232
J. A. ALLEN AND M. R. GARRETT
presence of taurine can be detectable at concentrations as low rn
0.1 pM/g by their chromatographic technique. These results would again
support the theory of taurine having an osmotic role. This work gave
no inforniation as to whether sulphur amino acids are metabolized
differently in marine molluscs as compared with freshwater molluscs.
However, Allen and Awapara (1960) carried out a number of experiments t d this end using Jlytilus edulis as an example of a marine
mollusc and Rangia cuneata as the freshwater species. Their experiments showed that both can convert methionine to cysteinc which in its
turn can be oxidized to cysteine sulphinic acid and which can give rise
to taurine and sulphate. Thus after injection of S35 methionine to both
species, Mytilus edulis, known to contain large quantities of taurine,
after 24 h contained labelled cystathione, an unknown which was
possibly methionine sulphate, hypotaurine, taurine and sulphate, and
a fairly small amount of cysteine/cystine while Rangia cuneata, which
normally contains no taurine, had labelled cysteic acid and cysteine
sulphinic acid (not found in Mytilus) but no cystathione or hypotaurine,
yet larger quantities of cysteinelcystine than in Mytilus. Labelled
taurine and sulphate were also present and in greater quantities than
Mytilus. In repeat experiments, analyses at different times up to 24 h
after administration of S35 methionine showed that in Rangia taurine
is quickly formed (within 5 h) but is not held and has largely disappeared after 24 h. In the case of Mytilus the high concentration of
endogenous taurine makes it difficult to establish the rate of taurine
formation. The significant difference between the two animals is
the rate at which taurine is disposed of; thus, Mytilus keeps it by
an unknown mechanism against a concentration gradient, while
Rangia cannot hold the taurine it produces. If there are any differences
in the metabolism of sulphur amino acids in the molluscs studied, the
differences are in the intermediates, thus in Rangia taurine is probably
formed mainly by decarboxylation of cysteic acid whereas in Mytilus
it is mainly formed by oxidation of hypotaurine (Fig. 1). Potts (1958)
also compared the adaptation of marine and freshwater bivalves to
different external concentrations. He found that the amino acids
contribute in major part t o the level of osmotic pressure in Mytilus.
He found, too, that in Mytilus adapted to half salinity that the amino
acid and taurine content of the adductor and byssus retractor muscles
was much reduced. However, in Anodonta amino acids play little part
in the maintenance of its lower osmotic pressure, instead here potassium
and phosphate ions make up the major contribution. Taurine is absent
in Anodonta. In both Anodonta and Mytilus, adaptation to a changed
osmolar concentration is brought about partly by water movement into
J. A. ALLEN AND M. R. GARRETT
presence of taurine can be detectable at concentrations as low rn
0.1 pM/g by their chromatographic technique. These results would again
support the theory of taurine having an osmotic role. This work gave
no inforniation as to whether sulphur amino acids are metabolized
differently in marine molluscs as compared with freshwater molluscs.
However, Allen and Awapara (1960) carried out a number of experiments t d this end using Jlytilus edulis as an example of a marine
mollusc and Rangia cuneata as the freshwater species. Their experiments showed that both can convert methionine to cysteinc which in its
turn can be oxidized to cysteine sulphinic acid and which can give rise
to taurine and sulphate. Thus after injection of S35 methionine to both
species, Mytilus edulis, known to contain large quantities of taurine,
after 24 h contained labelled cystathione, an unknown which was
possibly methionine sulphate, hypotaurine, taurine and sulphate, and
a fairly small amount of cysteine/cystine while Rangia cuneata, which
normally contains no taurine, had labelled cysteic acid and cysteine
sulphinic acid (not found in Mytilus) but no cystathione or hypotaurine,
yet larger quantities of cysteinelcystine than in Mytilus. Labelled
taurine and sulphate were also present and in greater quantities than
Mytilus. In repeat experiments, analyses at different times up to 24 h
after administration of S35 methionine showed that in Rangia taurine
is quickly formed (within 5 h) but is not held and has largely disappeared after 24 h. In the case of Mytilus the high concentration of
endogenous taurine makes it difficult to establish the rate of taurine
formation. The significant difference between the two animals is
the rate at which taurine is disposed of; thus, Mytilus keeps it by
an unknown mechanism against a concentration gradient, while
Rangia cannot hold the taurine it produces. If there are any differences
in the metabolism of sulphur amino acids in the molluscs studied, the
differences are in the intermediates, thus in Rangia taurine is probably
formed mainly by decarboxylation of cysteic acid whereas in Mytilus
it is mainly formed by oxidation of hypotaurine (Fig. 1). Potts (1958)
also compared the adaptation of marine and freshwater bivalves to
different external concentrations. He found that the amino acids
contribute in major part t o the level of osmotic pressure in Mytilus.
He found, too, that in Mytilus adapted to half salinity that the amino
acid and taurine content of the adductor and byssus retractor muscles
was much reduced. However, in Anodonta amino acids play little part
in the maintenance of its lower osmotic pressure, instead here potassium
and phosphate ions make up the major contribution. Taurine is absent
in Anodonta. In both Anodonta and Mytilus, adaptation to a changed
osmolar concentration is brought about partly by water movement into