AMPHIBIAN AND INVERTEBRATE DEVELOPMENT 101
phenomenon of induced enzymatic synthesis (Spiegelman, 1948, 1950)
is most evident in micro-organisms. Monod's (1955) classical experiment
shows that Esch. coli forms the enzyme which utilizes lactose, maltose,
or saccharose, whichever of these carbohydrates is present in the culture
medium; in other words, the micro-organism produces the enzymes
capable of utilizing one or more of these sugars.
The regular enzymatic supply of a species is under genetic control,
but it would seem that, to a certain extent, organisms can form entirely
new enzymes (Kruh, 1957). This new enzymatic aspect of the longdiscussed embryological problems concerning regulatory processes
opens up a wide field of speculation.
Several hypotheses have been formulated to explain the formation of
adaptive enzymes. It might be that in a population of micro-organisms
of the same species, mutants capable of utilizing a given substrate
develop in the culture; in other words, there would be selection among
the population. Or it might be that the enzymatic molecules are present
in an inactive state in the micro-organism and are brought into play by
the substrate, without actual synthesis (DubnofF, 1955; Yudkin, 1938).
According to another hypothesis, there would actually be a real synthesis of the enzymatic molecule starting from the amino acids: in
this case the substrate is the inducing agent. The scientists who have
looked into this problem seem to favour the latter theory (Pollock,
1952).
Enzymatic adaptation is also exhibited by animals of the highly
organized classes. For instance, ingestion of tryptophan in excess increases tryptophanoxidase activity ten-fold in the liver of the rabbit
(Knox and Meheler, 1955); similarly, an increase in threonin dehydrase
activity is shown by rats injected with threonin (Sare et al., 1956).
Adenosine deaminase is said to be an example of an adaptive enzyme
in embryonic material: Gordon (1952) and Gordon and Roder (1953)
observed that this enzyme is formed at a very early stage in the liver
of chick embryos after injecting adenosine into the yolk sac. But Solomon (1960) has been unable to confirm these findings and he concludes
that adenosine deaminase is a constitutive, not an inducible enzyme.
Mention should also be made of the work of Steams and Kostellow
(1958) and Kostellow (1961), who claim that frog eggs synthesize
tryptophan peroxidase and tryptophan pyrrolase upon the addition of
tryptophan. Independent confirmation seems to be needed before this
claim can be accepted.
In a few instances it was possible to show the dependence of adaptive
enzyme activity on the dose of the inducing agent used (Feigelson et al.,
1954).
The problem of enzymatic synthesis is but one aspect of the complex
phenomenon of induced enzymatic synthesis (Spiegelman, 1948, 1950)
is most evident in micro-organisms. Monod's (1955) classical experiment
shows that Esch. coli forms the enzyme which utilizes lactose, maltose,
or saccharose, whichever of these carbohydrates is present in the culture
medium; in other words, the micro-organism produces the enzymes
capable of utilizing one or more of these sugars.
The regular enzymatic supply of a species is under genetic control,
but it would seem that, to a certain extent, organisms can form entirely
new enzymes (Kruh, 1957). This new enzymatic aspect of the longdiscussed embryological problems concerning regulatory processes
opens up a wide field of speculation.
Several hypotheses have been formulated to explain the formation of
adaptive enzymes. It might be that in a population of micro-organisms
of the same species, mutants capable of utilizing a given substrate
develop in the culture; in other words, there would be selection among
the population. Or it might be that the enzymatic molecules are present
in an inactive state in the micro-organism and are brought into play by
the substrate, without actual synthesis (DubnofF, 1955; Yudkin, 1938).
According to another hypothesis, there would actually be a real synthesis of the enzymatic molecule starting from the amino acids: in
this case the substrate is the inducing agent. The scientists who have
looked into this problem seem to favour the latter theory (Pollock,
1952).
Enzymatic adaptation is also exhibited by animals of the highly
organized classes. For instance, ingestion of tryptophan in excess increases tryptophanoxidase activity ten-fold in the liver of the rabbit
(Knox and Meheler, 1955); similarly, an increase in threonin dehydrase
activity is shown by rats injected with threonin (Sare et al., 1956).
Adenosine deaminase is said to be an example of an adaptive enzyme
in embryonic material: Gordon (1952) and Gordon and Roder (1953)
observed that this enzyme is formed at a very early stage in the liver
of chick embryos after injecting adenosine into the yolk sac. But Solomon (1960) has been unable to confirm these findings and he concludes
that adenosine deaminase is a constitutive, not an inducible enzyme.
Mention should also be made of the work of Steams and Kostellow
(1958) and Kostellow (1961), who claim that frog eggs synthesize
tryptophan peroxidase and tryptophan pyrrolase upon the addition of
tryptophan. Independent confirmation seems to be needed before this
claim can be accepted.
In a few instances it was possible to show the dependence of adaptive
enzyme activity on the dose of the inducing agent used (Feigelson et al.,
1954).
The problem of enzymatic synthesis is but one aspect of the complex