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Electromagnetic window effects on proliferation rate of Corynebacterium glutamicum
Bioelectrochemistry and Bioenergetics 45 Ž1998. 261–265
Electromagnetic window effects on proliferation rate of Corynebacterium glutamicum Chenghong Lei 1, Hermann Berg
)
Laboratory of Bioelectrochemistry, Institute of Molecular Biotechnology, Jena, Germany Saxonian Academy of Sciences, Leipzig, Germany Received 8 January 1998; revised 3 March 1998; accepted 9 March 1998
Abstract To investigate the cell proliferation response to weak electromagnetic field, the determination of biomass and ATP pool of Corynebacterium glutamicum was performed under the electromagnetic windows with a varieties of amplitudes and frequencies by Helmholtz coils. The particular window with the frequency of 15 Hz and the amplitude of 3.4 mT increased the ATP level more than 20% compared with the control experiment after 8 h of continuous exposure. Under another window with the frequency of 50 Hz and the amplitude of 4.9 mT, the ATP level was more than 30% as much as with the control experiment after 6 h of continuous exposure. The condition for this result was the electrostimulation of the cell proliferation, however, the ATP concentration per cell remained constant. This result confirms the former experience with the proliferation of Saccharomyces cereÕisiae stimulated by analogous fields. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Proliferation; Helmholtz coils; Corynebacterium glutamicum
1. Introduction
Stimulation or inhibition of cellular processes by electromagnetic fields are of considerable interest in cell biology and biotechnology for not only establishing the basic mechanisms of this interaction but also its potential practical applications. Different targets including proliferation, enzyme reactions, biopolymer syntheses and membrane transport have been investigated with respect to their alteration by electromagnetic energy w1,2x. Exposing several types of cells to electric field pulses w3,4x or an oscillating field w5x has been shown to stimulate the ATP synthesis. Rectangular electric field pulses were effective in the case of a chloroplast suspension exposed in a 1 cm cuvette placed between two flat platinum electrodes
)
Corresponding author. Fax: q 49-3641-609818; e-mail: [email protected] 1 From Fudan University, Department of Chemistry, Shanghai 200433, China. 0302-4598r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 3 0 2 - 4 5 9 8 Ž 9 8 . 0 0 0 9 9 - 3
w3x. The total cytoplasmic ATP content was also increased in Escherichia coli when the cells were submitted to exponential pulses with field strength of 1–6 kV cmy1 and a decay time of 7–20 ms w4x. Platinum electrodes were used for red blood cells by an oscillating electric field of 20 V cmy1 for 60 min w4x. Treatment of human AMA cells and fibroblasts by 50 Hz, 0.80 mT for 30 min increased the proliferation rate up to 180% w6x. Concerning the yeast Saccharomyces cereÕisiae, the previous report of our group showed a positive response in the proliferation and ATP level at the ‘electromagnetic window’ with a frequency of 50 Hz and an amplitude of 0.5 mT produced by a pair of Helmholtz coils w7,8x. However, when the wet stationary yeast sediments were exposed to EMF with a frequency of 50 Hz, no window effect was observed w9x. Corynebacterium glutamicum, which can be cultured rapidly and used safely, is usually utilized for fermentation industry to produce glutamic acid by employing urea as a nitrogen source. The aim of this work was to test the possibility of alteration of its proliferation and ATP level by EMF applied to the culture of C. glutamicum during relatively long time periods.
262
C. Lei, H. Berg r Bioelectrochemistry and Bioenergetics 45 (1998) 261–265
2. Experimental 2.1. Corynebacterium glutamicum C. glutamicum 227 was obtained from the Genetics Institute of Fudan University. The medium for cultivation was composed of 1.0% peptone, 0.5% yeast powder, 1.0% sodium chloride. This medium was autoclaved at 1218C for 35 min. The solution was allowed to cool and the C. glutamicum was aseptically inoculated on this liquid medium. The inoculated medium was kept in a temperature-controlled vibratile culture case at 308C for 24 h. Then, 1 ml of such a cell suspension was resuspended in 20 ml of the fresh medium for this work. 2.2. Reagents ATP Bioluminescence Assay Kit CLS II Žbased on the ATP dependency of the light emitting luciferase catalyzed oxidation of luciferin. ŽBoehringer Mannheim, Germany.. 2.3. Apparatus and procedures A pair of specially designed Helmholtz coils of 5 cm diameter generated the mean magnetic flux intensities from 0 to 12 mT as measured by a digital Teslameter FM 210 ŽProject Electronik, Berlin. equipped with a transversal probe Ž2 = 3 mm.. The experiments were performed from 2 to 10 mT. The sinusoidal field frequency was selected within 10 to 100 Hz by means of a home made amplifier and a Tesla oscillator Ž10–100 MHz BM 492. and controlled with the Tektronix storage oscilloscope 5441. Two identical vessels of the cell suspension were maintained at constant temperature Ž28.5 " 0.18C. by circulated water from a thermostat. One is surrounded by the Helmholtz coils for the influence of EMF, another one was used for control experiments. Sterile air was bubbled through a filter in both two suspensions to realize stirring during fermentation. The experiments were performed for 8 h of cultivation time. The cell number was evaluated by microscopic determinations on the Counter System ŽCasy 1—from Scharfe ¨ System, Reutlingen, Germany.. ATP concentration was determined as follows: 0.1 ml of suspension was treated with 0.4 ml DMSO for 2 min stirring for fully extraction of ATP. In test tubes, 0.1 ml of such a mixture was added with 0.5 ml of distilled water and 0.2 ml of ATP reagent successively. The amplitude of the peak signal was recorded with Emilite Žthe tube luminometer EL 1003, Bio-Rad Vienna..
Fig. 1. Amplitude dependence of the ATP level in the cell suspension after 8 h of continuous exposure by an induced electromagnetic field with a frequency of 15 Hz.
performed for each field amplitude, showing the different ATP pool in the cell culture Žsee Fig. 1.. After 8 h of continuous exposure, a sharp peak of the ATP level being centered around the amplitude of 3.4 mT was detected.
3. Results and discussion To investigate the cell proliferation response to EMF, at a frequency of 15 Hz, a set of ATP determinations were
Fig. 2. Frequency dependence of the ATP level in the cell suspension after 8 h of continuous exposure by an induced electromagnetic field with an amplitude of 3.4 mT.
C. Lei, H. Berg r Bioelectrochemistry and Bioenergetics 45 (1998) 261–265
263
ous exposure Žsee Fig. 2.. It is worthy to mention that the negative effects for the ATP level were obtained for certain frequencies higher than 30 Hz. The stimulatory effect on faster proliferation as well as ATP synthesis is shown only for one electromagnetic window with the frequency of 15 Hz and the amplitude of 3.4 mT in Fig. 3A and B, respectively. The obvious acceleration effect was observed only after 4 h of continuous exposure, indicating the dependence of exposure time. The ATP level could increase more than 20% compared with the control experiment after 8 h of continuous exposure Žsee Fig. 3B.. The cell proliferation was stimulated in the same direction as the ATP level Žsee Fig. 3A.. The relative stimulatory effects on the proliferation of C. glutamicum are shown in Fig. 4, indicating that there was no obvious window effect on the ATP concentration per cell under such field conditions. This demonstrated that in principle the ATP determination could be regarded as an indicator of the accelerated cell proliferation w8x. Similarly, at a frequency of 50 Hz, the maximum ATP level was observed around the amplitude of 4.9 mT after 6 h of continuous exposure Žsee Fig. 5.. On the other hand, at the amplitude of 4.9 mT, the maximum ATP level was shown only around the frequency of 50 Hz Žsee Fig. 6.. The similar results were obtained after 8 h of continuous exposure Žnot shown.. It seems that the higher the frequency, the larger the amplitude for the maximum response of the EMF window, and vice versa. Under the electromagnetic window with the frequency of 50 Hz and the amplitude of 4.9 mT, the acceleration effect on the
Fig. 3. ŽA. Žv . Stimulatory effect on the cell proliferation at 15 Hz, 3.4 mT; ŽB. The control experiment. ŽB. Žv . Stimulatory effect on the ATP level in the cell suspension at 15 Hz, 3.4 mT; ŽB. The control experiment.
Moreover, the responses below 100% were observed around the amplitude of 2.4 mT and if the amplitudes exceeded 4.4 mT. Maintaining the 3.4 mT field amplitude, the frequency dependence of the ATP level in the cell suspension was investigated in detail. The maximum ATP level was obtained around the frequency of 15 Hz after 8 h of continu-
Fig. 4. Relative stimulatory effect on the culturing C. glutamicum at 15 Hz, 3.4 mT: ŽB. The ATP level; Žv . Cell number; Ž'. The ATP level per cell.
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C. Lei, H. Berg r Bioelectrochemistry and Bioenergetics 45 (1998) 261–265
Fig. 5. Amplitude dependence of the ATP level in the cell suspension after 6 h of continuous exposure by an induced electromagnetic field with a frequency of 50 Hz.
ATP level as the indicator for proliferation is shown in Fig. 7. More than 30% stimulatory effect on the ATP level, compared with the control experiment, was observed after 6 h of continuous exposure. As known, most of the ATP synthesis is carried out by the enzyme ATP synthase. The ATP synthase differs from the majority of all enzymes, which bind and release substrates and products spontaneously, but for which the overall catalytic reaction requires energy. This energy is required to bind ADP and the phosphate to the enzyme and to release ATP. In this respect, ATP synthase is called a molecular machine w10x. The ATP level served as one indicator of cell proliferation under EMF treatment, its increase or decrease might result from electromagnetic
Fig. 6. Frequency dependence of the ATP level in the cell suspension after 6 h of continuous exposure by an induced electromagnetic field with an amplitude of 4.9 mT.
Fig. 7. Žv .Stimulatory effect on the ATP level in the cell suspension at 50 Hz, 4.9 mT; ŽB. The control experiment.
window effects Žboth related to the amplitude and the frequency. on the running of such a ‘machine’.
4. Conclusion As we have shown with S. cereÕisiae w8x, also the proliferation rate and ATP synthesis of C. glutamicum could be electrostimulated and several electromagnetic windows after 4 h of cultivation were detected. Especially the ATP synthesis has proved to be a sensitive tool to measure the proliferation responses of microorganisms to the electromagnetic window with a frequency below 100 Hz and an amplitude below 10 mT. According to the relations of the a.c. electrostimulation of proliferation as a consequence of acceleration of all metabolic reactions including the ATP synthesis, several windows can be detected by measuring the response on broad regions of frequency, amplitude, cultivation time. In the small region, we found 2 windows: 15 Hz and 3.4 mT, 50 Hz and 4.9 mT. For this nonsynchronized culture, we cannot explain up to now why differences between the control and the experiment occurred after 4 h Žfor yeast it occurred after 2.5 h, also with 15 Hz and 50 Hz windows w8x.. Now the three open questions are: -what is the mechanism of electrostimulation? -where takes place the transformation from electrical to chemical energy? and -how can the cell distinguish so sensitively between different frequency and amplitude windows?
C. Lei, H. Berg r Bioelectrochemistry and Bioenergetics 45 (1998) 261–265
Acknowledgements Dr. Chenghong Lei would like to thank the Alexander von Humboldt Foundation ŽBonn, Germany. for awarding research fellowship and Dr. Ingeburg Hones for the kind ¨ assistance.
w4x
w5x
w6x
References w1x H. Berg, Possibilities and problems of low frequency weak electromagnetic fields in cell biology, Bioelectrochem. Bioenerg. 38 Ž1995. 153–159. w2x E.M. Goodman, B. Greenebaum, M.T. Marron, Effects of electromagnetic fields on molecules and cells, Int. Rev. Cytology 158 Ž1995. 279–338. w3x E. Schlodder, H.T. Witt, Relation between the initial kinetics of ATP synthesis and of conformational changes in the chloroplast
w7x w8x
w9x
w10x
265
ATPase studied by external field pulses, Biochim. Biophys. Acta 635 Ž1981. 571–579. X J. Teissie, Adenosine 5 -triphosphate synthesis in Escherichia coli submitted to a microsecond electric pulse, Biochemistry 25 Ž1986. 368–373. D-S. Liu, R. Dean Astumian, T.Y. Tsong, Activation of Naq and Kq pumping modes of ŽNa, K.-ATPase by an oscillating electric field, J. Biol. Chem. 265 Ž1990. 7260–7267. S. Kwee, P. Rastmark, Changes in cell proliferation due to environmental non-ionizing radiation, Bioelectrochem. Bioenerg. 36 Ž1995. 109–114. U. Fiedler, U. Grobner, H. Berg, Electrostimulation of yeast prolifer¨ ation, Bioelectrochem. Bioenerg. 38 Ž1995. 423–425. M. Mehedintu, H. Berg, Proliferation response of yeast Saccharomyces cereÕisiae on electromagnetic field parameters, Bioelectrochem. Bioenerg. 43 Ž1997. 67–70. S. Velizarov, H. Berg, An attempt to alter the ATP pool in Saccharomyces cereÕisiae cells by 50-Hz weak electromagnetic fields, Electro-Magnetobiol. 15 Ž1996. 209–212. P.D. Boyer, The ATP synthase-a splendid molecular machine, Ann. Rev. Biochem. 66 Ž1997. 717–749.
Electromagnetic window effects on proliferation rate of Corynebacterium glutamicum Chenghong Lei 1, Hermann Berg
)
Laboratory of Bioelectrochemistry, Institute of Molecular Biotechnology, Jena, Germany Saxonian Academy of Sciences, Leipzig, Germany Received 8 January 1998; revised 3 March 1998; accepted 9 March 1998
Abstract To investigate the cell proliferation response to weak electromagnetic field, the determination of biomass and ATP pool of Corynebacterium glutamicum was performed under the electromagnetic windows with a varieties of amplitudes and frequencies by Helmholtz coils. The particular window with the frequency of 15 Hz and the amplitude of 3.4 mT increased the ATP level more than 20% compared with the control experiment after 8 h of continuous exposure. Under another window with the frequency of 50 Hz and the amplitude of 4.9 mT, the ATP level was more than 30% as much as with the control experiment after 6 h of continuous exposure. The condition for this result was the electrostimulation of the cell proliferation, however, the ATP concentration per cell remained constant. This result confirms the former experience with the proliferation of Saccharomyces cereÕisiae stimulated by analogous fields. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Proliferation; Helmholtz coils; Corynebacterium glutamicum
1. Introduction
Stimulation or inhibition of cellular processes by electromagnetic fields are of considerable interest in cell biology and biotechnology for not only establishing the basic mechanisms of this interaction but also its potential practical applications. Different targets including proliferation, enzyme reactions, biopolymer syntheses and membrane transport have been investigated with respect to their alteration by electromagnetic energy w1,2x. Exposing several types of cells to electric field pulses w3,4x or an oscillating field w5x has been shown to stimulate the ATP synthesis. Rectangular electric field pulses were effective in the case of a chloroplast suspension exposed in a 1 cm cuvette placed between two flat platinum electrodes
)
Corresponding author. Fax: q 49-3641-609818; e-mail: [email protected] 1 From Fudan University, Department of Chemistry, Shanghai 200433, China. 0302-4598r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 3 0 2 - 4 5 9 8 Ž 9 8 . 0 0 0 9 9 - 3
w3x. The total cytoplasmic ATP content was also increased in Escherichia coli when the cells were submitted to exponential pulses with field strength of 1–6 kV cmy1 and a decay time of 7–20 ms w4x. Platinum electrodes were used for red blood cells by an oscillating electric field of 20 V cmy1 for 60 min w4x. Treatment of human AMA cells and fibroblasts by 50 Hz, 0.80 mT for 30 min increased the proliferation rate up to 180% w6x. Concerning the yeast Saccharomyces cereÕisiae, the previous report of our group showed a positive response in the proliferation and ATP level at the ‘electromagnetic window’ with a frequency of 50 Hz and an amplitude of 0.5 mT produced by a pair of Helmholtz coils w7,8x. However, when the wet stationary yeast sediments were exposed to EMF with a frequency of 50 Hz, no window effect was observed w9x. Corynebacterium glutamicum, which can be cultured rapidly and used safely, is usually utilized for fermentation industry to produce glutamic acid by employing urea as a nitrogen source. The aim of this work was to test the possibility of alteration of its proliferation and ATP level by EMF applied to the culture of C. glutamicum during relatively long time periods.
262
C. Lei, H. Berg r Bioelectrochemistry and Bioenergetics 45 (1998) 261–265
2. Experimental 2.1. Corynebacterium glutamicum C. glutamicum 227 was obtained from the Genetics Institute of Fudan University. The medium for cultivation was composed of 1.0% peptone, 0.5% yeast powder, 1.0% sodium chloride. This medium was autoclaved at 1218C for 35 min. The solution was allowed to cool and the C. glutamicum was aseptically inoculated on this liquid medium. The inoculated medium was kept in a temperature-controlled vibratile culture case at 308C for 24 h. Then, 1 ml of such a cell suspension was resuspended in 20 ml of the fresh medium for this work. 2.2. Reagents ATP Bioluminescence Assay Kit CLS II Žbased on the ATP dependency of the light emitting luciferase catalyzed oxidation of luciferin. ŽBoehringer Mannheim, Germany.. 2.3. Apparatus and procedures A pair of specially designed Helmholtz coils of 5 cm diameter generated the mean magnetic flux intensities from 0 to 12 mT as measured by a digital Teslameter FM 210 ŽProject Electronik, Berlin. equipped with a transversal probe Ž2 = 3 mm.. The experiments were performed from 2 to 10 mT. The sinusoidal field frequency was selected within 10 to 100 Hz by means of a home made amplifier and a Tesla oscillator Ž10–100 MHz BM 492. and controlled with the Tektronix storage oscilloscope 5441. Two identical vessels of the cell suspension were maintained at constant temperature Ž28.5 " 0.18C. by circulated water from a thermostat. One is surrounded by the Helmholtz coils for the influence of EMF, another one was used for control experiments. Sterile air was bubbled through a filter in both two suspensions to realize stirring during fermentation. The experiments were performed for 8 h of cultivation time. The cell number was evaluated by microscopic determinations on the Counter System ŽCasy 1—from Scharfe ¨ System, Reutlingen, Germany.. ATP concentration was determined as follows: 0.1 ml of suspension was treated with 0.4 ml DMSO for 2 min stirring for fully extraction of ATP. In test tubes, 0.1 ml of such a mixture was added with 0.5 ml of distilled water and 0.2 ml of ATP reagent successively. The amplitude of the peak signal was recorded with Emilite Žthe tube luminometer EL 1003, Bio-Rad Vienna..
Fig. 1. Amplitude dependence of the ATP level in the cell suspension after 8 h of continuous exposure by an induced electromagnetic field with a frequency of 15 Hz.
performed for each field amplitude, showing the different ATP pool in the cell culture Žsee Fig. 1.. After 8 h of continuous exposure, a sharp peak of the ATP level being centered around the amplitude of 3.4 mT was detected.
3. Results and discussion To investigate the cell proliferation response to EMF, at a frequency of 15 Hz, a set of ATP determinations were
Fig. 2. Frequency dependence of the ATP level in the cell suspension after 8 h of continuous exposure by an induced electromagnetic field with an amplitude of 3.4 mT.
C. Lei, H. Berg r Bioelectrochemistry and Bioenergetics 45 (1998) 261–265
263
ous exposure Žsee Fig. 2.. It is worthy to mention that the negative effects for the ATP level were obtained for certain frequencies higher than 30 Hz. The stimulatory effect on faster proliferation as well as ATP synthesis is shown only for one electromagnetic window with the frequency of 15 Hz and the amplitude of 3.4 mT in Fig. 3A and B, respectively. The obvious acceleration effect was observed only after 4 h of continuous exposure, indicating the dependence of exposure time. The ATP level could increase more than 20% compared with the control experiment after 8 h of continuous exposure Žsee Fig. 3B.. The cell proliferation was stimulated in the same direction as the ATP level Žsee Fig. 3A.. The relative stimulatory effects on the proliferation of C. glutamicum are shown in Fig. 4, indicating that there was no obvious window effect on the ATP concentration per cell under such field conditions. This demonstrated that in principle the ATP determination could be regarded as an indicator of the accelerated cell proliferation w8x. Similarly, at a frequency of 50 Hz, the maximum ATP level was observed around the amplitude of 4.9 mT after 6 h of continuous exposure Žsee Fig. 5.. On the other hand, at the amplitude of 4.9 mT, the maximum ATP level was shown only around the frequency of 50 Hz Žsee Fig. 6.. The similar results were obtained after 8 h of continuous exposure Žnot shown.. It seems that the higher the frequency, the larger the amplitude for the maximum response of the EMF window, and vice versa. Under the electromagnetic window with the frequency of 50 Hz and the amplitude of 4.9 mT, the acceleration effect on the
Fig. 3. ŽA. Žv . Stimulatory effect on the cell proliferation at 15 Hz, 3.4 mT; ŽB. The control experiment. ŽB. Žv . Stimulatory effect on the ATP level in the cell suspension at 15 Hz, 3.4 mT; ŽB. The control experiment.
Moreover, the responses below 100% were observed around the amplitude of 2.4 mT and if the amplitudes exceeded 4.4 mT. Maintaining the 3.4 mT field amplitude, the frequency dependence of the ATP level in the cell suspension was investigated in detail. The maximum ATP level was obtained around the frequency of 15 Hz after 8 h of continu-
Fig. 4. Relative stimulatory effect on the culturing C. glutamicum at 15 Hz, 3.4 mT: ŽB. The ATP level; Žv . Cell number; Ž'. The ATP level per cell.
264
C. Lei, H. Berg r Bioelectrochemistry and Bioenergetics 45 (1998) 261–265
Fig. 5. Amplitude dependence of the ATP level in the cell suspension after 6 h of continuous exposure by an induced electromagnetic field with a frequency of 50 Hz.
ATP level as the indicator for proliferation is shown in Fig. 7. More than 30% stimulatory effect on the ATP level, compared with the control experiment, was observed after 6 h of continuous exposure. As known, most of the ATP synthesis is carried out by the enzyme ATP synthase. The ATP synthase differs from the majority of all enzymes, which bind and release substrates and products spontaneously, but for which the overall catalytic reaction requires energy. This energy is required to bind ADP and the phosphate to the enzyme and to release ATP. In this respect, ATP synthase is called a molecular machine w10x. The ATP level served as one indicator of cell proliferation under EMF treatment, its increase or decrease might result from electromagnetic
Fig. 6. Frequency dependence of the ATP level in the cell suspension after 6 h of continuous exposure by an induced electromagnetic field with an amplitude of 4.9 mT.
Fig. 7. Žv .Stimulatory effect on the ATP level in the cell suspension at 50 Hz, 4.9 mT; ŽB. The control experiment.
window effects Žboth related to the amplitude and the frequency. on the running of such a ‘machine’.
4. Conclusion As we have shown with S. cereÕisiae w8x, also the proliferation rate and ATP synthesis of C. glutamicum could be electrostimulated and several electromagnetic windows after 4 h of cultivation were detected. Especially the ATP synthesis has proved to be a sensitive tool to measure the proliferation responses of microorganisms to the electromagnetic window with a frequency below 100 Hz and an amplitude below 10 mT. According to the relations of the a.c. electrostimulation of proliferation as a consequence of acceleration of all metabolic reactions including the ATP synthesis, several windows can be detected by measuring the response on broad regions of frequency, amplitude, cultivation time. In the small region, we found 2 windows: 15 Hz and 3.4 mT, 50 Hz and 4.9 mT. For this nonsynchronized culture, we cannot explain up to now why differences between the control and the experiment occurred after 4 h Žfor yeast it occurred after 2.5 h, also with 15 Hz and 50 Hz windows w8x.. Now the three open questions are: -what is the mechanism of electrostimulation? -where takes place the transformation from electrical to chemical energy? and -how can the cell distinguish so sensitively between different frequency and amplitude windows?
C. Lei, H. Berg r Bioelectrochemistry and Bioenergetics 45 (1998) 261–265
Acknowledgements Dr. Chenghong Lei would like to thank the Alexander von Humboldt Foundation ŽBonn, Germany. for awarding research fellowship and Dr. Ingeburg Hones for the kind ¨ assistance.
w4x
w5x
w6x
References w1x H. Berg, Possibilities and problems of low frequency weak electromagnetic fields in cell biology, Bioelectrochem. Bioenerg. 38 Ž1995. 153–159. w2x E.M. Goodman, B. Greenebaum, M.T. Marron, Effects of electromagnetic fields on molecules and cells, Int. Rev. Cytology 158 Ž1995. 279–338. w3x E. Schlodder, H.T. Witt, Relation between the initial kinetics of ATP synthesis and of conformational changes in the chloroplast
w7x w8x
w9x
w10x
265
ATPase studied by external field pulses, Biochim. Biophys. Acta 635 Ž1981. 571–579. X J. Teissie, Adenosine 5 -triphosphate synthesis in Escherichia coli submitted to a microsecond electric pulse, Biochemistry 25 Ž1986. 368–373. D-S. Liu, R. Dean Astumian, T.Y. Tsong, Activation of Naq and Kq pumping modes of ŽNa, K.-ATPase by an oscillating electric field, J. Biol. Chem. 265 Ž1990. 7260–7267. S. Kwee, P. Rastmark, Changes in cell proliferation due to environmental non-ionizing radiation, Bioelectrochem. Bioenerg. 36 Ž1995. 109–114. U. Fiedler, U. Grobner, H. Berg, Electrostimulation of yeast prolifer¨ ation, Bioelectrochem. Bioenerg. 38 Ž1995. 423–425. M. Mehedintu, H. Berg, Proliferation response of yeast Saccharomyces cereÕisiae on electromagnetic field parameters, Bioelectrochem. Bioenerg. 43 Ž1997. 67–70. S. Velizarov, H. Berg, An attempt to alter the ATP pool in Saccharomyces cereÕisiae cells by 50-Hz weak electromagnetic fields, Electro-Magnetobiol. 15 Ž1996. 209–212. P.D. Boyer, The ATP synthase-a splendid molecular machine, Ann. Rev. Biochem. 66 Ž1997. 717–749.