230
J. A. ALLEN AND M. R. GARRETT
tenance of internal osmotic pressure is important for the optimum
functioning of the nerves. But for taurine and alanine, the osmotic
deficit in Carcinus nerve would be considerable. Lewis (1952) showed
that amino acids are a conspicuous group of electrolytes in crustacean
and molluscan nerves-up to 20% of the dry weight of the nerveswhich exist in a free state in the axoplasm. As exemplified by the
rat (Awapara et al., 1950), vertebrate tissue contains much lower
concentrations of free amino acids, particularly the neutral ones alanine,
glycine and taurine. However, the relative distributions between muscle
and nerve in vertebrates when compared with invertebrates is closely
similar. Thus Carcinus muscle contains large quantities of taurine in
comparison with nerve (Table 9) and similarly rat muscle contains six
times more taurine than rat brain (Awapara et al., 1950). The higher
concentration of taurine in marine invertebrate excitable tissue is
certainly in part explained in terms of the maintenance of a higher
internal osmotic pressure. Apparent confirmation of this conclusion
came when Koechlin (1955) found relatively high concentrations of
taurine in squid axoplasm, but in fact the major anion appeared to be
isethionic acid which accounted for nearly 50% of the total anions.
The presence of both these substances in axoplasm was suggestive of
a metabolic relationship (Koechlin, 1955) and later Welty et al. (1962)
and Read and Welty (1962) were able t o demonstrate the conversion
of taurine to isethionic acid in dog heart tissue, presumably by the
agency of a deaminating enzyme. If this is so then taurine, as the amino
analogue, is also concerned in ionic balance as well as osmotic balance.
In chemical terms, taurine might form a cyclical structure by hydrogen
bonding between the amino group and the hydroxyl of the sulphonic
acid group and in this form it would have no charged groups and could
not attract cations. If it were then deaminated to isethionic acid a
charged group on the sulphonic acid would be released thus allowing it
to act as a strong anion. In the quantities that isethionic acid is found
in squid axon such a reaction would have a profound effect on the membrane potential (Koechlin, 1955; Read and Welty, 1962); Welty (1963)
thought that taurine and isethionic acid might be functioning in a feed
back system regulating potassium efflux from cardiac cells and the
excitability of cardiac tissue. A theoretical model implicating isethionic
acid in the conductance changes occurring in the squid axon during
excitation, has been proposed by Mullins (1959), although Robertson
(1965) rejected the view that isethionic acid has any specific role.
Returning to the theory that taurine is used in osmotic balance
Kossel and Edlbacher (1915) earlier suggested that this might explain
the large quantities of glycine and taurine present in the caeca and
J. A. ALLEN AND M. R. GARRETT
tenance of internal osmotic pressure is important for the optimum
functioning of the nerves. But for taurine and alanine, the osmotic
deficit in Carcinus nerve would be considerable. Lewis (1952) showed
that amino acids are a conspicuous group of electrolytes in crustacean
and molluscan nerves-up to 20% of the dry weight of the nerveswhich exist in a free state in the axoplasm. As exemplified by the
rat (Awapara et al., 1950), vertebrate tissue contains much lower
concentrations of free amino acids, particularly the neutral ones alanine,
glycine and taurine. However, the relative distributions between muscle
and nerve in vertebrates when compared with invertebrates is closely
similar. Thus Carcinus muscle contains large quantities of taurine in
comparison with nerve (Table 9) and similarly rat muscle contains six
times more taurine than rat brain (Awapara et al., 1950). The higher
concentration of taurine in marine invertebrate excitable tissue is
certainly in part explained in terms of the maintenance of a higher
internal osmotic pressure. Apparent confirmation of this conclusion
came when Koechlin (1955) found relatively high concentrations of
taurine in squid axoplasm, but in fact the major anion appeared to be
isethionic acid which accounted for nearly 50% of the total anions.
The presence of both these substances in axoplasm was suggestive of
a metabolic relationship (Koechlin, 1955) and later Welty et al. (1962)
and Read and Welty (1962) were able t o demonstrate the conversion
of taurine to isethionic acid in dog heart tissue, presumably by the
agency of a deaminating enzyme. If this is so then taurine, as the amino
analogue, is also concerned in ionic balance as well as osmotic balance.
In chemical terms, taurine might form a cyclical structure by hydrogen
bonding between the amino group and the hydroxyl of the sulphonic
acid group and in this form it would have no charged groups and could
not attract cations. If it were then deaminated to isethionic acid a
charged group on the sulphonic acid would be released thus allowing it
to act as a strong anion. In the quantities that isethionic acid is found
in squid axon such a reaction would have a profound effect on the membrane potential (Koechlin, 1955; Read and Welty, 1962); Welty (1963)
thought that taurine and isethionic acid might be functioning in a feed
back system regulating potassium efflux from cardiac cells and the
excitability of cardiac tissue. A theoretical model implicating isethionic
acid in the conductance changes occurring in the squid axon during
excitation, has been proposed by Mullins (1959), although Robertson
(1965) rejected the view that isethionic acid has any specific role.
Returning to the theory that taurine is used in osmotic balance
Kossel and Edlbacher (1915) earlier suggested that this might explain
the large quantities of glycine and taurine present in the caeca and