arenaria by about 47 % , and in Mercenaria mercenaria by as much as 91 % [234]. Various mammalian organs contain two ~-N-acetylhexosaminidases, A and B, which have many very similar properties but vary in their activities against
different gangliosides. These enzymes are tetramers, made up of two different subunits according to the formulae A = a2~2 and B = ~4' The
human genes for a and ~ are located on different
chromosomes. It has been shown for the liver
enzymes of various primates that the two genes
separated a long time ago, and their evolution is
particularly slow. The immunological distance,
determined by microcomplement fixation, between A and B of the same species is much greater
than the species-specific differences within A and
B. Isoenzymes of ~-N-acetyl-hexosaminidase
have also been recorded in protozoans and molluscs [203].
13.6.5 Lysozy~es
Based on the observation that bacteria were lysed
by chicken albumen, Fleming in 1922 postulated
the presence of an enzyme which he called lysozyme. It is now known that lysozymes cleave
~(1-4) bonds between N-acetylmuraminic acids
and N-acetylglucosamine in the cell wall material
of Gram-positive bacteria. The susceptibility of
Micrococcus lysodeikticus makes it particularly
suitable for lysozyme detection. According to
their origin, lysozymes may have a higher or
lower chitinase activity; the ratio of chitinase to
muraminidase activity for the enzymes from the
chicken egg or the edible mussel Mytilus edulis
are about 1: 100, whereas for the lysozyme from
the eggs of the Mediterranean fruit fly Ceratitis
capitata the ratio is much higher. Relevant here is
the fact that lysozymes are inhibited by free Nacetylglucosamine and even more so by chitin oligosaccharides. The primary function of the albumen lysozymes is apparently not defence against
microorganisms; many bacteria on the surface of
the egg are not attacked and the lysis products are
highly toxic to embryos. Lysozymes appear in fact
to be responsible for the gel structure of the albumen in freshly laid eggs. This is brought about by
lysozyme and ovomucin, which together produce
a cross-linked structure; the positively charged
lysine residues of the lysozyme interact electrostatically with the negatively charged terminal
sialic acid residues on the carbohydrate chains of
ovomucin. The temperature increase during
brooding causes a partial degradation of these
13.6.5 Lysozymes
503
carbohydrate chains and an increase in the fluidity of the egg white.
Lysozymes have been found in bacteriophages,
bacteria, lower fungi, higher plants, and both
invertebrates and vertebrates. Many of them
have been isolated and characterized and, consequently, they are amongst the most investigated
enzymes of comparative biochemistry. In the
mammals, they are found in tissues, blood and
urine, and also occur in secretions such as milk
and tears. In the ruminants and certain species of
ape, lysozymes function as digestive enzymes.
The lysozyme content of avian albumen varies
from 3-4 % of the total protein in chickens and
geese to only trace amounts in penguins [81].
Reptilian albumen also contains lysozymes. No
fewer than eight isozymes of lysozyme from various organs have been detected in the frog Rana
pipiens. Detailed investigations have been carried
out on the lysozymes of the plaice Pleuronectes
platessa, the polychaete Nephthys hombergi, the
starfish Asterias rubens, the edible mussel Mytilus
edulis and many insects [64, 116, 179,242, 296].
Haemolymph lysozymes can be induced in insects
by inoculation with bacteria; however, other haemolymph proteins, such as cecropins and attacips, have a much higher bactericidal activity than
the lysozymes.
Two different types of lysozyme found in avian
a1bu~en are designated, according to the first
examined species, as c type (chicken) and g type
(goose). The two lysozyme types are so different
in structural and functional properties that they
must be considered as two completely different
proteins. From the soluble peptidoglycan of
Micrococcus lysodeikticus, the chicken eggwhite lysozyme (HEWL) produces: (1) the
disaccharide GlcNAc~(1-4)MurNAc; (2) a tetrasaccharide made up of two such sequences; and
(3) a disaccharide-hexapeptide; the goose eggwhite lysozyme (GEWL) produces only (1) and
(3), and the lysozyme from phage T4 produces
only (3) [241]. Compared with the g type, the
c type has greater activity against chitin oligo saccharides and is more strongly inhibited by Nacetylglucosamine. The pH optimum of the
c type is weakly alkaline, and that of the g type
is strongly acidic [10]. HEWL has a molecular
mass of 14.5 kDa (129 amino acids) and GEWL
20 kDa (185 amino acids). Apart from certain
similarities in the active centre, there is no significant sequence agreement between the lysozymes of chicken and goose; however, the spatial
structures derived from the sequences are very
similar.
different gangliosides. These enzymes are tetramers, made up of two different subunits according to the formulae A = a2~2 and B = ~4' The
human genes for a and ~ are located on different
chromosomes. It has been shown for the liver
enzymes of various primates that the two genes
separated a long time ago, and their evolution is
particularly slow. The immunological distance,
determined by microcomplement fixation, between A and B of the same species is much greater
than the species-specific differences within A and
B. Isoenzymes of ~-N-acetyl-hexosaminidase
have also been recorded in protozoans and molluscs [203].
13.6.5 Lysozy~es
Based on the observation that bacteria were lysed
by chicken albumen, Fleming in 1922 postulated
the presence of an enzyme which he called lysozyme. It is now known that lysozymes cleave
~(1-4) bonds between N-acetylmuraminic acids
and N-acetylglucosamine in the cell wall material
of Gram-positive bacteria. The susceptibility of
Micrococcus lysodeikticus makes it particularly
suitable for lysozyme detection. According to
their origin, lysozymes may have a higher or
lower chitinase activity; the ratio of chitinase to
muraminidase activity for the enzymes from the
chicken egg or the edible mussel Mytilus edulis
are about 1: 100, whereas for the lysozyme from
the eggs of the Mediterranean fruit fly Ceratitis
capitata the ratio is much higher. Relevant here is
the fact that lysozymes are inhibited by free Nacetylglucosamine and even more so by chitin oligosaccharides. The primary function of the albumen lysozymes is apparently not defence against
microorganisms; many bacteria on the surface of
the egg are not attacked and the lysis products are
highly toxic to embryos. Lysozymes appear in fact
to be responsible for the gel structure of the albumen in freshly laid eggs. This is brought about by
lysozyme and ovomucin, which together produce
a cross-linked structure; the positively charged
lysine residues of the lysozyme interact electrostatically with the negatively charged terminal
sialic acid residues on the carbohydrate chains of
ovomucin. The temperature increase during
brooding causes a partial degradation of these
13.6.5 Lysozymes
503
carbohydrate chains and an increase in the fluidity of the egg white.
Lysozymes have been found in bacteriophages,
bacteria, lower fungi, higher plants, and both
invertebrates and vertebrates. Many of them
have been isolated and characterized and, consequently, they are amongst the most investigated
enzymes of comparative biochemistry. In the
mammals, they are found in tissues, blood and
urine, and also occur in secretions such as milk
and tears. In the ruminants and certain species of
ape, lysozymes function as digestive enzymes.
The lysozyme content of avian albumen varies
from 3-4 % of the total protein in chickens and
geese to only trace amounts in penguins [81].
Reptilian albumen also contains lysozymes. No
fewer than eight isozymes of lysozyme from various organs have been detected in the frog Rana
pipiens. Detailed investigations have been carried
out on the lysozymes of the plaice Pleuronectes
platessa, the polychaete Nephthys hombergi, the
starfish Asterias rubens, the edible mussel Mytilus
edulis and many insects [64, 116, 179,242, 296].
Haemolymph lysozymes can be induced in insects
by inoculation with bacteria; however, other haemolymph proteins, such as cecropins and attacips, have a much higher bactericidal activity than
the lysozymes.
Two different types of lysozyme found in avian
a1bu~en are designated, according to the first
examined species, as c type (chicken) and g type
(goose). The two lysozyme types are so different
in structural and functional properties that they
must be considered as two completely different
proteins. From the soluble peptidoglycan of
Micrococcus lysodeikticus, the chicken eggwhite lysozyme (HEWL) produces: (1) the
disaccharide GlcNAc~(1-4)MurNAc; (2) a tetrasaccharide made up of two such sequences; and
(3) a disaccharide-hexapeptide; the goose eggwhite lysozyme (GEWL) produces only (1) and
(3), and the lysozyme from phage T4 produces
only (3) [241]. Compared with the g type, the
c type has greater activity against chitin oligo saccharides and is more strongly inhibited by Nacetylglucosamine. The pH optimum of the
c type is weakly alkaline, and that of the g type
is strongly acidic [10]. HEWL has a molecular
mass of 14.5 kDa (129 amino acids) and GEWL
20 kDa (185 amino acids). Apart from certain
similarities in the active centre, there is no significant sequence agreement between the lysozymes of chicken and goose; however, the spatial
structures derived from the sequences are very
similar.