Naturwissenschaften 79, 361 - 365 (1992) © Springer-Verlag 1992

Abiotic Synthesis of Amino Acids under Hydrothermal Conditions and the Origin of Life: a Perpetual Phenomenon ? R. J.-C. Hennet 812 Carter Road, Rockville, MD 20852, USA N. G. Holm Department of Geology and Geochemistry, Stockholm University, S-106 91 Stockholm, Sweden M. H. Engel Department of Geology, University of Oklahoma, Norman, OK 73019, USA

Today, subaqueous hydrothermal activity is a very dynamic process. For example, the entire volume of water in the ocean is circulated in approximately 10 million years through the more than 70000 km length of global ridge systems scarring the bottom of the Earth's oceans [1]. Hydrothermal circulation also takes place off-ridges in the oceanic crust and the covering sediments [2, 3]. Early in the Earth's history (4 × 10 9 years ago or so) hydrothermal activity was even more intensive, mostly because of the lack of thick continental masses and higher heat fluxes cooling the planet. As soon as water was able to condense at the surface of the Earth, all processes associated with subaqueous hydrothermal activity began, and probably continued uninterrupted until the present. The physical and chemical conditions within submarine hydrothermal systems can vary greatly. Aqueous hydrothermal fluid temperature can range from cold (temperature of bottom seawater) in the recharge areas, to hot (e.g., 6 5 0 - 7 0 0 ° C , closest approach of aqueous fluid to magma [2]) closer to magmatic heat sources, deeper in the Earth's crust. Redox conditions (E h, which are of extreme importance in determining the direction of chemical reactions and the stability of chemical species, also vary, from relatively oxidizing in the hotter zones to more reducing in the colder zones of the system. For the redox couple CO2/CH4, the conditions are oxidizing (stability field of CO2) at higher temperatures (above 300 or 400 °C), and re-

ducing (stability field of CH4) at lower temperatures in a typical oceanic crust environment [4]. Many other parameters show variations, for example, mineral assemblages, chemical species activities, and pH, to name only a few [2]. Hydrothermal conditions are ubiquitous in the Earth's crust. Direct, observable expression of hydrothermal activity at the seafloor is in the form of hot aqueous fluids (at temperatures of up to 400°C or more) emanating directly from vents or seeps [5, 6]. However, hydrothermal fluids at the temperature conditions measured in vents and seeps represent only a very small portion of the entire mass of aqueous fluid perpetually circulating in the oceanic crust. An estimated 96 °7o of the mass of hydrothermal water circulating in the oceanic crust is at a temperature of around 150°C [3, 7, 8]. The E h of aqueous fluids circulating in the hydrothermally altered oceanic crust is reducing, and is generally controlled by the mineral assemblage pyrite/pyrrhotite/magnetite [9, 10]. The pH is close to neutral around 150°C, becoming more acidic as temperature increases towards 250°C, and then less acidic again at higher temperatures [t 1]. Chemical reactions take place in response to changes in chemical and physical conditions. For example, at 150°C, dissolved CO 2 introduced in a system consisting of an aqueous phase in contact with the pyrite/pyrrhotite/ magnetite mineral assemblage should be reduced to C H 4 a s the system responds to reach equilibrium [4]. However, stable chemical equilibrium be-

Naturwissenschaften 79 (1992) © Springer-Verlag 1992

and CO 2 is rarely achieved in a dynamic system where sharp chemical and physical gradients exist. Therefore, in such environments metastable equilibria predominate, implying the existence of many more chemical species than predicted by stable equilibrium calculations. For example, the reduction of CO2 to CH 4 is kinetically slow at low temperatures (100's or 1000's of years at temperatures of less than 500 or 600°C) and numerous metastable equilibria allow for a wide variety of chemical species to coexist [4]. Among them could be formaldehyde, carboxylic acids, and several amino acids (assuming that nitrogen compounds are present, note that metastable equilibria also apply to the reduction of N 2 to NH3). For this reason, among others, scientists have proposed that submarine hydrothermal systems are likely environments for the synthesis of amino acids on the prebiotic Earth [12]. Amino acids are the funamental building blocks of proteins that are required for the initiation of living systems. A controversy exists as to whether or not life could have originated under hydrothermal conditions. On the one hand, the high temperatures encountered are regarded as detrimental for the persistence of the necessary precursors for the origin of life (amino acids) [13]. On the other hand, such environments are regarded as adequate because of the sharp physical and chemical gradients encountered, the potential mineral catalysts present, the absence of destructive UV light from the sun, and the dynamic input of chemicals from the circulation of ocean water and from magmatic sources [12]. This study consisted of testing experimentally the hypothesis that amino acids can be synthesized under conditions representing hydrothermally altered oceanic crust (Eh, pH, temperature), in order to provide some experimental data to the on-going debate. The approach differs from previous work in that it consisted of investigating the formation of amino acids via abiotic synthesis under controlled conditions rather than studying synthesis or degradation reactions under uncontrolled conditions [13 - 17], or their theoretical thermodynamic disequilibrium [4]. The syntheses were carried out under controlled Eh, pH, and temperature to introduce the possibility of comparison tween CH 4


between the experimental setup and similar natural environments on Earth. The basic assumption of this work was that if amino acids could have been synthesized within the oceanic floor of the prebiotic Earth, the process should readily by going on today, and should occur under laboratory conditions at a measurable rate. The goal of the experiment was to realize the abiotic synthesis of amino acids under simulated hydrothermal conditions (reducing Eh, pH close to neutral, and 150°C), by reacting starting materials (formaldehyde, ammonia, and cyanide) in water, in the presence of mineral surfaces. Whether abiotic formaldehyde, ammonia, and cyanide are (or were in the distant past) available for reaction in hydrothermal fluids is not known at present and subject to debate. Hydrogen cyanide has been reported in volcanic emissions [18] and has been proposed to be a possible component of hydrothermal solutions [18, 19]; ammonia has been reported in hydrothermal fluids emanating at 30 °C at the Loihi Seamount, Hawaii [20, 21] and in hydrothermal fluids sampled at 185 °C from the Kolbeinsey Ridge hydrothermal system, Iceland [22]; and formaldehyde has been reported in hydrothermal samples from the Puertecitos hydrothermal system, Mexico (Ingmanson, pets. comm., 1991). However, no conclusive data are presently available to evaluate and quantify the presence of these chemicals in hydrothermal fluids, and to the knowledge of the authors no information exists on the presence of specifically abiotic formaldehyde, ammonia, and cyanide in hydrothermal systems. Despite the lack of data and general agreement among scientists, the assumption was made that the selected starting materials existed in the prebiotic Earth's crust and were available for chemical reaction at 150 °C. The reactant concentrations used in this work (see Table 1) are not to be interpreted as representative of natural hydrothermal fluids; the experiment focused only on simulating hydrothermal conditions (pH, E h, temperature) and on studying the possibility of abiotic synthesis of amino acids to occur under such conditions. The experimental conditions are listed in Table 1. The mineral phase consisted of titanium oxide (autoclave liners), 362

Table 1. Experimental conditions Temperature 150°C; pressure 10 atm. ; Volume (aq.) 700 ml; volume (tot.) 1000 ml Gas phase (Experiments A and B): CO2and H 2 (3 : 1 mixture) Aqueous phase (Experiments A and B): KCN (0.19M); NH4C1(0.23 M); HCHO (0.18 M, with 10- 15 °70 methanol) HC1 (conc., approx. 0.08 M) in Experiment A only: NariS (0.05 34) Mineral phase: Experiment A: pyrite + pyrrhotite + magnetite (1 : 1: 1, 10 g, coarse powder) Experiment B: illite (10 g, number 35 Illinois) in Experiments A and B: platinum powder (approx. 0.1 mg, fine powder) TiO2(autoclave liner material)

and pyrite/pyrrhotite/magnetite assemblage in Experiment A, and illite in Experiment B. For both cases A and B, a small amount (approximately 0.1 rag) of fine platinum powder was added to act as a sink for any excess oxygen possibly left in the system. Prior to introducing the aqueous solution containing the dissolved chemicals listed in Table 1, the autoclaves were sealed with the mineral phases inside. The sealed autoclaves were heated overnight to 240°C and evacuated to remove atmospheric air. A 2-1 HPLC-grade aqueous solution was prepared by first adding KCN (0.19 M) and NH4C1 (0.23 M), and 25 ml concentrated HC1 to bring the pH of the solution to approximately 8.8. The aqueous solution was then treated by bubbling with CO2; this had the effect to lower the pH of the solution to approximately 7. Formaldehyde (0.18 M) was then introduced as the last reactant just prior to aspirating 700 ml of solution into each of the autoclaves. N a r i s (0.05 M) was added to the solution of Experiment A before the addition of formaldehyde. The gas phase added to the autoclaves was a mixture of CO 2 and H2 (3:1 mixture). The total pressure in each autoclave at 150 °C was approximately 10 atm; the volume of the gas phase was 300 ml at the beginning of the experiments. The first sample was withdrawn at room

temperature and the autoclaves were heated to 150 °C. A background contamination run was obtained for the mineral phases by subjecting a sample of illite to 240°C (dry, under nitrogen atmosphere) for 24 h followed by heating in distilled water at 150°C for 24 h, prior to analysis for trace amino acid content. The experiments were carried out in 1-1 volume autoclaves allowing periodic sampling of the aqueous phase and the measurement of temperature and pressure. Aqueous samples (40 ml) were taken, at time 0 (room temperature), and after 7, 25, 31, 48, and 54 h at 150°C and kept on dry ice. A portion of each sample solution was evaporated to dryness, then hydrolyzed (6 N HC1, 24 h, 100°C) and prepared for amino acid analysis as previously reported [23]. Amino acid distributions and abundances were determined by highperformance liquid chromatography (HPLC) and amino acid identification was further confirmed by gas chromatography (GC); amino acid D/L values were also determined by GC [23]. The results are presented in Table 2 and representative gas chromatograms are presented in Fig. 1. The variety of amino acids that were detected and their relative abundance are roughly similar to that previously reported for electric spark discharge experiments [14] as well as from previous pioneer work which did not necessarily involve electrical discharges for synthesizing amino acids [24-26]. Remarkably, the relative yields of the amino acids detected, normalized to glycine (see Table 2), are significantly higher than in electric spark discharge experiments, and the overall yields (see Table 2) are at least one order of magnitude higher than in previously published results (e.g. [14]). Ratios of optical isomers (D/L) were measured by GC on a chiral column [23] on the three most abundant protein amino acids (alanine, aspartic acid, and glutamic acid). These protein amino acids were r a c e m i c (D/L = 1) in all samples (see Fig. 1 a, b). The apparent similarity in the distribution and abundance of amino acids in Experiments A and B might indicate that the mineral phases either provided similar catalytic effects or, alternatively, were not a direct, influencing factor in the syntheses. The experimental design did not permit a

Naturwissenschaften 79 (1992) © Springer-Verlag 1992

distinction between the different potential catalysts and further tests with individual minerals need to be conducted for this purpose. In addition to the amino acids listed in Table 2, traces of lysine, histidine, and /~-alanine were tentatively identified in Experiment A, and traces of phenylalanine, histidine, and /3-alanine were tentatively identified in Experiment B. Notice that the presence of glycine in the two samples taken at room temperature (t = 0, Table 2) is explained by the rapid formation of glycine (or aminonitrile) as an expected product of reaction at r o o m temperature (e.g. [14]), and is not related to biological contamination. The results for the mineral phase background test indicate that the m a x i m u m amount of amino acids possibly introduced as biological contamination in the autoclave with the minerals was small and usually at least one order o f magnitude lower than that detected in any of the samples analyzed (Table 3). Importantly, the amino acids extracted from the thermally treated mineral phase (exposure at 240°C for 24 h under nitrogen atmosphere followed by 24 h at 150°C in distilled water) were not racemic when analyzed (see Fig. 1 c and Table 3). The fact that the amino acids detected in the experiments were found to be completely racemic completes the demonstration that the amino acids produced during the experiments were synthesized abiotically and that biological contamination does not explain the observations. The exact synthetic pathways which led to the formation of the observed amino acids cannot be resolved based on the data collected so far. Further research





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Naturwissenschaften 79 (1992)

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Fig. 1. Gas chromatograms of the N,O-TFA-isopropyl esters of Dand L-amino acids. Replicate analyses were performed using a Hewlett Packard 5890 A gas chromatograph equipped with a 50 m × 0.25 mm i.d. fused silica capillary column coated with an optically active stationary phase (Chirasil-Val III), a nitrogen-phosphorous detector (NPD), and a Hewlett Packard integrator. Chromatograms are identified as follows: a) amino acids isolated from the acid hydrolysate of Experiment B (t = 54 h, 150 °C); b) amino isolated from the acid hydrolysate of Experiment A (t = 54 h, 150 °C); c) amino acids isolated from the pure illite used in Experiment B (treated as explained in the text). Note that the sensitivity for this chromatogram is approximately 2000 times higher than for the other cases in the figure; d) an illustrative standard chromatogram of racemic amino acids. It should be noted that the amino acids that could be isolated from the illite were not racemized (primarily L) in contrast to the racemic amino acids formed in Experiments A and B 363

Table 2: Analytical results: identified amino acids, measured concentrations (mM), and concentrations normalyzed to GLY (norm.). Present: present at relatively high concentrations but not quantified; t: time in hours t = 0 (200C) t = 7 (150°C) t = 25 (150°C) t = 31 (150°C) t = 48 (150°C) t = 54 (150°C) mM norm. mM norm. mM norm. mM norm. rnM norm. mM norm. ExperimentA (pyrite, pyrrhotite, magnetite,titanium oxide, platinum) ASP 0.187 0.0195 0.291 0.0310 0.355 0.0371 0.281 0.0303 0.362 0.0311 SER 0.025 0.0026 0.027 0.0029 0.011 0.0011 0.026 0.0028 0.007 0.0006 GLU 0.034 0.0036 0.064 0.0068 0.071 0.0074 0.081 0.0087 0.100 0.0086 GLY 4.029 1.0 9.571 1.0 9.400 1.0 9.574 1.0 9.275 1.0 11.630 1.0 ALA Trace Trace 0.545 0.0580 0.794 0.0829 0.847 0.0913 1.338 0.1150 CYS Present Trace Trace Trace Present Trace Trace Trace MET Trace ILE 0.022 0.0023 0.026 0.0028 0.050 0.0052 0.058 0.0050 ExperimentB (illite,titaniumoxide, platinum) ASP 0.142 0.0088 0.856 0.0426 0.523 0.0280 0.519 0.0256 0.367 0.0192 SER 0.003 0.0011 0.212 0.0131 0.229 0.0114 0.119 0.0064 0.125 0.0062 0.095 0.0050 GLU 0.002 0.0007 0.015 0.0009 0.195 0.0097 0.139 0.0074 0.183 0.0090 0.107 0.0056 GLY 2.806 1.0 16.146 1.0 20.073 1.0 18.677 1.0 20.252 1.0 19.032 1.0 ALA 0.515 0.0257 0.552 0.0296 0.592 0.0292 0.725 0.0380 CYS Present Present Present Present Present MET Trace Trace Trace ILE 0.006 0.0004 0.107 0.0053 0.067 0.0036 0.061 0.0030 0.157 0.0082

Table 3. Amino acid concentrations (mM) in illite (maximum concentration of amino acid which could have been liberated from illite to the water during Experiment B). Amino acids that could be detected in the sample were primarily L. D enantiomers, if present at all, were below detection limits (see Fig. 1 c) ASP SER GLU GLY ALA CYS MET ILE

0.022 0.059 0.004 0.057 0.024 Not detected Not detected 0.005

and additional experimental tests and analyses will have to be conducted. For example, it would be important to assess the possibility that some of the amino acids observed might have been formed as result of the hydrolysis and polymerization of hydrogen cyanide (e.g. [27]), even though the experimental conditions (neutral p H and high temperature) were quite different from the ones under which polymerization of HCN has been reported to take place [27], and this mechanism alone probably cannot explain the entirety of the amino acids synthesized. It is also probable that some of the amino acids observed started to be synthesized before a temperature of 150°C was 364

reached in the autoclaves, by reaction of glycine with formaldehyde, for example (e.g. [28]). This can be expected and does not diminish the significance of the observation that various amino acid syntheses can take place and be sustained under severe conditions (neutral pH, reducing Eh, 150°C). These severe conditions are similar to the conditions found in a vast portion of the hydrothermal systems in the Earth's crust. The synthesis of amino acids under the reducing conditions and temperature imposed in these two experiments is intriguing in light of previous theoretical predictions concerning organic synthesis in and around hydrothermal systems [4]. What remains to be assessed experimentally is the relationship between rates of amino acid production and rates of decomposition under the experimental conditions employed, the effects of temperature on these rates, and to determine whether other organic compounds that are essential for life can be formed under such conditions. The high yields obtained during both experiments indicate that the conditions (reducing, neutral pH, 150°C, and possibly mineral surfaces) are efficiently promoting the formation of amino acids at rates apparently higher than their decomposition. Further experimental work is required to assess more accurately the optimum condi-

tions for the formation of each individual amino acid and the catalytic effects of different mineral surfaces. In summary, the results of this study support the possibility that submarine hydrothermal systems (more generally, any subaqueous system under hydrothermal conditions) are environments in which life's building blocks could have first appeared in the distant past. The results also imply that if amino acids were synthesized in hydrothermal systems of the primordial Earth, they are likely still being produced under similar conditions. Thus, the concept of the origin of life may not be associated with a one time distant event but rather an on-going perpetual p h e n o m e n o n which might be taking place wherever subaqueous hydrothermal activity exists. This work is part of the activities of WG91 of the Scientific Committee on Oceanic Research. It was made possible by grant G-Gu3865 from the Swedish Natural Science Research Council, support from S. S. Papadopulos and Associates, Inc., and help and moral support from D. Crerar, M. Borcsik, A. Henry and B. McPhail (Princeton University). M. H. Engel acknowledges Y. Qian for his assistance with the amino acid analyses.

Received November 19, 1991 and April 9, 1992

1. Edmond, J. M., v. Damm, K. L., McDuff, R. E., Measures. C. I. : Nature 297, 187 (1982) 2. Rona, P. A., Bostrom, K., Laubier, L., Smith, Jr, K. L.: Hydrothermal Processes at Seafloor Spreading Centers. New York: Plenum 1983 3. Joint Oceanographic Institutions for Deep Earth Sampling/European Science Foundation: Report of the Second Conference on Scientific Ocean Drilling, Washington D.C./Strasbourg 1987 4. Shock, E. L.: Origins Life Evol. Biosph. 20, 331 (1990) 5. Spiess, F. N., Macdonald, K. C., Atwater, T., Ballard, R., Carranza, A., Cordoba, D., Cox, C., Diaz Garcia, V. M., Francheteau, J., Guerrero, J., Hawkins, J., Haymon, R., Hessler, R., Juteau, T., Kastner, M., Larson, R., Luyendyk, B., Macdougall, J. D., Miller, S., Normark, W., Orcutt, J., Rangin, C. : Science 207, 1421 (1980)

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6. Fouquet, Y., v. Stackelberg, U., Charlou, J. L., Donval, J. P., Erzinger, J., Foucher, J. P., Herzig, P., Muhe, R., Soakai, S., Wiedicke, M., Whitechurch, H.: Nature 349, 778 (1991) 7. Fehn, U. : Econ. Geol. 81, 1396 (1986) 8. Fehn, U., Cathles, L. M.: Tectonophysics 125, 289 (1986) 9. Alt, J. C., Honnorez, J., Laverne, C., Emmermann, R.: J. Geophys. Res. 91, 10309 (1986) 10. Alt, J. C., Anderson, T. F., Bonnell, L.: Geochim. Cosmochim. Acta 53, 1011 (1989) 11. Grichuk, D. V., Borisov, M. V., Mel'nikova, G. L. : Int. Geol. Rev. 1985, 1097 12. Corliss, J. B., Baross, J. A., Hoffman, S. E. : Oceanol. Acta SP 4, 59 (1981)

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13. Miller, S. L., Bada, J. L.: Nature 334, 609 (1988) 14. Miller, S. L., Orgel, L. E.: The Origins of Life on Earth. Englewood Cliffs, N. J. : Prentice-Hall 1974 15. Ivanov, Ch. P., Slavcheva, N. N..' Origins Life 8, 13 (1977) 16. White, R. H. : Nature 310, 430 (1984) 17. Bernhardt, G., Ludemann, H-D., Jaenicke, R., Konig, H., Stetter, K. O.: Naturwissenschaften 71, 583 (1984) 18. Mukhin, L. M.: Nature251, 50 (1974) 19. Ferris, J. P. : Origins Life Evol. Biosph. (in press) 20. Karl, D. M., McMurtry, G. M., Malahoff, A., Garcia, M. O.: Nature 335, 532 (1988) 21. Karl, D. M., Brittain, A. M., Tilbrook, B. D. : Deep-Sea Res. 36, 1655 (1989)

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Neural Inducing Factors in Neuroblastoma and Retinoblastoma Cell Lines Extraction with Acid Ethanol

G. V. Lopashov Institute o f Gene Biology, Russian A c a d e m y o f Sciences, Moscow 117334, Russia H. Selter, M. M o n t e n a r h and W. Kn0chel Abteilung Biochemie der Universit~t, W-7900 Ulm, F R G H. Grunz Abteilung Zoophysiologie der Universit~it G H S , W-4300 Essen, F R G H. Tiedemann and H. Tiedemann Institut for Biochemie und Molekularbiologie der Freien Universit~it, W-1000 Berlin, F R G

Factors which induce neural or mesodermal and endodermal tissues have been isolated from different sources. One family of mesoderm-inducing factors has been identified as activins or closely related homologues [ 1 - 4 ] . The factors are widely distributed in embryos, transformed cell lines, and in juvenile, and to some extent, also in adult tissues [5]. We have tested whether neural tissue and cells of neural origin can also induce mesodermal tissues or whether mesoderm-inducing factors are confined to tissues of mesodermal and endodermal origin. Cells of two neuNaturwissenschaften 79 (1992)

roblastoma and one retinoblastoma cell line were tested on ectoderm of amphibian gastrulae by the implantation method [6]. The cells o f all three cell lines and of retina induce foreheads with telencephalon, diencephalon, and eyes, some with lenses (archencephalic inductions, Fig. 1) which arise by secondary interactions, in which secondary factors are involved. Brain induces foreheads, hindheads with rhombencephalon and ear vesicles, and even small trunks and tails (Table 1). The mesoderm-inducing capacity in the brain originates p r o b a b l y from the

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22. Olafsson, J., Honjo, S., Thors, K., Stefansson, U., Jones, R. R., Ballard, R. D., in: Oceanography 1988 (A. Ayala-Castanares, W. Wooster, A. Yanez-Aranciba, eds.). Mexico, D. F.: Univ. Nacional de Mexico 1989 23. Silfer, J. A., Engel, M. H., Macko, S. A.: Appl. Geochem. 5, 159 (1990) 24. Oro, J., Kamat, S. S. : Nature 190, 442 (1961) 25. Oro, J., in: The Origins of Prebiological Systems and of their Molecular Matrices, p. 187 (S. W. Fox, ed.). New York: Academic Press 1965 26. Matthews, C. M., Nelson, J., Varma, P., Minard, R. : Science 198, 622 (1977) 27. Ferris, J. P., Hagan, W. J.: Tetrahedron 40, 1093 (1984) 28. Kamaluddin, Yanagawa, H., Egami, F.: J. Biochem. 85, 1503 (1979)

blood vessels and b l o o d elements in the brain. It is, on the other hand, not likely that long-term cultivation o f cells leads to a loss o f formerly existent mesoderm-inducing activity. Cell lines from mesoderm-inducing tissues did not lose their inducing activity [8, 9]. The conditioned media of the neuroblastoma and retinoblastoma cell lines were tested on isolated ectoderm from Triturus alpestris gastrulae [6] to show whether inducing factors were secreted by the cells. To this end the cells were cultivated without serum. Bovine serum albumin ( 0 . 1 % final concentration) was then added to the m e d i u m to prevent adsorption of factors to glass or plastic surfaces, the medium was concentrated about sevenfold by ultrafiltration, acidified to p H 2.5 to activate masked factors (1 h at 0°C), then neutralized to p H 7.4 and adjusted to the isotonicity o f frog Ringer solution. The conditioned medium of the neuroblastoma cell line SK-N-MC induces neural tissue in 3 1 % o f the cases. The neural inducing activity of the other two cell lines is lower ( 5 - 1 3 %). Tapetum was induced once. Other tissues were not induced. Conditioned media with fetal calf serum have no inducing activity because of the high dilution o f the factor by serum proteins. It has been shown that living tissues, in contrast to homogenized cells, can induce single tissues similar to the inducing tissue, i.e., adult lens epithelium in365

Abiotic synthesis of amino acids under hydrothermal conditions and the origin of life: a perpetual phenomenon?

Naturwissenschaften 79, 361 - 365 (1992) © Springer-Verlag 1992 Abiotic Synthesis of Amino Acids under Hydrothermal Conditions and the Origin of Life...
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