AMINO ACIDS IN THE MOLLUSCA

AM. ZOOLOCIST, 1:253-261 (1961). AM1NO ACIDS IN THE MOLLUSCA KENNETH ALLEN Dept. of Zoology, University of California, Los Angeles AMINO ACIDS IN MOLLUSCAN TISSUE The identification of "free" amino acids (i.e., in contrast to those bound in protein) is not a new phenomenon. Chemical isolation and quantification of several amino compounds was accomplished in the 19th century. One of the amino acids isolated from molluscan tissue was taurine, a sulfonic amino acid. Not only was this compound found in a number of molluscan species, but it was also found to be present in high concentration. The first report of this amino acid is recorded in a paper by Mendel and Bradley (1906). "The occurrence of taurin in the muscle of invertebrates has been known since 1845, when Karsten isolated a crystalline body from the watery extracts of certain molluscs, and identified it qualitatively with the taurin of ox bile." Later workers reported taurine from all of the major classes of the Mollusca. Krukenberg et al. (1885), Henze (1905, 1913, 1914), Suzuki et al. (1909), Okuda (1920) and Ackerman et al. (1924) found large quantities of taurine in cephalopods. Kelley (1904) extracted glycine and taurine from several species of invertebrates. Mytilus edulis, one of the species examined, was found to contain 1.6% taurine based on dry weight of tissue. Mendel (1904), Mendel and Bradley (1906), and Schmidt (1917) have likewise reported taurine from gastropods. It is of interest that all of the above investigators used marine species in their studies. Other componds, such as glycine and arginine, were also isolated by chemical means. Chittenden (1875) obtained pure glycine from the muscle of Pecten irradians. Ackerman (1922) reported arginine from Mytilus edulis. In the same year Ackerman et al. isolated adenine and arginine from the muscle of Eledone moschata, a cephalopod. Arnold and Luck in 1932 examined several species of vertebrates and invertebrates for arginine and arginine phosphate. High concentrations of both of these compounds were found in the tissues of the cephalopod Paroctopus apollyn. This latter study followed the work of Kutscher and Ackerman (1926) which indicated that arginine and arginine phosphate are present only in invertebrates. Creatine and creatinine phosphate were isolated from vertebrates and echinoderms. Reviews on this subject have been given by Baldwin (1933) and more recently by Ennor and Morrison (1958). No further discussion of the phosphagens will be given here, except to point out that the distribution of creatine phosphate in the animal kingdom is not nearly as limited as the earler workers suggested. The advent of microbiological assay and paper chromatography made possible more complete analyses of amino acids in animal tissue. Noland (1949) determined by mi- (253) The subject first suggested for this review was "Nitrogen Metabolism in the Mollusca." After examining the literature it seemed more appropriate to consider one of the basic groups of compounds in nitrogen metabolism: namely, the amino acids. There were several reasons for this. One, investigations dealing with the amino acids in Mollusca have not been reviewed, and these studies date back to 1845. Two, the role of amino acids as osmoregulators has been suggested for some time, and recent work on molluscs strongly supports this idea. Three, many areas of nitrogen metabolism in the Mollusca have not been studied. A considerable amount of work has been done on nitrogen catabolism, but even here the major problem of the biosynthesis of uric acid is yet unsolved. This paper therefore will discuss the work dealing with amino acid constituents of molluscan tissue, physiological studies involving amino acids, and the enzyme systems which have been studied that are pertinent to amino acid metabolism in the Mollusca. 254 KENNETH ALLEN Simpson et al. (1959) by the use of paper chromatography examined the amino acid content of numerous aquatic invertebrates. Their results on the molluscan species were interesting. The sulfonic amino acid taurine, which was found in high concentra- TABLE 1. Taurine content in various molluscs Species Gastropoda Lymnaea palustris Afarisa cornuarietis Pomacea bridgesi Rumina decollata Otala lactea Mesodon thyroidus Bulimulus alternates Murex fulvescens Littorina irrorala Oliva sayatia Polinices duplicala Buscycon perversum Siphonaria lineolata Fasciolaria distans Thais haemasloma huysae Pelecypoda Anadonta grandis Quadrula quadrula Lampsilis sp. Elliptio sp. Rangia cuneata Brachiodontes recurvus Crassostrea virginica Donax variabilis Venus mercenaria Dosinia discus Area incongrua Area campechiensis Noetia ponderosa Cephalopoda Loliguncula brevis Environment Taurine fresh water fresh water fresh water terrestrial terrestrial terrestrial terrestrial marine marine marine marine marine marine marine marine — — — — — — — -f-f+ + + + + -j- fresh water fresh water fresh water fresh water brackish-fresh water brackish-marine brackish-marine marine marine marine marine marine marine •— — •— -— — marine -f-j+ + -f-f -f" -j-f- tion by the earlier investigators, was detectable only in marine forms. This compound was not detectable by paper chromatography in the fresh water and terrestrial species. Their results are shown in Table 1. In this same study a /}-amino acid, /?alanine, was found in four species of pelecypods and in the cephalopod Loliguncula brevis. In the same year Awapara and Allen isolated /3-aminoisobutyric acid from Mytilus edulis and Volsella dimissus. These /J-amino acids are the products of thymine and uracil catabolism in mammalian tissue and thus may be of particular interest for future studies in pyrimidine metabolism in molluscs. Since taurine was detectable only in the marine molluscs, Allen and Awapara (1960) compared the metabolism of sulfur amino acids in two species of pelecypods. The) crobiological assay some of the amino acids in the pelycypod Mactra solidissima, the gastropod Buscycon canaliculntum, and the cephalopod Loligo peali. Relatively high values for tryptophane, tyrosine, and histidine were recorded for L. peali, while only traces of these compounds were found in the other two species. Duchateau and Florkin (1954), by the same method, determined values for fifteen amino acids in the foot of the marine gastropod Buccinum undatum. Alanine, arginine, glycine, and proline were the amino acids present in highest concentrations. Ramamoorthi (1958) investigated the amino acid composition of developing snail eggs. The eggs of Pila virens were extracted during various stages in development and amino acids were quantitated. In the one-cell stage only faint traces of alanine, threonine, glutamic acid, aspartic acid, serine, and glycine could be detected. In the two-cell stage hydroxyproline appeared, but disappeared in the tirree-cell stage. As development continued, the concentration of the amino acids increased. In the larval stage (...truncated)