NUCLEIC ACIDS AND SULPHYDRYL GROUPS
287
Acetabularia. Very few observations have been made so far concerning
the biochemical mode of action of lipoic acid.
The main findings with amphibian eggs (Brächet et al. 9 1961) can be
summarized as follows. First of all, it was observed that mercaptoethanol
(0-01 M) undergoes a fairly rapid autoxidation (about 60% oxidized in
24 h), which proceeds at the same rate whether or not embryos are
present. That mercaptoethanol really penetrates into the eggs was
demonstrated by comparing the acid-soluble-SH content of controls and
mercaptoethanol-treated eggs: a moderate increase (30%) was found in
treated eggs, indicating that the uptake of mercaptoethanol does not
occur on a very large scale. Mercaptoethanol inhibits the oxygen
consumption
of the treated embryos; again, the inhibitory effect is
shown to be a moderate one (25%) when suitable controls are used. In
the next series of experiments, the A TP content of normal and mercaptoethanol-treated embryos was compared. It was found that there is no
marked change in the ATP content during normal embryogenesis and
that mercaptoethanol, if anything, slightly increases the ATP content.
It can be concluded from these experiments that the overall energy
production remains essentially normal after a one day treatment with
0-01 M mercaptoethanol.
It was decided to study cathepsin activity in control and treated
embryos because cathepsin is a proteolytic enzyme which requires —SH
groups for full activity. It is conceivable that mercaptoethanol might
activate the enzyme and that enhanced proteolysis is responsible for the
block in morphogenesis. But the experiments do not substantiate this
hypothesis. Mercaptoethanol, under conditions which stop embryonic
development, has no measurable effect on the cathepsin activity of
gastrulae and neurulae: at later stages (hatching tadpoles), it reduces
catheptic activity 40%.
The total RNA content remains constant during the period of the
morphogenetic block induced by mercaptoethanol. This means that the
RNA synthesis, which normally occurs during development, is arrested.
But the same result can be obtained when embryogenesis is blocked by
cold treatment. Thus, the experiments only confirm that, as we already
know, RNA synthesis and morphogenesis are always closely linked.
Nucleic acid and protein synthesis have also been followed, using the
methods of autoradiography (Brächet et al., 1961) after incorporation of
specific precursors such as thymidine, uridine and leucine. The incorporation of these three precursors is unchanged after a 24-hour treatment
with M/100 mercaptoethanol, although such a treatment leads to a
considerable delay in development.
But, more recently, Quertier (1962 and unpublished) has made
interesting and unexpected observations on amphibian eggs treated with
M
287
Acetabularia. Very few observations have been made so far concerning
the biochemical mode of action of lipoic acid.
The main findings with amphibian eggs (Brächet et al. 9 1961) can be
summarized as follows. First of all, it was observed that mercaptoethanol
(0-01 M) undergoes a fairly rapid autoxidation (about 60% oxidized in
24 h), which proceeds at the same rate whether or not embryos are
present. That mercaptoethanol really penetrates into the eggs was
demonstrated by comparing the acid-soluble-SH content of controls and
mercaptoethanol-treated eggs: a moderate increase (30%) was found in
treated eggs, indicating that the uptake of mercaptoethanol does not
occur on a very large scale. Mercaptoethanol inhibits the oxygen
consumption
of the treated embryos; again, the inhibitory effect is
shown to be a moderate one (25%) when suitable controls are used. In
the next series of experiments, the A TP content of normal and mercaptoethanol-treated embryos was compared. It was found that there is no
marked change in the ATP content during normal embryogenesis and
that mercaptoethanol, if anything, slightly increases the ATP content.
It can be concluded from these experiments that the overall energy
production remains essentially normal after a one day treatment with
0-01 M mercaptoethanol.
It was decided to study cathepsin activity in control and treated
embryos because cathepsin is a proteolytic enzyme which requires —SH
groups for full activity. It is conceivable that mercaptoethanol might
activate the enzyme and that enhanced proteolysis is responsible for the
block in morphogenesis. But the experiments do not substantiate this
hypothesis. Mercaptoethanol, under conditions which stop embryonic
development, has no measurable effect on the cathepsin activity of
gastrulae and neurulae: at later stages (hatching tadpoles), it reduces
catheptic activity 40%.
The total RNA content remains constant during the period of the
morphogenetic block induced by mercaptoethanol. This means that the
RNA synthesis, which normally occurs during development, is arrested.
But the same result can be obtained when embryogenesis is blocked by
cold treatment. Thus, the experiments only confirm that, as we already
know, RNA synthesis and morphogenesis are always closely linked.
Nucleic acid and protein synthesis have also been followed, using the
methods of autoradiography (Brächet et al., 1961) after incorporation of
specific precursors such as thymidine, uridine and leucine. The incorporation of these three precursors is unchanged after a 24-hour treatment
with M/100 mercaptoethanol, although such a treatment leads to a
considerable delay in development.
But, more recently, Quertier (1962 and unpublished) has made
interesting and unexpected observations on amphibian eggs treated with
M