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243 ELECTRICAL MEMBRANE PHENOMENA in SPHERULES from PROTEINOID and LECITHIN*** Introduction One of the Most Studied Attributes O

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243 ELECTRICAL MEMBRANE PHENOMENA in SPHERULES from PROTEINOID and LECITHIN*** Introduction One of the Most Studied Attributes O BioSystems, 13 (lq81) 243--251 243 © Elsevier/North-Holland Scientific Publishers Ltd. ELECTRICAL MEMBRANE PHENOMENA IN SPHERULES FROM PROTEINOID AND LECITHIN*** YOSHIO ISHIMA*, ALEKSANDER T. PRZYBYLSKI** and SIDNEY W. FOXt Institute for Molecular and Cellular Evolution, University of Miami, 521 Anastasia Avenue, Coral Gables, Florida 33134, U.S.A. (Received August :[2th, 1980) (Revision received January 20th, 1981) Spontaneous and induced electrical phenomena resembling membrane and action potentials in natural excitable cells have been observed in artificial cells. These artificial cells were made from thermal proteinoid and lecithin in a solution of potassium acid phosphate with glycerol. Introduction Other related studies are numerous (e.g. Takahashi and Miyazaki, 1971; Kennedy et One of the most studied attributes of the al., 1977). Studies of synthetic bilayer mem- living state in modern organisms is that of branes indicate that polypeptides can form excitability. Because of an extensive literature ionic channels in excitable bilayers. on excitability in cells, and because of accu- The early examples and the results of mulated knowledge of properties of proteinoid chemical analysis of excitable cells (Condrea microparticles (Fox and Dose, 1977; Fox, and Rosenberg, 1968; Camejo et al., 1969; 1980), proteinoid-containing models for excit- Ishima and Waku, 1978) suggested that excit- able cells have been sought. ability might be produced in an artificial cell In somewhal; related studies, excitability composed of suitable polyamino acid and has been observed in internodal cells of plant phospholipid. tissue (Hill and Osterhout, 1934; Cole and The fact that proteinoid microparticles Curtis, 1938; F indlay, 1959) and in reassem- possess permselective membranes (Fox et al., bled vesicles. Droplets of protoplasm from an 1969) and are capable of junction formation internodal cell of NiteUa form semipermeable and communication (Hsu et al., 1971) also membranes on the surface under certain raised the possibility of electrical phenomena conditions (Yoneda and Kamiya, 1969) and in these units. This paper describes the results display excitabJ~lities when properly activated of experiments in making electrically active (Inoue et al., 1971; Takenaka et al., 1971). artificial cells from proteinoids and lecithin. Various proteinoids were prepared to contain *Present address: Laboratory of Neurophysiology, large proportions of individual amino acids Division of Neuroscience, Tomobe Hospital Medical Center, 654 .4~ahi-Mach, Tomobe-Cho, Nlshi- in an effort to identify which amino acids Ibaragigun, Ibaragi-Ken 319-03, Japan. most contribute to electrical activity. **Present addrem+: Medical Research Center, Polish Academy of Sciences, Dworkowa 3, 00-874 Warsaw, Materials and methods Poland. ***Some of these results were first presented at the Third Annual Meeting of the Society for Neuroscience Proteinoid in San Diego, Cal~[ornia, November 1973. tTo whom repriint requests should be addressed. The polyamino acids were prepared by 244 w 0 $ e~ 0 ¢JR °~ S 0 245 heating in an oil bath at 190°C for 6 h a mix- the surface of the crusts (Fig. 1). This mem- ture of 50 g of each individual amino acid brane formed from the crust in an external treated with 50 g of an equimolar mixture of solution containing mannitol at concentra- 18 amino acids under a nitrogen blanket tions between 150 mM and 300 mM. Mannitol (Vegotsky, 1972,). The products were mech- was used to maintain the osmotic balance of anically stirred with water for 2 h and then the spherule. If the mannitol in the external filtered. The dissolved fraction was dialyzed solution was concentrated to more than for 2 days in a cold room with two changes of 300 mM, it failed to produce semipermeable water per day. The non-diffusates were spherules. If the tonicity of the solution was lyophilized. Preliminary experiments showed too low the size of spherules increased rapidly that the dissolved fractions yielded the most until they burst. A concentration of 200 mM "active" particles. The leucine-rich protein- mannitol was found to prevent bursting of the oid that received most attention contained spherules and yet maintain suitable sized 14.9% leucine. spherules for several hours. The selected spherules used in this experiment were Phospholipid 60--150 ~m in diameter. The compositions of the inside and outside solutions for the Phospholipids were purified from Eastman spherules are shown in Table 1. practical grade vegetable lecithin by extrac- tion with 50% aqueous glycerol solution con- Electrical phenomena taining potassium phosphate (pH 5.8). A simple, standard intracellular microelec- Preparation of spherules trode technique was applied to monitor the electrical phenomena (Watanabe and Ishima, A suspension of 60 mg of proteinoid and 1972). Figure 2 shows a schematic outline of 60 rag of lecithin in 3.0 ml of potassium the equipment. The tip diameter of the glass phosphate solution with aqueous glycerol electrode was about 0.2/~m and the electrical (50:50 v/v) was heated to boiling. This resistance was approximately 20 M~2, with solution contained 80 mmol of phosphate 3 M KC1 inside. The reference electrode was a (PO4), and 50% of glycerol (pH 5.8). The silver-silver chloride-KC1 agar placed in the solution was allowed to cool to room tem- external solution. These are employed for perature, and then allowed to stand for a accurate measurements of the membrane period of several hours up to 2 days. Standing TABLE 1 was empirically observed to be helpful to the experiments. After incubation at room tem- Ionic concentrations within spherules. perature, crusts precipitated from the super- saturated solution. They were carefully Inside Outside removed by use of a pipet. The original K÷ 80 (mM) Na÷ 0.2 (mM) external solution was then replaced com- FF 130 K÷ 0.05 pletely by the e:cperimental external solution Mg ~ + 0.1 by a suction device. This was done by adding Ca 2÷ 0.5 (1.0) from the top and drawing from the bottom, PO~- 70 C1- 0.25 SO:- 0.1 while the level of the solution was kept NO~ 1.0 (2.0) constant. Approximately 30 min after the Glycerol 50% Mannitol 200.0 (mM) external solution was completely replaced, in pH 5.8 pH 7.0 (Na- the case of leucine-rich equimolar proteinoid, phos- phate for example, a very thin, transparent, almost buffer) invisible spherical membrane emerged from 246 proteinoids, four types proved suitable for experiment. These were leucine-rich equimolar proteinoid (AMX-9, dfl), threonine-rich equi- molar proteinoid (AMX-105, dfl), proline- rich equimolar proteinoid (AMX-105, dfl), and leucine-rich equimolar proteinoid (T-83, dfI). These water-soluble proteinoids formed semipermeable membranes with lecithin. Among these, the leucine-rich equimolar proteinoid alone produced pronounced elec- trical activities. While a proline-rich protein- 1 M ohm oid (49.8% proline) and a threonine-rich one (19.4% threonine) also formed mem- branes with the same lecithin, they showed Fig. 2. ;chematic diagram of the experimental setup lesser electrical activities. (see text). The individual amino acids included in proteinoids in large proportion represented potential with elimination of any possible quite fully the roster of amino acids common boundary potentials. For DC<lifferential to proteins. Although vesicles containing each recording of the potentials, a W-P Instrument amino acid were not tested for electrical Model 750 amplifier was used and, for current activity in an equalized comparison, three of monitor, a W-P Instrument Model 701 was the amino acids proved to offer encourage- used. These high-input impedance amplifiers ment for further investigation. These three: (20 000 M~2 work well for a range of electrical leucine, proline, and threonine are mainly measurements. The output of these preampli- amino acids with apolar sidechains. Such tiers was led to a cathode ray oscilloscope hydrophobicity may contribute directly or to (with Type 3A3 plug-in units, Tektronix) or aiding in the. complexing with lecithin. The a digital voltmeter (Elpo, Type V-529), one amino acid of the three that does not simultaneously. In order to obtain a constant contain a purely hydrocarbon sidechain is injection current for stimulations, a current threonine. It will be of interest to determine clamp circuit was applied with a Tektronix if the threonine hydroxyl group coordinates Type O amplifier. Stimulator (W-P Instrument ions in the manner that Kennedy et al. {1977) Series 800) was isolated from the ground with have suggested for serine hydroxyl. Model 850A Isolator (W-P Instrument). The The proteinoid crusts evidently act as injection current was led to a current injection ionophores because, without the proteinoid, glass microelectrode through a 100 M~ the lipid membrane itself has an extremely resistor, which stabilized the electrode con- high membrane resistance (Mueller and Rudin, siderably. Any current through the spherule 1969; Yoshida et al., 1972, cf. Grote and Fox, was monitored across a 1 M~ resistor placed 1979). Incubation is necessary for at least between the external solution and the ground. several hours and sometimes for 2 days. More Thus, any possible current leak was elimi- than 2 days of incubation improved the yields nated, by precisely balancing the equipment of the spherules suitable for the experiment. throughout the experiment. A Grass camera The period can be prolonged if there is no was used to take photographs of oscilloscope bacterial infection. traces. In various experiments, a glass micro- electrode was inserted in a spherule after it aged or about 40 min after the external Results and discussion solution was replaced; the membrane potential was measured. Of more than 100 different preparations of Various patterns of electrical activity along 247 with the steady state membrane potential current. The spherule was repeatedly stimu- were observed without any stimulating cur- lated by passing the current pulses in positive rent. These fell essentially into three cate- and negative directions, alternatively.
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