2. CHITIN AND MUCOSUBSTANCES
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carbon compound. Histochemical evidence suggests the possible involvement of a sterol in endocuticle.
Epicuticle does not give a positive chitosan test and in most groups is
characterized by the absence of chitin. The available evidence thus
indicates that lipid constituents of the cuticle are primarily associated
with modified protein structures rather than with chitin.
2. Protein Constituents
Accumulating evidence indicates that in situ native chitin is associated with proteins in a precise fashion. Much of the protein (arthropodin) in soft cuticle of arthropods is extractable by hot water (168)
and has been studied by Fraenkel and Rudall (68). These authors proposed that the protein was present as extended polypeptide chains,
oriented parallel to extended chitin chains. A further protein fraction
(sclerotin) is extractable by the action of dilute acids and alkalis, and in
hard cuticle this fraction is substantially greater than the water-soluble
material. Picken and Lotmar (169) subsequently showed that distinctive
features of X-ray diffraction patterns additional to that of chitin itself
exhibited by nonpurified chitins from Limulus, Palinurus, Aeschna, Sirex,
Aphrodite, and other species, were not present in alkali-extracted materials. These features, it was suggested (15), were attributable to the
possible presence of tanned oriented proteins. Picken points out that the
vigorous conditions required to deproteinize scleratized cuticles are not
necessarily indicative of tenacious chitin-protein interactions but may be
due to the high stability of a tanned protein constituent.
Chemical investigation of the protein extract obtained by aqueous
extraction at 0° of blow-fly puparia cuticles revealed that, whereas it was
soluble in hot water, it is precipitated by 10% trichloroacetic acid, by
ethanol (>50%) ammonium sulfate, and by saturated solutions of
sodium chloride.
Arthropodin appears to be chemically distinct from silk gelatin (sericin) and from egg shell proteins. It is poor in glycine but rich in
tyrosine (70, 170). It occurs in the unusual extended ^-configuration and
is not precipitated on heating. In such configurations, arthropodin could
readily participate in interactions with chitin. In relation to the chitin
structure, N-acetylglucosamine combines in vitro with arthropodin, peptides, and amino acids and notably with tyrosine, giving complexes
dissociable at acidic pH values but stable at higher values (171). Chitin
readily absorbs up to 8% of its weight of protein, giving temperaturestable complexes, sensitive, however, to the presence of electrolytes and
partly dissociable with varying pH (172). At pH 9, the dissociation is
119
carbon compound. Histochemical evidence suggests the possible involvement of a sterol in endocuticle.
Epicuticle does not give a positive chitosan test and in most groups is
characterized by the absence of chitin. The available evidence thus
indicates that lipid constituents of the cuticle are primarily associated
with modified protein structures rather than with chitin.
2. Protein Constituents
Accumulating evidence indicates that in situ native chitin is associated with proteins in a precise fashion. Much of the protein (arthropodin) in soft cuticle of arthropods is extractable by hot water (168)
and has been studied by Fraenkel and Rudall (68). These authors proposed that the protein was present as extended polypeptide chains,
oriented parallel to extended chitin chains. A further protein fraction
(sclerotin) is extractable by the action of dilute acids and alkalis, and in
hard cuticle this fraction is substantially greater than the water-soluble
material. Picken and Lotmar (169) subsequently showed that distinctive
features of X-ray diffraction patterns additional to that of chitin itself
exhibited by nonpurified chitins from Limulus, Palinurus, Aeschna, Sirex,
Aphrodite, and other species, were not present in alkali-extracted materials. These features, it was suggested (15), were attributable to the
possible presence of tanned oriented proteins. Picken points out that the
vigorous conditions required to deproteinize scleratized cuticles are not
necessarily indicative of tenacious chitin-protein interactions but may be
due to the high stability of a tanned protein constituent.
Chemical investigation of the protein extract obtained by aqueous
extraction at 0° of blow-fly puparia cuticles revealed that, whereas it was
soluble in hot water, it is precipitated by 10% trichloroacetic acid, by
ethanol (>50%) ammonium sulfate, and by saturated solutions of
sodium chloride.
Arthropodin appears to be chemically distinct from silk gelatin (sericin) and from egg shell proteins. It is poor in glycine but rich in
tyrosine (70, 170). It occurs in the unusual extended ^-configuration and
is not precipitated on heating. In such configurations, arthropodin could
readily participate in interactions with chitin. In relation to the chitin
structure, N-acetylglucosamine combines in vitro with arthropodin, peptides, and amino acids and notably with tyrosine, giving complexes
dissociable at acidic pH values but stable at higher values (171). Chitin
readily absorbs up to 8% of its weight of protein, giving temperaturestable complexes, sensitive, however, to the presence of electrolytes and
partly dissociable with varying pH (172). At pH 9, the dissociation is