Biomedical subjects
Publications and source records attributed to T Letellier.
Experimental values of flux control coefficients are usually small. We examine assumptions about the nature of metabolic networks or of the rate equations that might lead to this result. We show that a general result cannot be obtained and we discuss this question with some examples.
Threshold effects in the expression of metabolic diseases have often been observed in mitochondrial pathologies, i.e. the clinical demonstration of the disease appears only when the activity of a step has been reduced to a rather low level. We show experimentally that an inhibition of cytochrome c oxidase activity by cyanide, simulating a defect in this step, leads to a decrease in mitochondrial respiration which then exhibits a threshold behaviour similar to that observed in mitochondrial diseases. We discuss this behaviour in terms of metabolic control theory and construct a mathematical model simulating this behaviour.
The control of oxidative phosphorylation has been studied in normal skeletal muscle isolated from the hind legs of rats. The control coefficients of different steps of oxidative phosphorylation on the flux of O2 consumption were determined by the inhibitor method and calculation was done according to the model of Gellerich et al. (FEBS Lett. 274 (1990) 167-170) using a non-linear regression fitting procedure. The respiration was recorded with pyruvate (+malate) and palmitoyl-L-carnitine (+malate) as respiratory substrates, which are the main substrates oxidized in the muscle. It appears that the control is broadly distributed among the different complexes of the respiratory chain, and of the ATP synthesis system. Our results also provide an explanation for the threshold effects often evidenced in the clinical manifestation of mitochondrial diseases.
We report a case of mitochondrial myopathy (MM), assessed by histological and biochemical studies. This illness was diagnosed in a 69-year-old patient with myocardiopathy revealed by ventricular arrhythmias. The originality of this case lies in the patient's age, the mode of onset and the biochemical features (i.e. normal mitochondrial enzymatic complexes but very low respiration when using glutamate as a substrate).
The onset of severe muscle cramps in the course of road racing led a champion to undergo a muscle biopsy. Fuscinophilic deposits were detected around muscle cell fibers together with a few PAS positive deposits. These findings were related to the accumulation of abnormal mitochondria and glycogen granules. The subsequent study of the mitochondrial respiratory chain did not reveal any modification and so ruled out a genuine mitochondrial myopathy. Such mitochondrial modifications are probably related to the onset of muscle cramps, but no metabolic disturbance could be found to account for these two phenomena.
Substrate cycles are ubiquitous structures of the cellular metabolism (e.g. Krebs cycle, fatty acids beta-oxydation cycles, etc...). Moiety-conserved cycles (e.g. adenine nucleotides and NADH/NAD, etc...) are also important. The role played by such cycles in the metabolism and its regulation is not clearly understood so far. However, it was shown that these cycles can generate multistationarity (bistability), irreversible transitions, enhancement of sensitivity, temporal oscillations and chaotic motions (Hervagault & Canu, 1987; Hervagault & Cimino, 1989; Reich & Sel'kov, 1981; Ricard & Soulié, 1982). [formula: see text] Fig. 1: Scheme of the open binary substrate cycle under study. The substrate S is converted into P with a net rate v2. Substrate P is converted in turn into S with a net rate v3. Step v2 is inhibited by excess of the substrate, S. In addition, the cycle operates under open conditions, that is zero-order input of S at rates alpha 0(v1) and first order outputs of S and P at rates alpha S and alpha P(v4), respectively. The metabolic control theory (see also Fell, 1990), which shows how a metabolic network reacts to small perturbations in the vicinity of a steady state, and is formulated with the so-called "control coefficients", was applied to such a cycle in order to get a better knowledge on the importance of each step at the regulatory point of view. The behaviour of a binary substrate cycle (fig. 1) in which one of the enzymes may be subjected to inhibition by excess of its substrate (v2) was studied theoretically.(ABSTRACT TRUNCATED AT 250 WORDS)
Mitochondrial cytopathies are multisystemic diseases of extremely variable expression caused by a deficiency in oxidative phosphorylation. Only five cases of neonatal liver failure in the context of mitochondrial cytopathy have been reported, with incomplete morphological data of the liver in three. In the case presented here, ascites had been diagnosed prenatally and liver failure was particularly severe (factor V less than 15% with fatal coma the fourth day). Histologically there were incomplete cirrhosis, microvesicular steatosis, major canalicular cholestasis with proliferative neocholangioles, and bile duct thrombi. There were also some iron pigments in the periportal area and partial glycogen depletion. By electron microscopy, mitochondria in numerous hepatocytes appeared abnormal with occasional cristae in a fluffy matrix, some containing dense inclusions. Study of respiratory chain activity showed a defect in cytochrome c oxidase (complex IV), revealed by oxygraphic measurement on fresh muscle biopsy and confirmed by spectrophotometric enzymatic assays performed on muscle and liver homogenates. The association of neonatal liver failure with hyperlactacidemia warrants investigation into a deficiency in oxidative phosphorylation.
Respiratory parameters of skeletal muscle were determined in permeabilized muscle fibers by adapting a technique described by Veksler et al. for cardiac fibers (Biochim Biophys Acta, 892:191-196, 1987). This method consists of the permeabilization of muscle fibers by saponin by allowing respiratory substrates and inhibitors to reach the mitochondria. In this way, the mitochondria may be studied inside the fibers as if they were isolated. We have verified, using various techniques, that the mitochondria remain intact during this procedure. This method has been applied to the study of six newborn infants for whom a diagnosis of a mitochondrial defect was suspected. In all cases, the defect was to be found on the permeabilized fibers, and this was confirmed by an enzymatic study. The advantage of this new method, associated with the measurement of the enzymatic activities on a crude homogenate, is to enable a simple and rapid diagnosis on a small amount of sample without damaging the mitochondria during the isolation procedure.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
The program CONTROL is based on metabolic control theory and uses the method developed by Reder (1988). In this theory, two sets of parameters are defined in the vicinity of a steady-state: the elasticity coefficients which describe the local behaviour of the isolated enzymes, and the control coefficients which express the response of the whole metabolic network to perturbations at a given step. The theory shows that relationships exist between the control coefficients (summation relationships or structural relationships) and also between the two types of coefficients (control and elasticity coefficients: connectivity relationships). The program CONTROL is divided into two parts (sub-menus). The first one calculates all the control coefficients (flux and concentrations) of a metabolic network from the elasticity coefficients. Using the second menu, the symbolic relationships are obtained between the control coefficients (summation relationships) and between the control coefficients and the elasticity coefficients (connectivity relationships). These two sub-menus can be applied independently to any metabolic network (to date limited to 19 steps and 19 metabolites).
The definitions of flux control coefficients and elasticity coefficients as given in the Metabolic Control Theory are introduced by geometrical considerations. It is shown that a method to determine the (non-normalized) control coefficients from the elasticity coefficients as well as the well-known summation and connectivity relationships can easily be derived in a geometric way. This method is explained in the light of a simple example, but it can be applied to any metabolic system. A software implementation (program "control") and possible applications to biotechnology are briefly presented.
The expression of an enzymatic deficiency in a metabolic network can present a biochemical threshold. This threshold can be characterised thus: (1) a low activity of the enzyme can sustain a normal flux, but (2) a minute further decrease of its activity makes the flux collapse. We give simple mathematical models displaying such a behaviour, and we apply the models to some examples of oxidative phosphorylation dependency on respiratory chain complex deficiency.