Control analysis of muscle glycogen metabolism.

A minimal model of glycogen metabolism in muscle tissue is analyzed in accordance with metabolic control analysis. The model contains two branch points. Rather than contributing to complexity of the analysis, this branching allows expression of the control coefficients in a simplified form. Glucose 6-phosphate is the metabolite at the first branch point, and the analysis is simplified further by the fact that glucose 6-phosphate is the substrate for enzymes which catalyze near-equilibrium reactions. Control of the concentration of glucose 6-phosphate is of interest because of its pivotal location in the metabolic system, but also because it interacts with an allosteric site on glycogen synthase to stimulate glycogen synthase activity. It is shown that the control which the transporter and enzymes involved in glycogen synthesis exert on glycolytic flux is proportional to the control which these components exert on glucose 6-phosphate concentration. Thus, glycolysis plays a major role in control of glucose 6-phosphate concentration. It is concluded that control of glycogen synthesis is not a rigid parameter of any component of this metabolic system. Rather the distribution of control is flexible and shifts from one portion of the system to another in response to shifts in the physiological state. An important element in determining the distribution of control of glycogen synthesis is the change in the sensitivity of the allosteric site of glycogen synthase to glucose 6-phosphate which is brought about by conversion of glycogen synthase to the dephosphorylated, glucose 6-phosphate-independent, state.

Control analysis of single enzyme sequences with abortive complexes and random substrate binding.

Single-enzyme reactions involving abortive complexes, and random sequences, respectively, are subjected to control analysis. Explicit analytical expressions are presented which cover these kinetic behaviors. The latter are based (1) on the concept of control coefficients which measure the sensitivity of flux with respect to rate constants, and (2) on the classical steady state rate equations. The methods include both a graph theoretic approach and computer-aided derivation of algebraic expressions. Some conclusions are derived from the analysis of simple models. It is demonstrated (1) that abortive complexes exert no kinetic (as opposed to equilibrium) control over steady state flux; (2) the sum of the paired flux control coefficients for each step in the catalytic cycle, as well as the sum of the flux control coefficients for the unidirectional steps which emanate from each enzyme species, is equal to unity in a random sequence; (3) in the case of a random reaction sequence, the numerator terms of the rate equation exert an effect in the paired flux control coefficients for those steps in the random portion of the reaction sequence.

Kinetic selectivity of cholinephosphotransferase in mouse liver: the Km for CDP-choline depends on diacylglycerol structure.

The effects of different 1,2-diacyl-sn-glycerols on the kinetic properties of CDP-choline:1,2-diacylglycerol cholinephosphotransferase (EC 2.7.8.2) from mouse liver microsomes have been studied. Initial-velocity experiments were carried out with various concentrations of several species of diacylglycerol at different fixed concentrations of CDP-choline. Kinetic analysis of these data showed a family of intersecting lines consistent with a sequential kinetic mechanism of catalysis. The Km and Vmax. values derived from rate data revealed a pronounced effect of diacylglycerol species utilization on the Km value for CDP-choline. There was a biphasic relationship between diacylglycerol chain length and the Km for CDP-choline. Substitution of an unsaturated fatty acid in the sn-2 position of distearin also dramatically increased the CDP-choline Km value as well as the Vmax. 1,2-Dipalmitoyl-sn-glycerol was the preferred substrate over other disaturated species, but 1,2-dihexanoyl-sn-glycerol could not be utilized. These results demonstrate the kinetic mechanism of in vitro catalysis and suggest a regulatory role for CDP-choline concentration in the diacylglycerol species selectivity of cholinephosphotransferase resulting in the de novo biosynthesis of different molecular species of phosphatidylcholine.

Interpretation of nutrient-response relationships in rats.

Nutrient-response curves are nonlinear and, as is true of other curves that define the rates of biological reactions, these curves can be described by rational polynomials. These nutrient-response curves presumably reflect the metabolic fate of the nutrient. Data reported for the accumulation of body nitrogen in rats fed three different proteins are interpreted in terms of the biochemical systems theory and metabolic control theories. Analysis of the rational polynomials that define the nutrient-response curves permits partitioning the nutrient between a number of grossly defined metabolic pathways. The power law formalism of the biochemical systems theory is discussed with respect to the well-established relationship between body weight and basal energy requirement.

Algorithms for the derivation of Flux and Concentration Control Coefficients.

A computer program is described that derives the Flux and Concentration Control Coefficients for linear and branched metabolic pathways. The program prompts the user to enter a concise description of the metabolic pathway. From this description, the program constructs the appropriate equations in matrix form. The algorithm employed to obtain the symbolic determinants is described, and this algorithm also provides a convenient method for manual derivation of the Control Coefficients. The computer-based method will accommodate unlimited feed-forward and feed-backward loops and a maximum of two branches from each metabolite on the main pathway. The utility of the method is illustrated with a linear path with feed-forward and feed-backward loops, and with a substrate cycle as an example of a path with two branches.

Effects of product inhibition in metabolic pathways: stability and control.

The effect of product inhibition in metabolic pathways is examined using (a) an unbranched pathway in the absence of endproduct inhibition and (b) an unbranched pathway with endproduct inhibition. It is shown that product inhibition may be considered an alternative mechanism to endproduct inhibition for reducing the overall logarithmic gain of an unregulated pathway. When product inhibition and endproduct inhibition are both present, they act in concert with each other to lower the overall logarithmic gain and alleviate parameter sensitivities. Product inhibition is also found to exert a stabilizing influence that competes with the destabilizing effect of endproduct inhibition in controlling the dynamic behavior.

Energy metabolism in the whole animal revisited.

Dimensional analysis of animals with regard to those structures and functions which affect the rate of energy metabolism can be conducted on the basis of either geometric or elastic similarity. Predictions of the relationships between various structures and functions and body mass are dependent of the type of similarity assumed. Comparison of the relationships between numerous physiological variables and body mass reported in the literature and those predicted from geometric and elastic similarity indicates that the preponderance of predictions from elastic similarity are in better agreement with the observed values than are the predictions from geometric similarity. It appears that the components of the cardiovascular system are scaled in accordance with elastic similarity. For this reason, cardiac output is proportional to body mass raised to the 3/4 power. The biochemical basis for the relationship between cardiac output and the rate of energy metabolism is discussed.

Analysis of nutrient-response relationships.

A general mathematical model is proposed to relate biological response to nutrient intake. The mathematical expression is r = sigma k0 alpha ini/sigma l0 beta ini, k less than or equal to l, alpha 0 less than greater than or equal to 0, alpha 1 ... alpha k greater than or equal to 0, beta zero ... beta l greater than or equal to 0. The model provides for saturation behavior with respect to nutrient intake, but it also provides for a wide range of shapes to the nutrient-response curve. The characteristics of the nutrient-response curve, as well as the characteristics of transformations of the primary equation, are discussed in detail. Graphical methods are discussed for obtaining quantitative estimates of parameters that describe the nutrient-response relationship. The model is compared with other models used currently in nutrient-response analysis. The steps involved in the graphical procedure are summarized.

The purification and steady-state kinetic behaviour of rabbit heart mitochondrial NAD(P)+ malic enzyme.

The mitochondrial NAD(P)+ malic enzyme [EC 1.1.1.39, L-malate:NAD+ oxidoreductase (decarboxylating)] was purified from rabbit heart to a specific activity of 7 units (mumol/min)/mg at 23 degrees C. A study of the reductive carboxylation reaction indicates that this enzymic reaction is reversible. The rate of the reductive carboxylation reaction appears to be completely inhibited at an NADH concentration of 0.92 mM. A substrate saturation curve of this reaction with NADH as the varied substrate describes this inhibition. The apparent kinetic parameters for this reaction are Ka(NADH) = 239 microM and Vr = 1.1 mumol/min per mg at 23 degrees C. The steady-state product-inhibition patterns for pyruvate and NADH indicate a sequential binding of the substrates: NAD+ followed by L-malate. These data also indicate that NADH is the last product released. A steady-state kinetic model is proposed that incorporates NADH-enzyme dead-end complexes.

A comparative kinetic analysis of six substrates widely used for the detection and quantitation of phosphodiesterase I.

Six of the substrates currently used for the detection and quantitation of phosphodiesterase I (PDE I) were examined. Michaelis-Menten kinetic analysis of the hydrolysis of each of these by venom and bovine intestinal PDE I was performed to allow a comparison of the sensitivity of detection for each substrate. The results confirm that phenolic esters of phenylphosphonate are hydrolyzed 2- to 4-fold more rapidly than are the same esters of thymidine 5'-phosphate when both are present in saturating concentrations. The identification of the bovine intestinal enzyme as a PDE I is verified by its extensive similarity to the paradigmatical PDE I from rattlesnake venom. A new definition for PDE I activity is proposed which takes into account its ability to hydrolyze several types of substrates.

Simulation of energy metabolism in the simple-stomached animal.

1. A computer programme is described which simulates energy metabolism in the whole animal. Simulation was based on representation of the animal as a quasi-steady-state system. 2. Input for the programme consisted of the chemical composition of the diet and an estimate of either the maintenance energy requirement or an estimate of energy retention. 3. Simulation was performed by estimating the yield of adenosine triphosphate in the major metabolic pathways operative in simple-stomached animals, and on the utilization of adenosine triphosphate in major anabolic processes. 4. Results obtained from simulation were in close agreement with experimental observations reported by McCracken (1975).

Computer-based method for calculation of the available energy of proteins.

An aspect of protein nutrition that has not been resolved in a satisfactory manner is the utilizable energy equivalence of proteins. The existing methods overestimate the utilizable energy of proteins. A more accurate method fo calculating the utilizable energy of proteins is to calculate the moles of adenosine triphosphate formed during the complete oxidation of a given amount of proteins. The moles of adenosine triphosphate formed can be calculated from knowledge of the amino acid composition and knowledge of the metabolic pathway for each amino acid. A computer program is described that provides the bookkeeping required for these calculations. The available energy of a protein is calculated in this computer-based method by adjusting the energy value of the proteins so that it is equivalent to that of carbohydrates and fats in providing the energy for adenosine triphosphate formation. The computer-based method was used to calculate the available energy of a group of proteins of known amino acid composition. The available energy varied from 3.02 keal/g for colagen to 3