Biochem. J. (1997) 322, 213±221 (Printed in Great Britain)
Biosynthesis of ‘ essential ’ amino acids by scleractinian corals
Lisa M. FITZGERALD* and Alina M.
SZMANT‹Rosenstiel School of Marine and
Atmospheric Science, University of Miami, 4600 Rickenbacker Cswy., Miami, FL
33149, U.S.A.
DOI: 10.1042/bj3220213
Animals rely
on their diet for amino acids that they are incapable either of synthesizing or
of synthesizing in sufficient quantities to meet metabolic needs. These are the
so-called ` essential amino acids'. This set of amino acids is similar among
the vertebrates and many of the invertebrates. Previously, no information was available
for amino acid synthesis by the most primitive invertebrates, the Cnidaria. The
purpose of this study was to examine amino acid synthesis by representative
cnidarians within the Order Scleractinia. Three species of zooxanthellate reef
coral, Montastraea faŠeolata, Acropora cerŠicornis and Porites diŠaricata, and
two species of non-zooxanthellate coral, Tubastrea coccinea and Astrangia
poculata, were incubated with "%C-labelled glucose or with the "%C-labelled
amino acids glutamic acid, lysine or valine. Radiolabel tracer was followed
into protein amino acids.A total of 17 amino acids, including
hydroxyproline, were distinguishable by the techniques used. Of these, only threonine
was not found radiolabelled in any of the samples. We could not detect
tryptophan or cysteine, nor distinguish between the amino acid pairs glutamic
acid and glutamine, or aspartic acid and asparagine. Eight amino acids normally
considered essential for animals were made by the five corals tested, although some
of them were made only in small quantities. These eight amino acids are valine,
isoleucine, leucine, tyrosine, phenylalanine histidine, methionine and lysine.
The ability of cnidarians to synthesize these amino acids could be yet another
indicator of a separate evolutionary history of the cnidarians from the rest of
the Metazoa.
INTRODUCTION
Protein amino
acids are fundamental components of all life forms. Some extant groups are able
to synthesize all of the 20 protein amino acids (plants, and many fungi,
bacteria and Protista ; Table 1). All animals studied so far, however, lack a number
of amino acid synthetic pathways, or the rates of synthesis of these amino
acids are insufficient to meet metabolic needs. These amino acids, termed
essential, must be obtained by animals from their environment (e.g. food). All
vertebrates have the same nine essential amino acids (threonine, valine,
methionine, leucine, isoleucine, phenylalanine, histidine, lysine and tryptophan), and some have additional
essential amino acids(Table 1). More variability exists within the
invertebrates : some (but not all) representatives of the Crustacea, Mollusca
and Nematoda have been attributed with the ability to synthesize one or more of
the vertebrate essential amino acids (Table 1). One phylum for which
capabilities for amino acid synthesis had not yet been studied was the
Cnidaria. The purpose of this study was therefore to examine amino acid
synthesis within the order Scleractinia. Overall, available information on
amino acid synthesis shows that the pattern of essential compared with
non-essential amino acids is similar among most Metazoa. Possible explanations
for this similarity are: (1) animals evolved from early protozoan groups that
had already lost or never acquired all the amino acid synthetic pathways; (2)
the loss of these pathways occurred early in the evolution of the Metazoa; or
(3) certain amino acid synthetic pathways may be more prone to loss than
others. In general, essential amino acids are those with long, complex synthetic
pathways. Random deleterious mutations may be more likely to occur in pathways
with many steps than in those with few steps. The cnidarians are among the most
primitive extant metazoans in most phylogenetic schema (see, for example,
[1±3]). In this regard, their pattern of amino acid synthetic abilities might
be indicative of their divergence from other metazoan groups relative to the
loss of amino acid synthetic pathways. Glucose and amino acids can be
precursors for amino acid synthesis. Glucose is rapidly absorbed and used in
many metabolic pathways. Hence carbon atoms derived from glucose have a high
probability of being incorporated into all newly synthesized amino acids.
Glutamic acid is the direct precursor of several other amino acids via the
tricarboxylic acid cycle, from where, like glucose, its carbon atoms have a
high probability of being incorporated into newly synthesized amino acids.
Lysine and valine, on the other hand, are not direct precursors of other amino
acids. Although carbon atoms derived from these amino acids can feed into the
tricarboxylic acid cycle, their degradation pathways are long and complex. The
approach used in this study was to incubate corals with "%C-radiolabelled
d-glucose or one of three "%C-radiolabelled amino acids (glutamic acid,
lysine and valine), and follow the radiolabel into coral protein amino acids.
Five species of scleractinian coral were used: three that have dino¯agellate symbionts
(known as zooxanthellae) living within their gastrodermal cells, and two that
do not. The zooxanthellate species were Montastraea faŠeolata (¯M. annularis
[4]), AcroporacerŠicornis and Porites porites ; the two non-zooxanthellate species
were Tubastrea coccinea and Astrangia poculata (¯A. danae [5]).
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