THE ROLE OF DEUTERIUM IN MOLECULAR EVOLUTION
Oleg V. Mosin1
1 Department of Biotechnology M. V. Lomonosov State Academy of Fine Chemical Technology Vernadskogo Prospekt 86 117571 Moscow Russia
The role of deuterium in molecular evolution is most interesting question of nowdays science comprises two points mainly: the evolution of deuterium itself as well as the chemical processes going with participation of deuterium. It is believed the big bang produce the universe that was much denser and hotter than it is now and made almost entirely of two main elements - hydrogen and helium. Deuterium itself was made only at a second stage of the beginning of the universe namely through the collision of one neutron with one proton at a temperature of about one billion degrees; furthemore the two formed deuterons in turn stuck together into helium nuclei which contain two protons and two neutrons. It is considered that during the formation of helium nuclei almost all the deuterons combined to form helium nuclei leaving a tiny remant to be detected today so that only one in 10.000 deuterons remained unpaired.
Thus deuterium serves as a particularly important marker. The quantity of deuterium in contemporary nature is approximately small and measured as no more than 0.015% (from the whole number of hydrogen atoms) and depends strongly on both the uniformity of substance and the total amount of matter formed in course of early evolution. One may suggest that the very reliable source of producing of deuterium theoretically may to be the numerical explosions of nova stars but deuterium itself is very readily destroyed in those stars. If it was so perhaps this was the answer to the question why the quantity of deuterium increased slitely during the global changes of climate for worming conditions.
The second point is the chemical processing of deuterium as a result of this the 2H2O on the first hand may be formed from gaseous deuterium and atomic oxyden at very high temperature. Pretty interesting with chemical point of view seems our own idea proposed recently about the possible small enrichment of primodial environment with 2H2O. We supposed that this fact if really existed may be conditioned by a powerful electrical discharges taken place in premodial atmosphere laking the natural shield of ozone and may be resulting in electrolysis processes of H2O e.g. those ones are now used for the enrichment of 2H2O. But the realization of this process with practical point of view seems unlikely. Nevertheless if such process has really occured the some hydrophobic effects of 2H2O as well as chemical isotopic effects should be taken into account while discussing the chemico-physical properties of primodial environment. Perhaps it is also a big practical interest to study the properties of fully deuterated membraine structures composed for example from fully deuterated lipids and proteins. Either way or not the model of deuterium evolution provides a framework for predicting the biochemical consequences of such new fascinating ideas.
Deuterium (2H) the hydrogen isotope with nuclear mass 2 was discovered by Urey. In the years immediately following this discovery there developed a keen interest in development of methods for uniform biological enrichment of a cell with 2H that may be best achived via growing of an organism on medium with high content of 2H2O (99% 2H) which since yet resulted in a miscellany of rather confusing data (see as an example Katz J. Crespy H. L. 1972).
The main resolute conclusion that can be derived from the most competent and comprehensive of the early studies is that high concentrationsof 2H2O are incompatible with life and reproduction and furthemore could even causing even lethal effects on a cell. However today a many cells could be adapted to 2H2O either via employing a special methods of adaptation which of them we shall describe above or using selected (or/and resistent to 2H2O) strains of bacterial and other origin.
In this connection the main interesting question arises-what is the nature of this interesting phenomenon of biological adaptation to 2H2O and what is the role of life important macromolecules (particularly DNA individual proteins and/or enzymes) in this process? It is seems very likely that during adaptation to 2H2O the structure and conformation of [U -2H]labeled macromolecules undergoing some modifications that are more useful for the working in 2H2O-conditions. Unfortunately there are a small number of experiments carried out with fully deuterated cells that could confirmed that during the growth on 2H2O [U-2H]labeled macromolecules with difined isotopical structures and conformations are formed so that a discussion about the role of deuterium on the structure and the conformation of [U-2H]labeled macromolecules in course of biolodical adaptation to 2H2O is still actual through more than four decades of years after the first description of the biological consequences of hydrogen replacement by deuterium.
To further discuss the matter we should distingueshed mainly three aspects of biological enrichment with deuterium: chemical biological and biophysical aspects all of them are connected in some way with the structure of [U -2H]labeled macromolecules. Theoretically the presence of deuterium in biological systems certainly could be manifested in more or less degree by changes in the structure and the conformation of macromolecules. Nevertheless it is important namely what precise position in macromolecule deuterium ocupied and dipending from that the primary and secondary isotopic effects are distingueshied. For example most important for the structure of macromolecule the hydrogen (deuterium) bonds form between different parts of the macromolecule and play a major part in determining the structure of macromolecular chains and how these structures interact with the others and also with 2H2O environment. Another important weak force is created by the three-dimentional structure of water (2H2O) which tends to force hydrophobic groups of macromolecule together in order to minimize their disruptive effect on the hydrogen (deuterium)-bonded network of water (2H2O) molecules.
On the other side the screw parameters of the proton helix are changed by the presence of deuterium so that ordinary proteins dissolved in 2H2O exhibit a more stable helical structure (Tomita K. Rich A. et all. 1962). While 2H2O probably exerts a stabilizing effect upon the three-dimentional hydrogen (deuterium)-bonded helix via forming many permanent and easily exchangeable hydrogen (deuterium) bonds in macromolecule in the presence of 2H2O (as an example the following types of bonds -COO2H; -O2H; -S2H; -N2H; N2H2 et.) the presence of nonexchangeable deuterium atoms in amino acid side chains could only be synthesized de novo as the species with only covalent bonds -C2H causes a decrease in protein stability.
These opposing effects do not cancel with the case of protein macromolecule and fully deuteration of a protein often results in the destabilization. As for the deuteration of DNA macromolecule today there are not reasonable considerations that such negative effect of 2H2O on the structure and function is really existiting. Nevertheless deuterium substitution can thus be expected to modify by changes in the structure and the conformation of both [U- 2H]labeled DNA and protein not only the reproductionl and division systems of a cell and cytological or even mutagenical alterations of a cell but to a greater or lesser degree of an order of a cell.
It should be noted however that not only these functions but also the lipid composition of cell membrane are drastically changed during deuteration. The lipid composition of deuteriated tissue culture cells has been most complitely investigated by a certain scientists (Rothblat et all. 1963 1964). As it is reported in these articles mammalian cells grown in 30% (v/v) 2H2O contain more lipid than do control cells. THe increase in the lipids of 2H2O grown cells is due primarily to increased amounts of triglycerids and sterol esters. Radioisotope experiments indicate that the differens are due to an enhanced synthesis of lipid. Monkey kidney cells grown in 25% (v/v) 2H2O and or irradiated with X-rays likewise showed increases of lipid. The 2H2O grown cells contained more squalene sterol esters sterols and neutral fat than did either the control of X-irradiated cells. Phospholipid levels were equal for all groups of cells. Thus the effects of 2H2O on lipid synthesis are qualitatively quite similar to those of radiation damade. An interisting observation that deserves further scrutiny relates to the radiation sensitivity of deuterated cells. Usually cells grown and irradiated in 2H2O shown much less sensivity to radiation than ordinary cells suspended in water. Suspension of ordinary cells in 2H2O did not have any effect on the reduced sensitivety became apparent.
A serious alteration in cell chemistry must be reflected in the ability of the cells to divide in the presence of 2H2O and in the manner of its division. However a many statements suggesting that 2H2O has a specific action on cell division are common since today. Probably it may be true that rapidly proliferating cells are highly sensitive to 2H2O but that deuterium acts only to prevent cell division is unlikely.
The rabbit cells grown on medium containing the various concentrations of 2H2O shown that 2H2O caused a reduction in cell division rate and this effect increased as the concentration of 2H2O or duration of exposure or both were increased (Lavillaureix et all. 1962). With increasing concentration of 2H2O the frequency of early metaphases increased accompanied by proportional decreases in the other phases.
It was suggested that 2H2O blocks mitosis in the prophase and the early metaphase of many cells grown in 2H2O. The blockage however was overcome if the initial concentration of 2H2O was not too high and the exposure time not too long. In experiments with eggs of the fresh water cichlid fish Aequidens portalegrensis they observed that in 30% 2H2O only one-fifth of the eggs hathed and in 50% (v/v) 2H2O none did so. Segmentation in fertilized frog eggs developed normally for 24 hours in 40% (v/v) 2H2O after which the embryos died. It was also found by Tumanyan and Shnol that 2H2O disturbed embryogenesis in Drosophila melanogaster eggs (Lavillaureix et all. 1962. Feeding female flies with 20% (v/v) 2H2O caused a significant increase in the proportion of nondeveloped eggs whether males were deuterated or not.
As pointed out by many researches carried elsewhere the reason for the cessation of mitotic activity from exposure to 2H2O is not clear. Certain microorganisms have been adapted to grow on fully deuterated media. However higher plants and animals resist adaptation to 2H2O. Even in microorganisms however cell division appears initially to be strongly inhibited upon transfer to highly deuterated media.
After the adaptation however cellular proliferation proceeds more or less normally in 2H2O but this stage is not reached in higher organisms. No ready explanation in terms of the present understanding of mitosis suggests itself. In Arbacia eggs antimitotic action of 2H2O is manifested almost immediately at all stages of the mitotic cycle and during cytokinesis (Gross P. R. et all. 1963 1964).
Table. Isotope components of growth media and characteristics of bacterial growth of Brevibacterium methylicum
Media components, % (v/v)
H2O 2H2O MetOH [U -2H]
|Generation time (h)||Production of phenylalanine (%)|
A stabilizing action on the nuclear membrane and gel structures i.e. aster spindle and peripheral plasmagel layer of the cytoplasm can be detected. Prophase and metaphase cells in 80% (v/v) 2H2O remain frozen in the initial state for at least 30 minutes. Furrowing capacity probably is not abolished by 2H2O. The 2H2O-block is released on immersion in 2H2O although cells kept in deuterium-rich media for long periods show multipolar and irregular divisions after removal to 2H2O and may subsequently cytolyze. The inhibition of mitosis in the fertilized egg is not the only interesting effect of deuterium. The unfertilized egg also responds. It was described by Gross that deuterium parthenogenesis in Arbacia in the following graphic terms: if an unfertilized egg is placed in 2H2O there appear in the cytoplasm after half an hour a number of cytasters. The number then increases with time. If after an hours immersion in 2H2O eggs are transferred to normal sea water a high proportion (80% of the population) raises a fertilization membrane which gives evidence that activation has occurred.
Deuterium genetics is for the most part like genetics itself conveniently divisible into dipteran mutation studies the genetics of microorganisms and miscellaneous studies of which those of Gross and Harding and Flaumenhaft et al. are examples. The customary procedure in most of the dipteran and bacterial investigations so far reported has been to administer 2H2O to the organism and then to test it for mutation or other chromosomal change. The results obtained by such an investigation have seldom been striking. For example many researchers found an increase in sex-linked lethals in the sperm of flies that had been exposed to deuterium either by way of injection into their pupae or by the inclusion of 2H2O in their food. They introduced 2H2O into Drosophila melanogaster larvae both by feeding and by injection. The males which matured from these larvae were tested for mutation by CIB method. But the test showed no increase in the mutation rate. It was assumed by these scientists that the deuterium which was used in dilute form entered the DNA molecule.
De Giovanni and Zamenhof have carried out the most comprehensive investigations on the genetic effects of deuterium in bacteria. The results are of considerable interest. For example they found a several mutants of E. coli including a so called rough mutant 1/D which is more resistant to 2H2O than its parent strain were isolated from E. coli grown in 2H2O media. The spontaneous frequency of occurerence of this mutant was 10-4 and the mutation rate could be increased 300-fold by ultraviolet irradiation. This mutant was derived only from the strain E. coli 15 thymidine and no similar mutant was observed in other strains of E. coli or B. subtilis. By application of a fluctuation test De Giovanni then was able to show convincingly that this mutation to increased deuterium resistance occurred spontaneously and not in response to the mutagenic effect of 2H2O. Back mutations in some instances do seem to occur at higher rates in 2H2O. Reversion from streptomycin dependence to streptomycin sensitivity in E. coli strain Sd/4 or from thymine dependence to thymine independence in strain 1 occurs with higher frequency in 2H2O but 2H2O does not cause a discernible increase in mutation in the wild type.
De Giovanni further found that deuteriated purines and pryrimidines had no effect upon the growth and back mutation rates of specific base-requiring strains. Thymine containing deuterium in two of the four nonexchangeable positions adequately supplied the requirement for thymine with no concominant genetic changes. It would appear therefore that the preponderance of the evidence from these studies with bacteria is in favor of the view that 2H2O is not a strong mutagenic agent.