262
RUTH BELLAIRS
hydrolysed the phosphate linkages in the phosvitin molecule, although
this reaction proceeded very slowly.
Cathepsin is present in the yolk sac (Goldstein and Ginstburg, 1936;
Mystkowski, 1936). Ito (1957) has suggested that his proteinase which
acts at p H 5-8 is cathepsin. This becomes more active in the yolk sac
than in the yolk after 4 days and increases until the 10th day; it then
slowly decreases.
Other enzymes affecting breakdown of the yolk proteins have been
reported. They include: cysteinlyase which activates the j8-carbon of
cysteine (Chapeville and Fromageot, 1961) ; trimetaphosphatase which is a
catalyst for the hydrolytic conversion of cyclic trimetaphosphate to linear
inorganic tripolyphosphate. According to Berg and Szekerczes (1960)
this enzyme first appears in the area vasculosa and by the 12th day is
found throughout the yolk sac. They conclude that although its precise
significance is obscure it may be the site of active uptake of nutrients ;
cysteine desulphydrase (see Section V, E) which has maximal activity in
the yolk sac at 7-11 days of incubation, but falls to half that value by
16 days (Solomon, 1963) ; lactic and malic dehydrogenases (see Section V,
E) which also have a high level of activity in the yolk sac in the early
stages (Solomon, 1958, 1959); glutamotransferase is very active during
the first 12-13 days of incubation (Rudnick et ai., 1954). In addition to
the enzymes co-enzyme Q is present in the yolk in decreasing amounts
until it disappears by the 15th day (Ramasarma et al., 1961).
Proteins are also present in the egg-white (see Romanoff and Romanoff, 1949) and these enter the yolk sac. Mclndoe (1960) found that they
cause a marked increase in the protein content of the yolk about the
14th-16th days of incubation. He estimated that 30% of the total eggwhite may enter at this time. The egg-white proteins appear to be
utilized mainly during the second half of incubation. From about the
9th to the 14th days, the embryo selectively absorbs methionine and
cystine which have been supplied by the egg-white even though the
same amino acids are present in the yolk (Nonani, 1959).
As the egg-white decreases during incubation the proportion of the
various proteins in it remains constant (Marshall and Deutsch, 1950,
1951). By the time of hatching the egg-white has disappeared.
2. Lipases
Despite the importance of the fat content of the hen's egg the lipases
have received considerably less attention than the proteases. Accounts
of the earlier work are given by Needham (1931, 1950). Lipases are
present in the unincubated egg (Needham, 1950), and Zacks (1954)
found aliesterases in the area opaca at 21 h of incubation. Similarly,
Buno and Gonzales (1952) have reported lipase activity in the early
RUTH BELLAIRS
hydrolysed the phosphate linkages in the phosvitin molecule, although
this reaction proceeded very slowly.
Cathepsin is present in the yolk sac (Goldstein and Ginstburg, 1936;
Mystkowski, 1936). Ito (1957) has suggested that his proteinase which
acts at p H 5-8 is cathepsin. This becomes more active in the yolk sac
than in the yolk after 4 days and increases until the 10th day; it then
slowly decreases.
Other enzymes affecting breakdown of the yolk proteins have been
reported. They include: cysteinlyase which activates the j8-carbon of
cysteine (Chapeville and Fromageot, 1961) ; trimetaphosphatase which is a
catalyst for the hydrolytic conversion of cyclic trimetaphosphate to linear
inorganic tripolyphosphate. According to Berg and Szekerczes (1960)
this enzyme first appears in the area vasculosa and by the 12th day is
found throughout the yolk sac. They conclude that although its precise
significance is obscure it may be the site of active uptake of nutrients ;
cysteine desulphydrase (see Section V, E) which has maximal activity in
the yolk sac at 7-11 days of incubation, but falls to half that value by
16 days (Solomon, 1963) ; lactic and malic dehydrogenases (see Section V,
E) which also have a high level of activity in the yolk sac in the early
stages (Solomon, 1958, 1959); glutamotransferase is very active during
the first 12-13 days of incubation (Rudnick et ai., 1954). In addition to
the enzymes co-enzyme Q is present in the yolk in decreasing amounts
until it disappears by the 15th day (Ramasarma et al., 1961).
Proteins are also present in the egg-white (see Romanoff and Romanoff, 1949) and these enter the yolk sac. Mclndoe (1960) found that they
cause a marked increase in the protein content of the yolk about the
14th-16th days of incubation. He estimated that 30% of the total eggwhite may enter at this time. The egg-white proteins appear to be
utilized mainly during the second half of incubation. From about the
9th to the 14th days, the embryo selectively absorbs methionine and
cystine which have been supplied by the egg-white even though the
same amino acids are present in the yolk (Nonani, 1959).
As the egg-white decreases during incubation the proportion of the
various proteins in it remains constant (Marshall and Deutsch, 1950,
1951). By the time of hatching the egg-white has disappeared.
2. Lipases
Despite the importance of the fat content of the hen's egg the lipases
have received considerably less attention than the proteases. Accounts
of the earlier work are given by Needham (1931, 1950). Lipases are
present in the unincubated egg (Needham, 1950), and Zacks (1954)
found aliesterases in the area opaca at 21 h of incubation. Similarly,
Buno and Gonzales (1952) have reported lipase activity in the early