Biomedical subjects

M J Achs

Publications and source records attributed to M J Achs.

At least 19 recordsLinked to original sources

PubMed1991

A computer model of pancreatic islet glycolysis.

We have modeled an experiment with perifused pancreatic islet cells using our BIOSSIM language. The experiment and the resulting model are concerned with glucose uptake and glycolysis by the beta-cells of pancreatic islets. Although glycolysis appears to be involved in insulin release, we do not have enough information to represent insulin release in detail. The rapid entry of glucose into the beta-cell is promoted by a carrier having a very high tissue capacity. Phosphorylation of glucose by the low affinity enzyme glucokinase appears to be limiting for glycolysis. The effects of several hexose diphosphate activators of phosphofructokinase are modeled. Model behavior is described. The kinetic parameters of the enzyme submodels are given. Because of the difficulties of preparing large amounts of experimental material, information on pancreatic islet metabolism is limited. This model is a plausible explanation of the experimental results. Recent work on the genetically engineered glucose transporter and glucokinase is discussed.

Animals↗

PubMed1988

Pancreatic islet discrimination of hexose anomers. I. Steady-state computer simulation.

Pancreatic islets detect glucose level by phosphorylating it and converting the glycolytic rate to a signal to secrete insulin. Insulin secretion is greater from the alpha- than from the beta-anomer when the D-glucose level is below 22 mM. D-mannose behaves similarly but at nearly twofold higher concentrations. Two explanations have been proposed: 1) glucokinase, which has the same anomeric preference, is the principal hexose phosphorylating enzyme and limits glycolytic rate. 2) Phosphofructokinase limits glycolysis and hexokinase is the principal enzyme phosphorylating hexose; hexosediphosphate activators of phosphofructokinase are more readily synthesized from alpha-anomers of hexose phosphates. We have simulated both alternatives with a detailed anomerically specific model of the hexose-metabolizing glycolytic enzymes. The pathway preference for alpha-anomer of both hexoses was adequately reproduced with anomerically active limiting glucokinase. The other mechanism did not reproduce the observed pathway preference.

Animals↗

PubMed1988

Pancreatic islet discrimination of hexose anomers. II. Transient computer simulation.

We have previously modeled pancreatic islet glycolysis under idealized steady-state conditions where the input is a pure hexose anomer and there is no mutarotation and reproduced the known preference for the alpha-anomers of glucose and mannose as substrates. This model is here extended to simulate real experiments, where the hexoses mutarotate and measurements may be taken over time. The behavior of our model system agrees with available experimental data. The hexose diphosphate activators of phosphofructokinase, whose effect was seen as not important in the preceding steady-state analysis, are found here to have a modest (approximately 10-15%) effect on its flux. The previous conclusion that the anomeric preference of the glycolytic pathway follows from that of glucokinase continues to hold in the real experimental situation.

Animals↗

PubMed1987

Modeling and artificial intelligence approaches to enzyme systems.

Modeling is a means of formulating and testing complex hypotheses. Useful modeling is now possible with biological laboratory microcomputers with which experimenters feel comfortable. Artificial intelligence (AI) is sufficiently similar to modeling that AI techniques, now becoming usable on microcomputers, are applicable to modeling. Microcomputer and AI applications to physiological system studies with multienzyme models and with kinetic models of isolated enzymes are described. Using an IBM PC microcomputer, we have been able to fit kinetic enzyme models; to extend this process to design kinetic experiments by determining the optimal conditions; and to construct an enzyme (hexokinase) kinetics data base. We have also used a PC to do most of the constructing of complex multienzyme models, initially with small simple BASIC programs; alternative methods with standard spreadsheet or data base programs have been defined. Formulating and solving differential equations in appropriate representational languages, and sensitivity analysis, are soon likely to be feasible with PCs. Much of the modeling process can be stated in terms of AI expert systems, using sets of rules for fitting and evaluating models and designing further experiments. AI techniques also permit critiquing and evaluating the data, experiments, and hypotheses being modeled, and can be extended to supervise the calculations involved.

Artificial Intelligence↗

PubMed1982

Computer simulation of energy metabolism in acidotic cardiac ischemia.

Construction and fit to experimental data of a computer model of glycolysis, the tricarboxylic acid cycle, and related metabolism in the perfused rat heart involving 63 enzyme submodels is described. The experimental preparation simulated is a rat heart perfused with Krebs bicarbonate solution containing glucose and insulin whose pH was lowered to 6.6 by equilibration with 35% CO2-65% O2. The glycolytic rate falls sharply and ischemia results, becoming apparent after 3.5 min. The model initially ascribes the fall in glycolysis largely to inhibition of hexokinase by accumulated glucose 6-phosphate and inactivation of phosphofructokinase by the low pH and subsequently to cytoplasmic glucose depletion owing to limitation of glucose uptake by the external acidosis. At the same time there is insufficiently deep hypoxia to trigger substantial mobilization of endogenous fuels (e.g., glycogenolysis or fatty acid mobilization, so that these hearts become ischemic primarily owing to a shortage of metabolic fuel.

Acidosis↗

PubMed1982

Computer simulation of metabolism of glucose-perfused rat heart in a work-jump.

A computer model of glycolysis, the tricarboxylic acid cycle, and related amino acid metabolism, is described for a glucose-perfused experimental rat heart preparation suddenly switched from low work load (Langendorff perfusion) to high work load (left atrial perfusion). Glycolytic intermediate measurements suggest activation of phosphofructokinase within a few seconds. This activation, and also that of other glycolytic enzymes, is calculated as due to a sharp increase in cytoplasmic Mg2+ level, which overcomes the inhibitory effects of a rapid fall in cytoplasmic pH to 6.77 (calculated from a rapid fall in creatine phosphate). Increased glycolytic substrate is initially supplied by glycogenolysis mediated by phosphorylase b (activated by an early rise in cytoplasmic AMP), followed by increased glucose uptake from the perfusate. Testable predictions are made by the model, especially that lactate production rate should peak early. Additional experiments are described that verify these predictions and fill gaps in the original measurements. The role of modeling in interpreting such experiments is discussed.

Amino Acids↗

PubMed1979

Metabolism of the acutely ischemic dog heart. I. Construction of a computer model.

Construction of a computer model of glycolysis, the Krebs cycle, and related metabolism in acutely ischemic dog heart, involving 122 metabolites, 65 enzymes, and 406 chemical reactions, is described. A previous model of the same metabolism in normal-flow rat heart was modified to fit ischemic dog heart experimental data to within experimental error. The result resembles other models of ischemic heart preparations, implying common underlying mechanisms. The principal change made was reduction of enzyme amounts, consistent with the generally slower metabolism of large animals, suggesting that differing enzyme amounts are a major component of interspecies metabolic difference. Glycolytic intermediates oscillate, asynchronously and with large changes in level; pyridine nucleotides become highly reduced; pH falls, but mitochondria stay alkaline relative to cytoplasm even after oxidative phosphorylation stops.

Animals↗

PubMed1979

Metabolism of the acutely ischemic dog heart. II. Interpretation of a model.

The glycolytic oscillations occurring in an acutely ischemic dog heart are analyzed with a computer model. The major regulations of the glycolytic pathway flux occur at phosphohexose isomerase, which is inhibited by accumulated pentose shunt intermediates; at phosphorylase, which shapes the first cycle of the oscillation; and at aldolase, which shapes the last two cycles. Aldolase is not under normal substrate control. Its activity, and that of some subsequent glycolytic enzymes, appears to be regulated by known interactions with the muscle proteins. The mitochondria become reduced as a result of anoxia, and their metabolism reorganizes to export rather than import reducing equivalents. It is in general feasible to account for the behavior of this preparation in terms of the known metabolism of less severely perturbed hearts, especially (but not completely) in terms of effects of anoxia. The reasons for the inapplicability of the crossover theorem previously used to analyze this preparation are described.

Animals↗

PubMed1979

Computer simulation of metabolism in pyruvate-perfused rat heart. I. Model construction.

A computer model of energy metabolism was constructed for the pyruvate-perfused rat heart subjected to a sudden increase in work load. The model construction techniques are explained and justified. Hypotheses defining a plausible sequence of physiological events resulting in the observed behavior of the organ were arrived at from the measured metabolite time profiles. There is a lag in increasing pyruvate utilization following the work-jump. A transient increase in fatty acid oxidation is required to account for the observed respiration rate. The observed increase in lactate production of this preparation is due to a transient "burst" of glycogenolysis after which the remaining lactate is imported from the interstitium and oxidized, reversing lactate dehydrogenase and hence the alpha-ketoglutarate-malate portion of the malate-aspartate shuttle.

Animals↗

PubMed1979

Computer simulation of metabolism in pyruvate-perfused rat heart. II. Krebs cycle.

A realistic metabolic model of the tricarboxylic acid cycle in the perfused rat heart was constructed to help explain the sequence of biochemical events regulating the metabolism of exogenous pyruvate following a large increase in work load. The unchelated Mg2+ level was the most important controlling factor. The resulting mixture of chelated and unchelated nucleotides and tribasic acids effected coordinated control of citrate synthase, aconitase, isocitrate dehydrogenase, succinyl CoA synthetase, fumarase, and nucleoside diphosphokinase, because Mg2+-chelates are generally substrates whereas unchelated species are inhibitors. Succinate dehydrogenase is largely controlled by the ubiquinone redox potential. The fluxes through alpha-ketoglutarate and malate dehydrogenases are largely dependent on thepyridine nucleotide redox potential, but the succinyl CoA-to-CoASH ratio strongly affects the former enzyme as well. The model predicts an accumulation of succinate during the transition to higher work output.

Animals↗

PubMed1979

Computer simulation of metabolism in pyruvate-perfused rat heart. III. Pyruvate dehydrogenase.

A physiologically and biochemically realistic model of the regulation of pyruvate dehydrogenase complex (PDH) was constructed for the perfused rat heart. It includes conversion between inactive (phospho) and active (dephospho) forms by a specific protein kinase (PDHK) and phosphoprotein phosphatase (PDHP). The activity of the tightly bound PDHK is influenced by synergistic activation/inhibition by acetyl CoA/CoASH and NADH/NAD. PDHK in this simulation was more sensitive to the fraction of ADP that was Mg2+-chelated than to the ATP-to-ADP ratio. Ca2+ stimulates binding of Mg2+-dependent PDHP to the complex; the bound enzyme was considered to be the active species. The fraction of PDH in the active form, rather than substrate and inhibitor levels, determines PDH activity under these conditions. This fraction depends on the present value and recent history of the difference between PDHK and PDHP activities. Both of these are active continuously and continuously control PDH.

Animals↗

PubMed1979

Computer simulation of metabolism in pyruvate-perfused rat heart. IV. Model behavior.

The behavior of a computer model of energy metabolism was determined for perfused rat hearts utilizing pyruvate as sole exogenous fuel and subjected to a rapid increase in work load. Computer-generated metabolite profiles, which are solutions of the differential equations for 1 min elapsed time, closely match 12 experimental curves (involving 120 concentration measurements) and exhibit the following properties. The computed cytosolic pyruvate level oscillates due to large changes in the rates of the processes that produce and consume this metabolite. Cytosolic Mg2+ seems to act as a coordinated controller of glycolytic enzymes; its transient increase permits a transient increase of glycolysis without an accumulation of glucose 6-phosphate. Lactate is exported to the interstitium by a lactate permease and then reimported and oxidized. As a result, the malate-aspartate shuttle reverses direction, and the Krebs cycle is "unspanned."

Animals↗

PubMed1979

Computer simulation of metabolism in pyruvate-perfused rat heart. V. Physiological implications.

The results of a simulation of metabolism in the pyruvate-perfused rat heart subjected to a sudden increase in work load are interpreted to provide a coherent explanation for the observed physiology. Respiration is most closely correlated with the mitochondrial phosphate potential, calculated from the MgATP and MgADP levels. No correlation between respiration and the pH gradient across the mitochondrial membrane was found. The transient falls in pH in the cytosol and perhaps the mitochondria are due largely to carbonic and lactic acidosis and appear to be only weakly coupled. The heart maintains a high ATP level during the transition to increased work by utilizing its energy reserves in order of decreasing availability in response to physiological signals mediated by Mg2+, Ca2+, and cAMP.

Animals↗

PubMed1979

Metabolism of totally ischemic excised dog heart. I. Construction of a computer model.

Construction and fit to the experimental data of a computer model of glycolysis, the Krebs cycle, and related metabolism in an ischemic dog heart preparation, involving 122 metabolites, 65 enzymes, and 406 chemical reactions, is described. The experimental preparation simulated is a dog heart excised from the body, placed in a beaker of Tyrode's solution, and sampled for 100 min; the model required only moderate modification from models representing perfused rat hearts, and little modification from a model of another ischemic dog heart preparation. Common underlying mechanisms for the ischemia are indicated, although this preparation appears to evolve more slowly with time, perhpas owing to heavy sedation and diffusion-limited transport. Lactate is, at first, exported and then accumulates intracellularly; pH falls, but not as much in the mitochondria as the cytoplasm; redox couples go reduced, but with counterintuitive time courses; calcium phosphate is calculated to precipitate, as often observed in cardiac ischemia.

Adenine Nucleotides↗

PubMed1979

Metabolism of totally ischemic excised dog heart. II. Interpretation of a computer model.

Analysis of the ischemic dog heart preparation described in the preceding paper indicates that it is an analogue in slow motion of the tissue in the center of a cardiac infarct. It is respiring very slowly and not capable of performing mechanical work. Glycolysis starts up with both glucose and glycogen as inputs. Later hexokinase and to some extent phosphofructokinase become limiting owing to inhibitor accumulation or acidosis. Metabolism then results primarily from cAMP-driven glycogenolysis, largely limited by the glycogen debranching enzymes at later times, with accumultion not only of lactate and alpha-glycerophosphate but of glucose as well. Amino acid levels oscillate with time while fatty acids accumulate at late times. The elevation of cAMP at later times may involve disturbances in its metabolism as well as mechanisms such as adenosine accumulation that are more important in cardiac ischemia than in normal heart. The clinical implications of this behavior are discussed.

Amino Acids↗

PubMed1977

Distribution of adenine nucleotides in the perfused rat heart.

A computer technique for determination of the distribution of adenine nucleotides among compartmented, protonated, and metal-chelated species has been developed for the perfused rat heart. This procedure requires knowledge of tissue levels of creatine, creatine phosphate, ATP, ADP, and AMP and the glycolytic and respiration rates. The method is applicable to any physiological state of the organ and has been applied to transient behavior in aerobic, anoxic, and ischemic hearts. The results suggest that ADP uptake and ATP export by mitochondria are normally linked and equal in rate during aerobic metabolism or short-term anoxia but become separate and unequal during ischemia, so that mitochondrial adenine nucleotides, primarily AMP, accumulate.

Actins↗

PubMed1977

Computer simulation of energy metabolism in anoxic perfused rat heart.

We have modeled the energy metabolism of the perfused rat heart in order to elucidate the interaction of physiological and biochemical control mechanisms. This model which includes glycolysis, the Krebs cycle, and related metabolism, contains 68 submodels of individual enzymes and transport mechanisms including both cytosolic and mitochondrial reactions. The method of model construction, which relies heavily on fitting observed in situ behavior to known algebraic rate laws for isolated enzymes, and its data requirements and necessary assumptions are described. Simulation of a CO-induced anoxic preparation is described in detail. Here glycolysis increases sharply, due to both increased glucose uptake and phosphorylase activation (there is rapid interconversion between a and b forms, both of which are active here); this causes a damped glycolytic oscillation originating with the glycogen-handling enzymes rather than phosphofructokinase. The behavior and physiological consequences of ATPase activity and of a lactate permease which exports lactate to the perfusate are discussed.

Adenine Nucleotides↗

PubMed1977

Computer simulation of rat heart metabolism after adding glucose to the perfusate.

An experiment where perfused rat hearts receiving no substrate are suddenly given glucose with insulin in the perfusate is simulated with a computer model of cardiac energy metabolism. Mitochondrial metabolism is quantitatively reorganized under cytoplasmic control, with fatty acid oxidation undergoing a two-step decrease. There is an unspanning of the Krebs cycle (different reactions going at different rates) due primarily to slowing of alpha-ketoglutarate dehydrogenase; this ends when cytoplasmic glucose reaches a new steady state. Mitochondria in vitro are known to have higher pH than their surroundings; it is found here that this also holds in situ. Under these conditions, glycolysis is coherently substrate controlled, as is phosphofructokinase, usually considered the typical example of an allosteric enzyme. Limitations on simple methods of analyzing metabolic data of this type, e.g., use of lactate/pyruvate ratios to calculate NADH/NAD ratios, are discussed. Here a large volume of enzyme and other biochemical information has been integrated into a physiologically meaningful system.

Amino Acids↗