Development and implementation of standardized respiratory chain spectrophotometric assays for clinical diagnosis

Diversity of respiratory chain spectrophotometric assays may lead to difficult comparison of results between centers. The French network of mitochondrial diseases diagnostic centers undertook comparison of the results obtained with different protocols in the French diagnostic centers. The diversity of protocols was shown to have striking consequences, which prompted the network to undertake standardization and optimization of the protocols with respect to clinical diagnosis, i.e. high velocity while maintaining linear kinetics relative to time and enzyme concentration. Assays were set up on animal tissues and verified on control human muscle and fibroblasts. Influence of homogenization buffer and narrow range of optimal concentration of phosphate, substrate and tissue were shown. Experimental data and proposed protocols have been posted on a free access website. Their subsequent use in several diagnostic centers has improved consistency for all assays.

Gradual alteration of mitochondrial structure and function by beta-amyloids: importance of membrane viscosity changes, energy deprivation, reactive oxygen species production, and cytochrome c release

Intracellular amyloid beta-peptide (A beta) accumulation is considered to be a key pathogenic factor in sporadic Alzheimer's disease (AD), but the mechanisms by which it triggers neuronal dysfunction remain unclear. We hypothesized that gradual mitochondrial dysfunction could play a central role in both initiation and progression of sporadic AD. Thus, we analyzed changes in mitochondrial structure and function following direct exposure to increasing concentrations of A beta(1--42) and A beta(25--35) in order to look more closely at the relationships between mitochondrial membrane viscosity, ATP synthesis, ROS production, and cytochrome c release. Our results show the accumulation of monomeric A beta within rat brain and muscle mitochondria. Subsequently, we observed four different and additive modes of action of A beta, which were concentration dependent: (i) an increase in mitochondrial membrane viscosity with a concomitant decrease in ATP/O, (ii) respiratory chain complexes inhibition, (iii) a potentialization of ROS production, and (iv) cytochrome c release.

A novel Y243S mutation in the pyruvate dehydrogenase El alpha gene subunit: correlation with thiamine pyrophosphate interaction

We identified a new Y243S mutation in the X-linked E1 alpha-PDH gene in a patient with pyruvate dehydrogenase complex (PDHc) deficiency. The activity in cultured fibroblasts was very low even in the presence of high thiamine pyrophosphate (TPP) concentrations, indicating that the defect could be due to decreased affinity of PDHc for TPP.

Relationships between muscle mitochondrial metabolism and stress-induced corticosterone variations in rats

In order to determine the effect of chronic and acute stress on muscle mitochondrial metabolism, two strains of rats were selected on the basis of their different hypothalamo-pituitary-adrenal (HPA) axis responses to different stressors [Spontaneous Hypertensive Rats (SHR) and Lewis rats]. For 8 weeks animals were stressed by daily exposure to either a novel environment (SHR: n=16, Lewis: n=16) or forced exercise (SHR: n=16, Lewis: n=16). An unstressed group was left undisturbed (SHR: n=5, Lewis: n=5). Half of the stressed animals (n=32) were submitted to an acute stress (1-h immobilization). The mitochondrial responses of plantaris muscle [cytochrome-c-oxidase (COX), citrate synthase and succinate dehydrogenase activities, the latter two being measured as indices of functional mitochondrial amount] in the presence of different physiological plasma corticosterone (CORT) concentrations were analyzed. The novel environment and forced exercise stress induced different levels of plasma CORT which were negatively correlated with the amount of functional mitochondria in the plantaris muscle. Therefore, a chronic intermittent stress is able to induce an increase in plasma CORT which may be related to deleterious changes in muscle mitochondrial metabolism. Lastly, the acute stress was not associated with a decrease in functional mitochondria but with an increase in COX activity. This suggests that the relationship between CORT and muscle mitochondrial metabolism depends both on the level and duration of endogenous glucocorticoids exposure.

Effect of 'binary mitochondrial heteroplasmy' on respiration and ATP synthesis: implications for mitochondrial diseases

Respiratory-chain-complex subunits in mitochondria are encoded by nuclear or mitochondrial DNA. This property might have profound implications for the phenotypic expression of mutations affecting oxidative phosphorylation complexes. The aim of this paper is to study the importance of the origin of the mutation (nuclear or mitochondrial) on the expression of mitochondrial defects. We have therefore developed theoretical models illustrating three mechanisms of nuclear or mitochondrial DNA mutation giving rise to a deficiency in the respiratory-chain complex: (1) a partial deficiency, homogeneously distributed in all of the mitochondria; (2) a complete deficiency, only affecting some of the mitochondria ('binary mitochondrial heteroplasmy'); and (3) a partial deficiency, affecting only some of the mitochondria. We show that mutations affecting oxidative phosphorylation complexes will be expressed in different ways depending on their origins. Although the expression of nuclear or mitochondrial mutations is evidence of a biochemical threshold, we demonstrate that the threshold value depends on the origin and distribution of the mutation (homogeneous or not) and also on the energy demand of the tissue. This last prediction has been confirmed in an experimental model using hexokinase for the simulation of the energy demand and a variation in mitochondrial concentration. We also emphasize the possible role of 'binary mitochondrial heteroplasmy' in the expression of mitochondrial DNA mutations and thus the importance of the origin of the deficit (mutation) for the diagnosis or therapy of mitochondrial diseases.

What do mitochondrial diseases teach us about normal mitochondrial functions...that we already knew: threshold expression of mitochondrial defects

This paper shows how metabolic control analysis (MCA) can help to explain two important features of mitochondrial diseases: (i) the existence of a threshold in the expression of the complex deficiencies on the respiratory flux or on ATP synthesis, i.e. the fact that it is necessary to have a large complex deficiency in order to observe a substantial decrease in these fluxes; (ii) the tissue specificity, i.e. the fact that all tissues are not affected, even if the complex deficiency is present in all of them. We also show the limits of MCA, particularly when considering the in vivo situation. However, MCA offers a new way to consider mitochondrial diseases. The fact that fluxes only slightly change, when a complex is affected, is done at the expense of great changes in intermediate metabolite concentrations; intermediate metabolites situated upstream from the deficient complex are more reduced, leading to a greater generation of free radicals. This could bring an explanation for the diseases observed in conditions where the mitochondrial rate of ATP synthesis is only slightly affected.

Simple models of threshold curves in the expression of inborn errors of metabolism: application to some experimental observations

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.

Absence of stereospecific effects of bupivacaine isomers on heart mitochondrial bioenergetics

BACKGROUND

Highly lipophilic local anesthetics interfere with mitochondrial energy metabolism. These metabolic effects could, in part, explain some toxic effects of local anesthetics, such as bupivacaine-induced myocardial depression. The purpose of this study was to compare the optically pure isomers of bupivacaine on heart mitochondrial bioenergetics.

METHODS

Both bupivacaine enantiomers were tested on rat heart isolated mitochondria. Oxygen consumption, adenosine triphosphate synthesis, and enzymatic activities of the four complexes of the respiratory chain were measured.

RESULTS

No significant differences were found between R(+)- and S(-)-bupivacaine on mitochondrial oxidative phosphorylation with a similar dose-dependent decrease in adenosine triphosphate synthesis. Complex I (nicotinamide adenine dinucleotide ubiquinone reductase) was the enzymatic complex of the respiratory chain most sensitive to the bupivacaine isomers. Half-inhibitory concentrations for R(+)- and S(-)-bupivacaine were not statistically different (3.3 +/- 0.4 mm and 2.8 +/- 0.6 mm, respectively).

CONCLUSIONS

No stereospecific effects of bupivacaine enantiomers were shown in the inhibition of complex I activity and uncoupling of oxidative phosphorylation. This can be correlated with the lack of stereospecific effects of bupivacaine on myocardial depression. The lipid solubility of local anesthetics appears to be the principal physicochemical factor affecting the potency of these tertiary amines on mitochondrial bioenergetics.

Statistical analysis of mitochondrial pathologies in childhood: identification of deficiencies using principal component analysis

Mitochondrial pathologies are a heterogeneous group of metabolic disorders that are frequently characterized by anomalies of oxidative phosphorylation, especially in the respiratory chain. The identification of these anomalies may involve many investigations, and biochemistry is a main tool. However, considering the whole set of biochemical data, the interpretation of the results by the traditionally used statistical methods remains complex and does not always lead to an unequivocal conclusion about the presence or absence of a respiratory chain defect. This arises from three main problems: (a) the absence of an a priori-defined control population, because the determination of the control values are derived from the whole set of investigated patients, (b) the small size of the population studied, (c) the large number of variables collected, each of which creates a wide variability. To cope with these problems, the principal component analysis (PCA) has been applied to the biochemical data obtained from 35 muscle biopsies of children suspected of having a mitochondrial disease. This analysis makes it possible for each respiratory chain complex to distinguish between different subsets within the whole population (normal, deficient, and, in between, borderline subgroups of patients) and to detect the most discriminating variables. PCA of the data of all complexes together showed that mitochondrial diseases in this population were mainly caused by multiple deficits in respiratory chain complexes. This analysis allows the definition of a new subgroup of newborns, which have high respiratory chain complex activity values. Our results show that the PCA method, which simultaneously takes into account all of the concerned variables, allows the separation of patients into subgroups, which may help clinicians make their diagnoses.

Modification of mitochondrial metabolism in fibroblasts from mice with a skeletal muscle mutation (muscular dysgenesis). Evidence of embryonic communication between myoblasts and fibroblasts

Muscle development during embryogenesis is a complex process involving many mechanisms. It requires a close communication among the different cellular types of the muscle, especially the fibroblasts and myoblasts. Indeed, any abnormality in one cell type might influence the differentiation of the other. Thus, any disturbance altering the metabolism of the myoblasts might lead to modifications in the fibroblasts. To study this phenomenon, we used the dysgenic mouse (mdg-"muscular dysgenesis") carrying a homozygous recessive lethal mutation expressed only in skeletal muscle cells. First, we found that fibroblasts isolated from such mutant muscle (and not from mutant skin tissue) and grown in culture exhibited an altered metabolism. Secondly, muscle fibroblasts showed a lower capacity for proliferation. We also observed that respiration and ATP synthesis of dysgenic muscle fibroblasts were deficient, while respiratory chain enzymatic activities were normal. Finally, intracellular [Ca2+] levels of dysgenic fibroblasts are 50% of those of normal fibroblasts. These results support the hypothesis that certain characteristics of fibroblasts are determined by the surrounding cellular environment during embryonic organogenesis, and that such modifications are stable when the fibroblasts are isolated in vitro. Since fibroblast differentiation was disrupted permanently, this suggests, in the case of myopathies, that the modified cells, surrounding the muscle tissue, could contribute to the muscle pathology. Synergistic activities of this type should be considered when studying the course of pathologies in different types of muscle diseases.

Tissue variation in the control of oxidative phosphorylation: implication for mitochondrial diseases

Metabolic control analysis has often been used for quantitative studies of the regulation of mitochondrial oxidative phosphorylations (OXPHOS). The main contribution of this work has been to show that the control of mitochondrial metabolic fluxes can be shared among several steps of the oxidative phosphorylation process, and that this distribution can vary according to the steady state and the tissue. However, these studies do not show whether this observed variation in the OXPHOS control is due to the experimental conditions or to the nature of the mitochondria. To find out if there actually exists a tissue variation in the distribution of OXPHOS control coefficients, we determined the control coefficients of seven OXPHOS complexes on the oxygen-consumption flux in rat mitochondria isolated from five different tissues under identical experimental conditions. Thus in this work, only the nature of the mitochondria can be responsible for any variation detected in the control coefficient values between different tissues. The analysis of control coefficient distribution shows two tissue groups: (i) the muscle and the heart, controlled essentially at the level of the respiratory chain; and (ii) the liver, the kidney and the brain, controlled mainly at the phosphorylation level by ATP synthase and the phosphate carrier. We propose that this variation in control coefficient according to the tissue origin of the mitochondria can explain part of the tissue specificity observed in mitochondrial cytopathies.

Threshold effect and tissue specificity. Implication for mitochondrial cytopathies

Mitochondrial cytopathies present a tissue specificity characterized by the fact that even if a mitochondrial DNA mutation is present in all tissues, only some will be affected and induce a pathology. Several mechanisms have been proposed to explain this phenomenon such as the appearance of a sporadic mutation in a given stem cell during embryogenesis or mitotic segregation, giving different degrees of heteroplasmy in tissues. However, these mechanisms cannot be the only ones involved in tissue specificity. In this paper, we propose an additional mechanism contributing to tissue specificity. It is based on the metabolic expression of the defect in oxidative phosphorylation (OXPHOS) complexes that can present a biochemical threshold. The value of this threshold for a given OXPHOS complex can vary according to the tissue; thus different tissues will display different sensitivities to a defect in an OXPHOS complex. To verify this hypothesis and to illustrate the pathological consequences of the variation in biochemical thresholds, we studied their values for seven OXPHOS complexes in mitochondria isolated from five different rat tissues. Two types of behavior in the threshold curves can be distinguished corresponding to two modes of OXPHOS response to a deficiency. We propose a classification of tissues according to their type of OXPHOS response to a complex deficiency and therefore to their threshold values.

Menkes disease: study of the mitochondrial respiratory chain in three cases

Mitochondrial oxidative metabolism in three patients with typical Menkes disease was studied. In two cases, a general decrease in all of the respiratory chain complex activities (I, II, III and IV) was observed. However, in the most severe case, these activities were entirely normal. Our results emphasize the diversity of the cellular expression of Menkes disease which can, in some cases, be associated with a mitochondrial encephalomyopathy.

Metabolic control analysis and mitochondrial pathologies

One of the main salient features recognized in mitochondrial diseases is the existence of a threshold in the degree of a mitochondrial deficit for the expression of the disease. When expressed as a function of the degree of heteroplasmy, the value of the threshold can be very high, around 90% (mutated DNA/total DNA). This means that 10% of normal DNA is enough to sustain a quasi normal mitochondrial oxidative phosphorylating flux. We have shown that most of the compensation is done at the metabolic level: for instance a 70% deficit of cytochrome oxidase decreases the oxidative flux by just 10%. Similar patterns are observed for the other complexes. Using Metabolic Control Analysis (MCA), we have shown that this kind of result is inescapable: the threshold value can be correlated to the control coefficient of the deficient step. The value of the threshold is reinforced by slight increases at the transcriptional and translational level as we show in a simple mathematical model. Finally we associate the threshold in the expression of a deficit, to the threshold in the energy demand of different tissues, in order to describe various patterns of onset of mitochondrial diseases (double threshold hypothesis).

Comparison of the effects of bupivacaine and ropivacaine on heart cell mitochondrial bioenergetics

BACKGROUND

High lipophilic local anesthetics interfere with mitochondrial energy metabolism. These metabolic effects could in part explain some of the toxic effects of local anesthetics, such as bupivacaine-induced myocardial depression. The aim of this study was to compare the bioenergetic effects of the local anesthetics bupivacaine and ropivacaine.

METHODS

The effects of both local anesthetics on mitochondrial energy metabolism were studied in rat heart isolated mitochondria and in saponin-skinned left ventricle fibers. Oxygen consumption, adenosine triphosphate synthesis, and enzymatic activities of the complexes of the respiratory chain were measured.

RESULTS

Bupivacaine and ropivacaine acted, in isolated mitochondria, as uncouplers between oxygen consumption and phosphorylation of adenosine diphosphate. Further, an inhibitory effect of mitochondrial respiration was evidenced with both anesthetics during maximal respiration and was assigned to a direct inhibition of complex I of the respiratory chain. Mitochondrial adenosine triphosphate synthesis was decreased by both mechanisms. However, both in isolated mitochondria and in permeabilized heart fibers, ropivacaine was less potent than bupivacaine. Adenosine triphosphate synthesis was completely suppressed at 3 mM (approximately 0.1%) bupivacaine, whereas 3 mM ropivacaine induced only about a 40% inhibition.

CONCLUSIONS

Ropivacaine disturbs mitochondrial energy metabolism less than bupivacaine does. The lower lipid solubility of ropivacaine may be responsible for the lesser dose-dependent effects of this drug on mitochondrial bioenergetics.

Identification of mitochondrial deficiency using principal component analysis

The mitochondrial pathologies are a heterogeneous group of metabolic disorders that are characterized by anomalies of oxidative phosphorylation, especially in the respiratory chain. The diagnosis of these pathologies involves many investigations among which biochemical study is at present the main tool. However, the analysis of the results obtained during such study remains complex and often does not make it possible to conclude clearly if a patient is affected or not by a biochemical and/or bioenergetic deficiency. This arises from two main problems: 1. The determination of control values from the whole set of variable values (affected and unaffected people). 2. The small size of the population studied and the large number of variables collected which present a rather large variability. To cope with these problems, the principal component analysis method is applied to the results obtained during our biochemical studies. This analysis makes it possible for each respiratory chain complex, to distinguish clearly two subsets of the whole population (affected and unaffected people) as well as to detect the variables which are the most discriminative.

Metabolic control analysis and threshold effect in oxidative phosphorylation: implications for mitochondrial pathologies

We have shown that the Metabolic Control Analysis (MCA) can explain the threshold effect observed in the expression of mitochondrial diseases. As a matter of fact, the effect of a specific inhibitor on the flux of O2 consumption mimics a defect in a step of oxidative phosphorylation. The observed threshold is correlated to the value of the control coefficient of the inhibited step. For this reason, we have studied the repartition of the control coefficients of different steps in oxidative phosphorylation on various tissues (liver, kidney, brain, skeletal muscle and heart). We discuss the results in terms of metabolic control theory and provide a possible explanation for the heterogeneous phenotype of those pathologies. We present the double threshold hypothesis of both a threshold in the energy demand of a tissue and in the energy supply by oxidative phosphorylation.

The kinetic basis of threshold effects observed in mitochondrial diseases: a systemic approach

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.

Merrf family with 8344 mutation in tRNA (lys). Evidence of a mitochondrial vasculopathy in muscle biopsies

This article reports a new MERRF family. The mother, regarded as suffering from Ramsay-Hunt Syndrome, and her three daughters, had the same clinical pattern: myoclonic epilepsy and ataxia. Two daughters were studied on morphological, biochemical and molecular genetic levels. Muscle biopsies showed ragged-red fibres and mitochondrial vasculopathy. Arterioles were strongly SDH-reactive and COX-negative. By electron microscopy, abnormal mitochondria were observed in skeletal muscle fibres, in smooth muscle fibres of intramuscular vessels and in sweat gland epithelium. The study of the respiratory chain showed complex IV and I + IV deficiency, respectively. Mitochondrial tRNA (lys) mutation at position 8344 was pointed out as previously reported in the MERRF syndrome.

Phosphatidylethanolamine: ceramide-ethanolaminephosphotransferase activity in synaptic plasma membrane vesicles. Influence of some cations and phospholipid environment on transferase activity. Further proof of its location

1. Synaptic plasma membrane vesicles (SPMV) from rat brain synthesized ceramide-phosphoethanolamine (SpE), an analogue of sphingomyelin (SpC) from phosphatidylethanolamine (PE) and ceramide. 2. This reaction was catalyzed by PE: ceramide-phosphotransferase. 3. The presence of PC did not modify the SpE synthesis and PI and PS at twice PE concentration seemed to be activators; only PG was an inhibitor at all concentrations. 4. Some cations (Mg2+, Mn2+) were without effect, while Ca2+ increased transferase activity, so was interesting to study. 5. Transferase was compared with sialidase (external enzyme). 6. Kinetics other than those already performed by us were undertaken in order to confirm its location.

Control of oxidative phosphorylation in rat muscle mitochondria: implications for mitochondrial myopathies

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.

Fatal neonatal liver failure and mitochondrial cytopathy: an observation with antenatal ascites

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.

Mitochondrial myopathy studies on permeabilized muscle fibers

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.

Evidence for the biosynthesis of ceramide-phosphoethanolamine in brain synaptic plasma membrane vesicles and in sciatic nerve microsomes from normal and Trembler mice

Synaptic plasma membrane vesicles (SPMV) from brains of normal and Trembler mice synthesized ceramide-phosphoethanolamine, and analogue of sphingomyelin from phosphatidylethanolamine (PE) and ceramide. The rate of this synthesis (6 nmol/mg of protein/h) and the (Na+)-K+)ATPase activity (about 70 mumol Pi per mg of protein/h) were very similar in normal and Trembler. The synthesis increased as a linear function of protein when endogenous PE was taken into account, but the addition of exogenous ceramide was without effect. Likewise, PE was the donor of phosphoethanolamine for the synthesis of the ceramide-phosphoethanolamine in normal and Trembler mouse sciatic nerves and this synthesis was 3.5 times greater in the mutant than in controls.

Sidedness of ceramide-phosphoethanolamine synthesis on rat liver plasma membrane

Phosphatidylethanolamine:ceramide-ethanolaminephosphotransferase catalyzes the synthesis of ceramide-ethanolamine, a sphingomyelin analogue. Its transverse localization in rat liver plasma membrane was studied by treating intact and deoxycholate- or Triton X-100-disrupted membrane vesicles with trypsin or bacterial protease. The latency of ATPase was preserved during protease treatment; its value was 80% in the membrane vesicles obtained by sucrose gradient procedure alone and 91.2% in the vesicles isolated after sucrose gradient plus two-phase partitioning. This suggested that membrane integrity was not altered and that 90% of the vesicles were right-side out. When the sucrose gradient was followed by the two-phase procedure, 62% of phosphatidylethanolamine:ceramide-ethanolamine-phosphotransferase was accessible to the protease action, but only 45% in vesicles obtained by sucrose gradient alone. Our results suggest that at least a sizable portion of the active center of the enzyme responsible of biosynthesis of ceramide-phosphoethanolamine is located on the external side of liver plasma membrane and that the other is embedded in the membrane interior and is not accessible to trypsin, even in the presence of detergent.

Sidedness of ceramide-phosphoethanolamine synthesis on rat liver and brain microsomal membranes

Phosphatidylethanolamine:ceramide-ethanolamine-phosphotransferase catalyzes the synthesis of ceramide-phosphoethanolamine, a sphingomyelin analogue. Its localization was studied in rat liver and brain microsomes. After testing the integrity and the sidedness of microsomal vesicles, trypsin treatment of intact or deoxycholate-disrupted microsomes made it possible to conclude that both the transferase and the ceramide-phosphoethanolamine are located in the cisternal leaflet of the membrane bilayer. Using trinitrobenzenesulfonic acid as a probe, no trace of newly synthesized ceramide-phosphoethanolamine was detectable on the cytoplasmic side of the microsomes.

Sphingomyelin and ceramide-phosphoethanolamine synthesis by microsomes and plasma membranes from rat liver and brain

Pulse-chase experiments showed that phosphatidylethanolamine (PE) was the direct precursor for ceramide-phosphoethanolamine, a sphingomyelin analogue, in the same way as phosphatidylcholine was for sphingomyelin. Ceramide-phosphoethanolamine could be identified by incorporation of radioactivity from labeled PE, as well as by its stability in alkaline methanolysis and its ability to be methylated by S-adenosyl-methionine. Ceramide-phosphoethanolamine synthesis from labeled exogenous PE seemed to be independent of exogenous ceramide; it was proportional to the amount of incubated membrane, when taking into account the isotopic dilution of labeled precursor by endogenous PE. Sphingomyelin synthesis, which was demonstrated using natural PC as a substrate, was not possible using dipalmitoyl-PC. The formation of sphingomyelin and ceramide-phosphoethanolamine was demonstrated in microsomes and plasma membranes from rat brain and liver.