116
P. W. KENT
charides. Chitobiase promoted the hydrolysis of synthetic substrates
(e.g., phenyl N-acetyl-/?-D-glucosaminide) as well as di-N-acetylchitobiose and the trisaccharide, in a similar fashion to chitobiase in emulsin.
Further fractionation of a Streptomyces chitinase into three fractions has
been reported by Jeuniaux (141-143).
Chitinolytic enzymes have been
found (144) in the digestive juices, washed glands, and mucosae of a
number of species (Tables I-A and I-B) of insectivorous or chitinconsuming animals. Weak chitinolysis is very widespread even in herbivores. The pancreas of Lacerta, gastric mucosae of Turdus and Bhinolophus are comparatively rich in chitinase while containing little chitobiase.
B. BIOSYNTHETIC PATHWAYS
Comparatively little is known of the metabolic transformations occurring in cuticle, in particular in the epidermis, leading to the formation
of chitin. It is however considered unlikely that reversal of chitinase has
any significance (145) in the synthesis of di- or oligosaccharides, notwithstanding the demonstration in vitro of its ability to synthesize a
^-glycoside from N-acetylglucosamine (146).
In a survey of the nucleotides of the hypodermal tissues (147) of
Carcinus maenas, Homarus vulgaris, and Maia squinado, ion-exchange
chromatography of hot water extracts revealed the presence of 5'phosphates of adenosine, cytosine, uridine, and guanosine, together
with adenosine triphosphate. Uridine diphosphate N-acetylglucosamine
(UDPAG) was present in the hepatopancreas of the lobster (0.150.75 /xmole per gram tissue, wet weight) and of C. maenas (0.22 jumole
per gram tissue, wet weight); it is possible therefore that this nucleotide,
here too, may be a precursor of chitin. Evidence for the biosynthesis of
chitin by the desert locust has been presented by Kilby and Candy (148).
Glaser and Brown (149, 150) succeeded in obtaining an enzyme
system from Neurospora crassa which catalyzed the incorporation of C
14
uridine diphosphate N-acetyl-D-glucosamine (UDPAG) into chitin in
vitro. The amino sugar moiety, fully [C
14 ] labeled, was enzymatically
transferred to an added chitodextrin primer without which no significant
incorporation occurred. The [C
14 ] product retained its labeling throughout extensive purification procedures and was recovered as [C
14 ]
2-amino-2-deoxy-D-glucose when the product was hydrolyzed by acid.
Evidence for the chitin structure of the labeled product was obtained
from its degradation by emulsin chitinase to [C
14 ] N-acetyl-D-glucosamine. Curiously, N-acetyl-D-glucosamine stimulated the incorporation
of label into product although neither this sugar nor its 6- or 1-phosphate
could replace UDPAG. The biosynthesis is apparently irreversible since
incubation of radioactive chitin with UDPAG resulted in no exchange
P. W. KENT
charides. Chitobiase promoted the hydrolysis of synthetic substrates
(e.g., phenyl N-acetyl-/?-D-glucosaminide) as well as di-N-acetylchitobiose and the trisaccharide, in a similar fashion to chitobiase in emulsin.
Further fractionation of a Streptomyces chitinase into three fractions has
been reported by Jeuniaux (141-143).
Chitinolytic enzymes have been
found (144) in the digestive juices, washed glands, and mucosae of a
number of species (Tables I-A and I-B) of insectivorous or chitinconsuming animals. Weak chitinolysis is very widespread even in herbivores. The pancreas of Lacerta, gastric mucosae of Turdus and Bhinolophus are comparatively rich in chitinase while containing little chitobiase.
B. BIOSYNTHETIC PATHWAYS
Comparatively little is known of the metabolic transformations occurring in cuticle, in particular in the epidermis, leading to the formation
of chitin. It is however considered unlikely that reversal of chitinase has
any significance (145) in the synthesis of di- or oligosaccharides, notwithstanding the demonstration in vitro of its ability to synthesize a
^-glycoside from N-acetylglucosamine (146).
In a survey of the nucleotides of the hypodermal tissues (147) of
Carcinus maenas, Homarus vulgaris, and Maia squinado, ion-exchange
chromatography of hot water extracts revealed the presence of 5'phosphates of adenosine, cytosine, uridine, and guanosine, together
with adenosine triphosphate. Uridine diphosphate N-acetylglucosamine
(UDPAG) was present in the hepatopancreas of the lobster (0.150.75 /xmole per gram tissue, wet weight) and of C. maenas (0.22 jumole
per gram tissue, wet weight); it is possible therefore that this nucleotide,
here too, may be a precursor of chitin. Evidence for the biosynthesis of
chitin by the desert locust has been presented by Kilby and Candy (148).
Glaser and Brown (149, 150) succeeded in obtaining an enzyme
system from Neurospora crassa which catalyzed the incorporation of C
14
uridine diphosphate N-acetyl-D-glucosamine (UDPAG) into chitin in
vitro. The amino sugar moiety, fully [C
14 ] labeled, was enzymatically
transferred to an added chitodextrin primer without which no significant
incorporation occurred. The [C
14 ] product retained its labeling throughout extensive purification procedures and was recovered as [C
14 ]
2-amino-2-deoxy-D-glucose when the product was hydrolyzed by acid.
Evidence for the chitin structure of the labeled product was obtained
from its degradation by emulsin chitinase to [C
14 ] N-acetyl-D-glucosamine. Curiously, N-acetyl-D-glucosamine stimulated the incorporation
of label into product although neither this sugar nor its 6- or 1-phosphate
could replace UDPAG. The biosynthesis is apparently irreversible since
incubation of radioactive chitin with UDPAG resulted in no exchange