not in tetrapods, suggests that
these genes were lost during tetrapod species evolution. Double gene knockdown of
And1 and And2 in zebrafi sh embryos results in the absence of actinotrichia and
impaired fi n folds. Gene expression profi les in embryos lacking and1 and and2 function are consistent with pectoral fi n truncation, and may offer a potential explanation
for the polydactyly observed in early tetrapod fossils. It was proposed that the loss of
both actinodins and actinotrichia during evolution may have led to the loss of lepidotrichia, and may have contributed to the fi n-to-limb transition,” (Zhang et al. 2010 ).
7.3 Chemistry of Fish Fin: Elastoidin