College of Science and Engineering
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Item Switching phenomena in Lithium Drifted Silicon p i n Diodes(Rice University, 1971-10) Harman, Thomas L.Abstract not available.Item Resensitization to gibberellin of grand rapids lettuce seeds treated with continuous far red light(Journal of the Tennessee Academy of Science, 1973) Mills, RonaldItem Numerical Computation of the External Potential Field of the Isolated Active Purkinge Strand in a Volume Conductor(SWIEECO Proceedings, 1973) Harman, Thomas L.Abstract not available.Item Effects of Intrinsic Region Width in SI(Li) p i n Diodes(Solid State Electronics, 1974) Harman, Thomas L.Description of certain effects related to the width of the intrinsic region produced in a silicon p-i-n diode by lithium ion drifting. It is found that for wider intrinsic regions at large forward biases there is a larger ohmic drop across the region and a correspondingly smaller current. Moreover, the recovery time decreases with increasing intrinsic region width. Conversely, the decay phase time increases with increasing intrinsic region width and with increasing current levels.Item A Comparison of Two Methods of Determining the Extracellular Potential Field of an Isolated Purkinge Strand in a Volume Conductor(IEEE Transactions, 1975-05) Harman, Thomas L.; Harman, Thomas L.The models considered in this study are those of Spach et al., [1], and Clark and Plonsey [6], [7]. Both assume circular cylindrical geometry for the isolated Purkinje strand and input information to the models consists mainly of the recorded transmembrane action potential, the ratio of conductivities of the intra- and extracellular media, the conduction velocity of the action potential, and the radius of the strand. In general the extracellular potentials computed using both methods agree with measured potential data and with each other. However, the Clark-Plonsey method provides a more accurate prediction of both the peak-to-peak magnitude and the separation between peaks of the bipolar extracellular potential waveform, particularly at field points close to the strand.Item Item Volume Conductor Fields of the Isolated Axon(Elsevier B.V., 1977) Greco, E. C.; Clark, J. W.; Harman, Thomas L.A solution of Laplace's equation relating the transmembrane potential distribution of an active fiber in a volume conductor to its extracellular field distribution utilizing a Fourier-transform method [4] has been reformulated as a one-dimensional linear filtering problem. Formulation of the solution in this manner allows the application of well-known techniques in linear system theory and optimal linear filtering, thereby facilitating the solution for both the forward (from transmembrane to field potential distribution) and inverse (from field to transmembrane potential distribution) problems. The forward problem is shown to be a simple two-stage filtering process composed of a membrane and medium filter. In the inverse case, the field potential distribution is considered in the presence of additive measurement noise, and the best estimate in the least-mean-square sense is obtained for the transmembrane potential distribution. Discrete Fourier-transform techniques are applied to this reformulated Fourier-transform method, resulting in a fast, efficient algorithm for solution of the forward and inverse field problems.Item Experience with a Fourier Method for Determining the Extracellular Potential Fields of Excitable Cells with Cylindrical Geometry(CRC Press, Inc., 1978) Clark, J. W.; Harman, Thomas L.; Greco, E. C.In this chapter, well-known solutions that utilize a Fourier transform method for determining the extracellular, volume-conductor potential distribution surrounding elongated excitable cells of cylindrical geometry are reformulated as a discrete Fourier transform (DFT) problem, which subsequently permits the volume-conductor problem to be viewed as an equivalent linear-filtering problem. This DFT formulation is fast and computationally efficient. In addition, it lends itself to the application of some rather well-known techniques in linear systems theory (e.g., the DFT for convolution and least mean-square (Wiener) filtering for optimal prediction of a signal in random noise). Two specific examples are employed to demonstrate the utility of this discrete Fourier method: (1) the single, isolated, active nerve fiber in an essentially infinite volume conductor and (2) the isolated, active nerve trunk in a similar type of extracellular medium. In each of these, our DFT method is employed to obtain both the classical "forward" and "inverse" potential solutions for each volume conductor problem. In the case where the single, active nerve fiber is the bioelectric source in the volume conductor, simulated action-potential data from an invertebrate giant axon is utilized, and potentials at various points in the extracellular medium are calculated. The calculated potential distributions in axial distance z, at various radial distances r, are consistent with well-known experimental fact. When the active nerve trunk acts as the bioelectric source, the DFT method provides calculated potential distributions that are fairly consistent with experimental data under a variety of experimental conditions. For example, in these experiments, a special, isolated frog spinal cord preparation is used that permits separate or combined stimulation of the motor and sensory nerve fiber components of the attached sciatic nerve trunk. By manipulating the stimulus intensity applied to the motor (ventral) or appropriate sensory (dorsal) roots of the spinal cord, a variety of multiphasic extracellular volume-conductor potentials can be recorded from the sciatic nerve. The excellent agreement of model-generated and experimental data, regardless of the complexity of surface potential waveform, tends to validate the modeling assumptions and offer encouragement that this computationally efficient DFT method may be usefully employed in volume-conductor problems where both the bioelectric source, and the surrounding volume conductor, are of a much more complicated nature.Item Effects of lysine, threonine, and methionine on light-driven protein synthesis in isolated pea (Pisum sativum L.) chloroplasts(Planta, 1978) Mills, RonaldLight-driven incorporation of [14C]leucine (LEU) into protein in isolated pea chloroplasts was inhibited by 0.1 and 1 mM lysine (LYS), 0.1–10 mM threonine (THR) and 1 mM methionine (MET). Equimolar combinations of LYS plus THR were inhibitory at both 0.1 and 0.5 mM. Incorporation of [14C] aspartic acid (ASP) and [3H]tyrosine (TYR) was also reduced by 1 mM LYS or THR. In the cases tested, LYS and/or THR inhibitions were partially or fully reversed by 0.1 mM MET. [35S]MET incorporation was unaffected or stimulated by LYS and THR. These data are consistent with the hypothesis that MET is biosynthesized in isolated chloroplasts and that its synthesis is regulated by LYS and/or THR. Of 16 other amino acids tested at 1 mM, isoleucine, phenylalanine, tryptophan, tyrosine and valine inhibited protein synthesis.Item Amino acid biosynthesis in isolated pea chloroplasts: metabolism of labeled aspartate and sulfate(FEBS Letters, 1978) Mills, RonaldAccumulating evidence points to chloroplasts as the site of nitrite and sulfate reduction and incorporation into carbon skeletons [I] Furthermore, several enzymes of aspartate-family amino acid biosynthesis recently have been localized in plastids. These are : aspartate kinase [2] , homoserine dehydrogenase [3] , diaminopimelate decarboxylase [4] , acetolactate synthetase [S] and homocysteine-dependent 5-methyltetrahydrqpteroyl glutamate transmethylase [6]. In whole plants Lys and/or Thr have been shown to regulate the production of the aspartatederived amino acids [7]. Biosynthesis of this family of amino acids and its regulation in isolated chloroplasts has, however, received little attention. In this report we describe the biosynthesis of amino acids in pea chloroplasts from labeled aspartate and sulfate.Item Itracellular localization of aspartate kinase in spinach (Spinacea oleracea)(FEBS Letters, 1979) Mills, Ronald1. Enzyme distribution between chloroplasts and the nonchloroplast parts of green leaf cells of Spinacia oleracea, Nicotiana rustica, Vicia faba, and Phaseolus vulgaris have been investigated by use of the nonaqueous chloroplast isolation technique. Whereas pyruvate kinase and peroxidase were located only or mainly outside of the chloroplasts, the other enzymes studied, isocitric dehydrogenase, glutathione reductase, NAD- and NADP-dependent pyridine nucleotide quinone reductase, malic dehydrogenase, NAD- and NADP-dependent glyoxylate reductase, glutamate-oxaloacetate transaminase, NAD-dependent glutamic dehydrogenase, and NADP-dependent aspartic dehydrogenase were both inside and outside of the plastids. In contrast, NADP-dependent glyceraldehyde-3-phosphate dehydrogenase is located only within the chloroplasts. 2. Intact isolated spinach chloroplasts incorporated only a very small amount of labeled carbon from 14CO2 into amino acids in the light. The addition of NH4Cl did not increase the amount of labeled amino acids and had no effect on the total amount of 14C fixed during short time photosynthesis. However, NH4⊕ caused changes in the pathway of carbon during photosynthesis. In the presence of NH4⊕, more 14C was incorporated into sugar monophosphates and phosphoglyceric acid than in the absence of NH4⊕. 3. 14C-labeled glycine and serine fed to intact isolated spinach chloroplasts were neither accumulated nor transformed into other compounds, but 14C-labeled glutamic acid was converted into glutamine. This transformation took place only in the light in chloroplasts containing an intact outer envelope. The addition of NH4⊕ and certain substrates and cofactors did not increase the rate of transformation. 4. The penetration of some amino acids and substrates through the outer envelope of the chloroplasts was investigated on aqueously isolated spinach plastids. It was found that a-ketoglutarate, oxaloacetate, pyruvate, aspartate, and alanine are able to penetrate the envelope although at least for some of these compounds the outer membrane of the chloroplasts acts as a partial barrier. 5. From the experiments reported here and in connection with the results published by other investigators it can be concluded that the most common amino acids such as glutamic acid, aspartic acid, alanine, glycine, and serine are able to penetrate through the outer envelope of the chloroplasts and the synthesis of these amino acids can occur in the leaf cells inside as well as outside of the chloroplasts.Item The isolation of a lysine sensitive aspartate kinase from pea leaves and its involvement in homoserine biosynthesis in isolated chloroplasts(FEBS Letters, 1979) Mills, RonaldHomoserine is an intermediate in the biosynthesis of the essential amino acids Thr and Met in microorganisms [I] and higher plants [2]. Pea seedlings synthesise massive amounts of Hse, mainly in the roots [3-51, although more recent studies suggest that Hse metabolism also takes place in the leaf [6]. Aspartate kinase (EC 2.7.2.4) catalyses the first reaction in Hse biosynthesis, and is subject to complex feedback re~lation by Lys, Thr and Met in microorganisms [7]. In various plants aspartate kinase is also inhibited by the same end products (reviewed [S I), but in pea seedlings the enzyme has only been shown to be inhibited by Thr [9]. This paper reports the light-dependent synthesis of Hse from Asp by isolated pea chloroplasts. The enzyme aspartate kinase has been isolated from pea leaves and chloroplasts and found to be sensitive to both Lys and Thr. We believe this is the first report of a Lys-sensitive aspartate kinase in legume plants.Item A rapid method for isolation of purified physiologically active chloroplasts, used to study the intracellular distribution of amino acids in pea leaves(Planta, 1980) Mills, RonaldA procedure is described for the rapid (<5 min) isolation of purified, physiologically active chloroplasts from Pisum sativum L. Mitochondrial and microbody contamination is substantially reduced and broken chloroplasts are excluded by washing through a layer containing a treated silica sol. On average the preparations contain 93% intact chloroplasts and show high rates of (14)CO2 fixation and CO2-dependent O2 evolution (over 100 μmol/mg chlorophyll(chl)/h); they are also able to carry out light-driven incorporation of leucine into protein (4 nmol/mg chl/h). The amino-acid contents of chloroplasts prepared from leaves and from leaf protoplasts have been determined. Asparagine is the most abundant amino acid in the pea chloroplast (>240 nmol/mg chl), even thought it is proportionately lower in the chloroplast relative to the rest of the cell. The chloroplasts contain about 20% of many of the amino acids of the cell, but for individual amino acids the percentage in the chloroplast ranges from 8 to 40% of the cell total. Glutamic acid, glutamine and aspartic acid are enriched in the chloroplasts, while asparagine, homoserine and β-(isoxazolin-5-one-2-yl)-alanine are relatively lower. Leakage of amino acids from the chloroplast during preparation or repeated washing was ca. 20%. Some differences exist between the amino-acid composition of chloroplasts isolated from intact leaves and from protoplasts. In particular, γ-aminobutyric acid accumulates to high levels, while homoserine and glutamic acid decrease, during protoplast formation and breakage.Item Identification and measurement of homoserine by gas-liquid chromatography(Analytical Biochemistry, 1980) Mills, RonaldHomoserine has been analyzed quantitatively by gas-liquid chromatography of its N-heptafluorobutyryl isopropyl ester. The method was confirmed by analysis of the soluble amino acid fraction of pea leaves. The possible use of the method for analysis of methionine is discussed.Item Photosynthetic formation of the aspartate family of amino acids in isolated chloroplasts(Plant Physiology, 1980) Mills, RonaldThe metabolism of 14C-labeled aspartic acid, diaminopimelic acid, malic acid and threonine by isolated pea (Pisum sativum L.) chloroplasts was examined. Light enhanced the incorporation of [14C] aspartic acid into soluble homoserine, isoleucine, lysine, methionine and threonine and protein-bound aspartic acid plus asparagine, isoleucine, lysine, and threonine. Lysine (2 millimolar) inhibited its own formation as well as that of homoserine, isoleucine and threonine. Threonine (2 millimolar) inhibited its own synthesis and that of homoserine but had only a small effect on isoleucine and lysine formation. Lysine and threonine (2 millimolar each) in combination strongly inhibited their own synthesis as well as that of homoserine. Radioactive [1,7-14C]diaminopimelic acid was readily converted into [14C]threonine in the light and its labeling was reduced by exogenous isoleucine (2 millimolar) or a combination of leucine and valine (2 millimolar each). The strong light stimulation of amino acid formation illustrates the point that photosynthetic energy is used in situ for amino acid and protein biosynthesis, not solely for CO2 fixation.Item Trends in Computer Based Energy Management Systems(University of Houston Clear Lake, 1980-12) Harman, Thomas L.Abstract not available.Item Amino acid biosynthesis in chloroplasts(Balaban International Science Service, 1981) Mills, RonaldItem The assimilation of nitrogen and the synthesis of amino acids in chloroplasts and blue-green bacteria(Elsevier, 1982) Mills, RonaldItem Item The Design of a Microprogrammed Controller Using the AM29116 Bipolar Microprocessor(University of Houston Clear Lake, 1982-11) Harman, Thomas L.; Kadri, R.Abstract not available.