easy to understjarid. Goodbod,v (1965) suggested that it was entirely
due to a deficicnncy in thc uricolytic: eneyme system and showed that
uricase, allantoinase, al1;~ntoic~r~se and urease were all absent from
hornogenat,es of’ a large number of species of ascidian. A fuller understartding of thc problem requires t i knowledge of the biosynthetic
pathways in t h c ascidiaii by which uric acid is formed. Nolfi (1970)
has shown that de novo synthesis of uric acid in Molgula from smaller
molecules does not occur but that it must be entirely produced from
pre-formed purines. He has also shown that uric acid makes up about
3% of the total nitrogen excreted by Molgula and thus confirms Goodbody’s (1965) suggestion that the uric acid is probably derived from
nucleic acid nietabolism and not from protein metabolism.
There remuins the question as to what functional purpose is served
by the uccumulation of uric acid in uscidians. Nolfi (Zoc. cit.) has pointed
out that thcre are four possible biological functions which could be
served by purine storage, viz. ( I ) storage excretion ; (2) pigmentation ;
(3) a source of reserve of carbon or nitrogen, and (4) a stable purine
source for nucleic acid synthesis.
It is urilikcly that pigmentation is the primary reason for uric acid
storage as in ;nost species it is stored deep in the tissues away from
surface aroas of pigmentation, atid most authors have assumed that it is
solely a meam of removing solid wastes. However, there are a number
of pieces of evidence which suggest that the uric acid may be mobile and
involved i n some active metabolic process in the animal. Hertwig
(1871 ) found that tlie concretions of l’hallusia niammillata were soluble
in acids with an intense evolution of gas and he suggested that they
were coniposccd primarily of carbonates. Goodbody (1 965) working
with the S I L I I I C species found alrnoxt no rarhonate and fi2.5% of uric
acid in the concretion. Goodbody also found that the concretions of
AxidieEia aspersa wete largely composed of calcium carbonate in the
form of calcite and Nrarrevang ( I 966) found dense granules of glycogen
and acid mucopolysaccharides in the renal vesicles of the same Rpecies
but noted that secLsona1 differences in the stat(> of the vesicle occur.
Nolfi (Eoc. cit.) found that uric acid content of the vesicle in Molyula
manhuttensis varies from 3 to 75% of the dry weight in the concretion.
Nolfi quotes Bwhner (1953) as having reported the presence of fungi
in the renal vesicle of M. manhntterLai8 which he considered engulfed
and digestcd the concretions in R it umoebocytic fashion. Kolfi suggests
that if some of the metabolic intermediates “ leaked ” back to the
ascidian host thiN rould be a miitualistic relationship. It is difficult to
see how such a rchtionship could function and how the fungus could
initially invtide the closed coelomic vesicle. My own histological
4 \I H -12
4
due to a deficicnncy in thc uricolytic: eneyme system and showed that
uricase, allantoinase, al1;~ntoic~r~se and urease were all absent from
hornogenat,es of’ a large number of species of ascidian. A fuller understartding of thc problem requires t i knowledge of the biosynthetic
pathways in t h c ascidiaii by which uric acid is formed. Nolfi (1970)
has shown that de novo synthesis of uric acid in Molgula from smaller
molecules does not occur but that it must be entirely produced from
pre-formed purines. He has also shown that uric acid makes up about
3% of the total nitrogen excreted by Molgula and thus confirms Goodbody’s (1965) suggestion that the uric acid is probably derived from
nucleic acid nietabolism and not from protein metabolism.
There remuins the question as to what functional purpose is served
by the uccumulation of uric acid in uscidians. Nolfi (Zoc. cit.) has pointed
out that thcre are four possible biological functions which could be
served by purine storage, viz. ( I ) storage excretion ; (2) pigmentation ;
(3) a source of reserve of carbon or nitrogen, and (4) a stable purine
source for nucleic acid synthesis.
It is urilikcly that pigmentation is the primary reason for uric acid
storage as in ;nost species it is stored deep in the tissues away from
surface aroas of pigmentation, atid most authors have assumed that it is
solely a meam of removing solid wastes. However, there are a number
of pieces of evidence which suggest that the uric acid may be mobile and
involved i n some active metabolic process in the animal. Hertwig
(1871 ) found that tlie concretions of l’hallusia niammillata were soluble
in acids with an intense evolution of gas and he suggested that they
were coniposccd primarily of carbonates. Goodbody (1 965) working
with the S I L I I I C species found alrnoxt no rarhonate and fi2.5% of uric
acid in the concretion. Goodbody also found that the concretions of
AxidieEia aspersa wete largely composed of calcium carbonate in the
form of calcite and Nrarrevang ( I 966) found dense granules of glycogen
and acid mucopolysaccharides in the renal vesicles of the same Rpecies
but noted that secLsona1 differences in the stat(> of the vesicle occur.
Nolfi (Eoc. cit.) found that uric acid content of the vesicle in Molyula
manhuttensis varies from 3 to 75% of the dry weight in the concretion.
Nolfi quotes Bwhner (1953) as having reported the presence of fungi
in the renal vesicle of M. manhntterLai8 which he considered engulfed
and digestcd the concretions in R it umoebocytic fashion. Kolfi suggests
that if some of the metabolic intermediates “ leaked ” back to the
ascidian host thiN rould be a miitualistic relationship. It is difficult to
see how such a rchtionship could function and how the fungus could
initially invtide the closed coelomic vesicle. My own histological
4 \I H -12
4