Enrichment of carnitine palmitoyltransferases I and II in the contact sites of rat liver mitochondria

Personalised modelling of clinical heterogeneity between medium-chain acyl-CoA dehydrogenase patients

BACKGROUND

Monogenetic inborn errors of metabolism cause a wide phenotypic heterogeneity that may even differ between family members carrying the same genetic variant. Computational modelling of metabolic networks may identify putative sources of this inter-patient heterogeneity. Here, we mainly focus on medium-chain acyl-CoA dehydrogenase deficiency (MCADD), the most common inborn error of the mitochondrial fatty acid oxidation (mFAO). It is an enigma why some MCADD patients-if untreated-are at risk to develop severe metabolic decompensations, whereas others remain asymptomatic throughout life. We hypothesised that an ability to maintain an increased free mitochondrial CoA (CoASH) and pathway flux might distinguish asymptomatic from symptomatic patients.

RESULTS

We built and experimentally validated, for the first time, a kinetic model of the human liver mFAO. Metabolites were partitioned according to their water solubility between the bulk aqueous matrix and the inner membrane. Enzymes are also either membrane-bound or in the matrix. This metabolite partitioning is a novel model attribute and improved predictions. MCADD substantially reduced pathway flux and CoASH, the latter due to the sequestration of CoA as medium-chain acyl-CoA esters. Analysis of urine from MCADD patients obtained during a metabolic decompensation showed an accumulation of medium- and short-chain acylcarnitines, just like the acyl-CoA pool in the MCADD model. The model suggested some rescues that increased flux and CoASH, notably increasing short-chain acyl-CoA dehydrogenase (SCAD) levels. Proteome analysis of MCADD patient-derived fibroblasts indeed revealed elevated levels of SCAD in a patient with a clinically asymptomatic state. This is a rescue for MCADD that has not been explored before. Personalised models based on these proteomics data confirmed an increased pathway flux and CoASH in the model of an asymptomatic patient compared to those of symptomatic MCADD patients.

CONCLUSIONS

We present a detailed, validated kinetic model of mFAO in human liver, with solubility-dependent metabolite partitioning. Personalised modelling of individual patients provides a novel explanation for phenotypic heterogeneity among MCADD patients. Further development of personalised metabolic models is a promising direction to improve individualised risk assessment, management and monitoring for inborn errors of metabolism.

Does skeletal muscle carnitine availability influence fuel selection during exercise?

Fat and carbohydrate are the major fuel sources utilised for oxidative, mitochondrial ATP resynthesis during human skeletal muscle contraction. The relative contribution of these two substrates to ATP resynthesis and total energy expenditure during exercise can vary substantially, and is predominantly determined by fuel availability and exercise intensity and duration. For example, the increased ATP demand that occurs with an increase in exercise intensity is met by increases in both fat and carbohydrate oxidation up to an intensity of approximately 60-70 % of maximal oxygen consumption. When exercise intensity increases beyond this workload, skeletal muscle carbohydrate utilisation is accelerated, which results in a reduction and inhibition of the relative and absolute contribution of fat oxidation to total energy expenditure. However, the precise mechanisms regulating muscle fuel selection and underpinning the decline in fat oxidation remain unclear. This brief review will primarily address the theory that a carbohydrate flux-mediated reduction in the availability of muscle carnitine to the mitochondrial enzyme carnitine palmitoyltransferase 1, a rate-limiting step in mitochondrial fat translocation, is a key mechanism for the decline in fat oxidation during high-intensity exercise. This is discussed in relation to recent work in this area investigating fuel metabolism at various exercise intensities and taking advantage of the discovery that skeletal muscle carnitine content can be nutritionally increased in vivo in human subjects.

Effects of Copper and/or Cholesterol Overload on Mitochondrial Function in a Rat Model of Incipient Neurodegeneration

Copper (Cu) and cholesterol (Cho) are both associated with neurodegenerative illnesses in humans and animals models. We studied the effect in Wistar rats of oral supplementation with trace amounts of Cu (3 ppm) and/or Cho (2%) in drinking water for 2 months. Increased amounts of nonceruloplasmin-bound Cu were observed in plasma and brain hippocampus together with a higher concentration of ceruloplasmin in plasma, cortex, and hippocampus. Cu, Cho, and the combined treatment Cu + Cho were able to induce a higher Cho/phospholipid ratio in mitochondrial membranes with a simultaneous decrease in glutathione content. The concentration of cardiolipin decreased and that of peroxidation products, conjugated dienes and lipoperoxides, increased. Treatments including Cho produced rigidization in both the outer and inner mitochondrial membranes with a simultaneous increase in permeability. No significant increase in Cyt C leakage to the cytosol was observed except in the case of cortex from rats treated with Cu and Cho nor were there any significant changes in caspase-3 activity and the Bax/Bcl2 ratio. However, the A β (1-42)/(1-40) ratio was higher in cortex and hippocampus. These findings suggest an incipient neurodegenerative process induced by Cu or Cho that might be potentiated by the association of the two supplements.

A matter of life, death and diseases: mitochondria from a proteomic perspective

Mitochondria are highly ordered, integrated organelles that energize cellular activities and contribute to programmed death by initiating disciplined apoptotic cascades. This review seeks to clarify our understanding of mitochondrial structural-functional integrity beyond the resolved nuclear genome by unraveling the dynamic mitochondrial proteome and elucidating proteome/genome interplay. The roles of mechanochemical coupling between mitoskeleton and cytoskeleton and crosstalk with other organelles in orchestrating cellular outcomes are explained. The authors also review the modulation of mitochondrial-related oxidative stress on apoptosis and cancer development and the context is applied to interpret pathogenetic events in neurodegenerative disorders and cardiovascular diseases. The accumulated proteomics evidence is used to describe the integral role that mitochondria play and how they influence other intracellular organelles. Possible mitochondrial-targeted therapeutic interventions are also discussed.

L-carnitine is essential to beta-oxidation of quarried fatty acid from mitochondrial membrane by PLA(2)

Mitochondrial beta-oxidation is an important system involved in the energy production of various cells. In this system, the function of L-carnitine is essential for the uptake of fatty acids to mitochondria. However, it is unclear whether or not endogenous respiration, ADP-induced O(2) consumption without substrates, is caused by L-carnitine treatment. In this study, we investigated whether L-carnitine is essential to the beta-oxidation of quarried fatty acids from the mitochondrial membrane by phospholipase A(2) (PLA(2)) using isolated mitochondria from the liver of rats. Intact mitochondria were incubated in a medium containing Pi, CoA and L-carnitine. The effect of L-carnitine treatment on ADP-induced mitochondrial respiration was observed without exogenous respiratory substrate. Increase in mitochondrial respiration was induced by treatment with L-carnitine in a concentration-dependent manner. Treatment with rotenone, a complex I blocker, completely inhibited ADP-induced oxygen consumption even in the presence of L-carnitine. Moreover, the L-carnitine dependent ADP-induced mitochondrial oxygen consumption did not increase when PLA(2) inhibitors were treated before ADP treatment. The L-carnitine-dependent ADP-induced oxygen consumption did contribute to ATP productions but not heat generation via an uncoupling system. These results suggest that L-carnitine might be essential to the beta-oxidation of quarried fatty acids from the mitochondrial membrane by PLA(2).

Lipids, mitochondria and cell death: implications in neuro-oncology

Polyunsaturated fatty acids (PUFAs) are known to inhibit cell proliferation of many tumour types both in vitro and in vivo. Their capacity to interfere with cell proliferation has been linked to their induction of reactive oxygen species (ROS) production in tumour tissues leading to cell death through apoptosis. However, the exact mechanisms of action of PUFAs are far from clear, particularly in brain tumours. The loss of bound hexokinase from the mitochondrial voltage-dependent anion channel has been directly related to loss of protection from apoptosis, and PUFAs can induce this loss of bound hexokinase in tumour cells. Tumour cells overexpressing Akt activity, including gliomas, are sensitised to ROS damage by the Akt protein and may be good targets for chemotherapeutic agents, which produce ROS, such as PUFAs. Cardiolipin peroxidation may be an initial event in the release of cytochrome c from the mitochondria, and enriching cardiolipin with PUFA acyl chains may lead to increased peroxidation and therefore an increase in apoptosis. A better understanding of the metabolism of fatty acids and eicosanoids in primary brain tumours such as gliomas and their influence on energy balance will be fundamental to the possible targeting of mitochondria in tumour treatment.

Structural insight into function and regulation of carnitine palmitoyltransferase

The control of fatty acid translocation across the mitochondrial membrane is mediated by the carnitine palmitoyltransferase (CPT) system. Modulation of its functionality has simultaneous effects on fatty acid and glucose metabolism. This encourages use of the CPT system as drug target for reduction of gluconeogenesis and restoration of lipid homeostasis, which are beneficial in the treatment of type 2 diabetes mellitus and obesity. Recently, crystal structures of CPT-2 were determined in uninhibited forms and in complexes with inhibitory substrate-analogs with anti-diabetic properties in animal models and in clinical studies. The CPT-2 crystal structures have advanced understanding of CPT structure-function relationships and will facilitate discovery of novel inhibitors by structure-based drug design. However, a number of unresolved questions regarding the biochemistry and pharmacology of CPT enzymes remain and are addressed in this review.

Self-association of transmembrane domain 2 (TM2), but not TM1, in carnitine palmitoyltransferase 1A: role of GXXXG(A) motifs

Carnitine palmitoyltransferase 1 (CPT1) controls the rate of entry of long-chain fatty acids into the mitochondrial matrix for beta-oxidation and has been reported to exist as an oligomer. We have investigated the in vivo oligomerization of full-length rat CPT1A (rCPT1A) along with those of the N-terminal truncation/deletion mutants Delta(1-82), Delta(1-18), and Delta(19-30) expressed in yeast mitochondria. The data indicate that in liver mitochondria in vivo CPT1A exists as a hexamer but that during preparation and storage of mitochondria the order of oligomerization is rapidly reduced to the trimer, such that a mixture of hexamer and trimer is observed in isolated mitochondria in vitro. Mutants bearing deletions of different segments of the N terminus (including the more N-terminal of the two transmembrane domains) have the same pattern of oligomerization when expressed in yeast mitochondria. The self-association of the individual rCPT1A transmembrane (TM) domains (TM1, TM2) was also studied using the TOXCAT assay (which measures TM self-association in the Escherichia coli inner membrane). There was minimal self-association of the sequence corresponding to TM1 but significant self-association of TM2 in TOXCAT. Chemical cross-linking and analytical ultracentrifugation of a TM2-derived synthetic peptide showed oligomerization with a similar trimer/hexamer equilibrium to that observed for native rCPT1A in isolated mitochondria. Therefore, there was a correlation between the oligomerization behavior of TM2 peptide and that of the full-length protein. In silico molecular modeling of rCPT1A TM2 highlighted the favorable orientation of GXXXG and GXXXA motifs in the formation of the TM2 hexamer.

Malonyl-CoA, a key signaling molecule in mammalian cells

Malonyl-CoA can be formed within the mitochondria, peroxisomes, and cytosol of mammalian cells. Besides being an intermediate in the pathways of de novo fatty acid biosynthesis and fatty acid elongation, malonyl-CoA has an important signaling function through its allosteric inhibition of carnitine palmitoyltransferase 1, the enzyme that normally exerts flux control over mitochondrial beta-oxidation. Malonyl-CoA is rapidly turned over in mammalian cells, and the activities of acetyl-CoA carboxylase and malonyl-CoA decarboxylase are important determinants of its cytosolic concentration. It is now recognized that malonyl-CoA participates in a diverse range of physiological or pathological responses and systems. These include the ketogenic response of the liver to fasting and diabetes, carbohydrate versus fat fuel selection in muscle tissues, metabolic changes in muscle during contracture, alterations in fatty acid metabolism during cardiac ischemia and postischemic reperfusion, stimulation of B cell insulin secretion by glucose, and the hypothalamic control of appetite.

Rat liver mitochondrial carnitine palmitoyltransferase-I, hepatic carnitine, and malonyl-CoA: effect of starvation

Hepatic mitochondrial fatty acid oxidation and ketogenesis increase during starvation. Carnitine palmitoyltransferase I (CPT-I) catalyses the rate-controlling step in the overall pathway and retains its control over beta-oxidation under fed, starved and diabetic conditions. To determine the factors contributing to the reported several-fold increase in fatty acid oxidation in perfused livers, we measured the V(max) and K(m) values for palmitoyl-CoA and carnitine, the K(i) (and IC(50)) values for malonyl-CoA in isolated liver mitochondria as well as the hepatic malonyl-CoA and carnitine contents in control and 48 h starved rats. Since CPT-I is localized in the mitochondrial outer membrane and in contact sites, the kinetic properties of CPT-I also was determined in these submitochondrial structures. After 48 h starvation, there is: (a) a significant increase in K(i) and decrease in hepatic malonyl-CoA content; (b) a decreased K(m) for palmitoyl-CoA; and (c) increased catalytic activity (V(max)) and CPT-I protein abundance that is significantly greater in contact sites compared with outer membranes. Based on these changes the estimated increase in mitochondrial fatty acid oxidation is significantly less than that observed in perfused liver. This suggests that CPT-I is regulated in vivo by additional mechanism(s) lost during mitochondrial isolation or/and that mitochondrial oxidation of peroxisomal beta-oxidation products contribute to the increased ketogenesis by bypassing CPT-I. Furthermore, the greater increase in CPT-I protein in contact sites as compared to outer membranes emphasizes the significance of contact sites in hepatic fatty acid oxidation.

Mitochondrial glycerol-3-P acyltransferase 1 is most active in outer mitochondrial membrane but not in mitochondrial associated vesicles (MAV)

Glycerol 3-phosphate acyltransferase-1 (GPAT1), catalyzes the committed step in phospholipid and triacylglycerol synthesis. Because both GPAT1 and carnitine-palmitoyltransferase 1 are located on the outer mitochondrial membrane (OMM) it has been suggested that their reciprocal regulation controls acyl-CoA metabolism at the OMM. To determine whether GPAT1, like carnitine-palmitoyltransferase 1, is enriched in both mitochondrial contact sites and OMM, and to correlate protein location and enzymatic function, we used Percoll and sucrose gradient fractionation of rat liver to obtain submitochondrial fractions. Most GPAT1 protein was present in a vesicular membrane fraction associated with mitochondria (MAV) but GPAT specific activity in this fraction was low. In contrast, highest GPAT1 specific activity was present in purified mitochondria. Contact sites from crude mitochondria, which contained markers for both endoplasmic reticulum (ER) and mitochondria, also showed high expression of GPAT1 protein but low specific activity, whereas contact sites isolated from purified mitochondria lacked ER markers and expressed highly active GPAT1. To determine how GPAT1 is targeted to mitochondria, recombinant protein was synthesized in vitro and its incorporation into crude and purified mitochondria was assayed. GPAT1 was rapidly incorporated into mitochondria, but not into microsomes. Incorporation was ATP-driven, and lack of GPAT1 removal by alkali and a chaotropic agent showed that GPAT1 had become an integral membrane protein after incorporation. These results demonstrate that two pools of GPAT1 are present in rat liver mitochondria: an active one, located in OMM and a less active one, located in membranes (ER-contact sites and mitochondrial associated vesicles) associated with both mitochondria and ER.

Muscle intermediate filaments and their links to membranes and membranous organelles

Intermediate filaments (IFs) play a key role in the integration of structure and function of striated muscle, primarily by mediating mechanochemical links between the contractile apparatus and mitochondria, myonuclei, the sarcolemma and potentially the vesicle trafficking apparatus. Linkage of all these membranous structures to the contractile apparatus, mainly through the Z-disks, supports the integration and coordination of growth and energy demands of the working myocyte, not only with force transmission, but also with de novo gene expression, energy production and efficient protein and lipid trafficking and targeting. Desmin, the most abundant and intensively studied muscle intermediate filament protein, is linked to proper costamere organization, myoblast and stem cell fusion and differentiation, nuclear shape and positioning, as well as mitochondrial shape, structure, positioning and function. Similar links have been established for lysosomes and lysosome-related organelles, consistent with the presence of widespread links between IFs and membranous structures and the regulation of their fusion, morphology and stabilization necessary for cell survival.

An improved enzyme assay for carnitine palmitoyl transferase I in fibroblasts using tandem mass spectrometry

Carnitine palmitoyl transferase I (CPTI), which converts acyl-CoA and carnitine into acyl-carnitine and free CoASH, is the rate limiting enzyme of hepatic mitochondrial beta-oxidation. CPTI-deficiency is a severe disorder characterized by Reye-like attacks with hypoketotic hypoglycemia, hepatomegaly, elevated liver enzymes and hyperammonemia. We developed a simple tandem-MS-based assay to measure CPTI activity in human fibroblasts. Surprisingly, a large part of the palmitoyl-carnitine formed in our assay by CPTI was degraded into C14- to C2-acyl-carnitines. Degradation of the product of CPTI leads to under estimation of the CPTI activity. When we used potassium cyanide to inhibit enzymes downstream of CPTI and thereby degradation of the product, we measured four times more CPTI activity than the previous methods. This inhibition is essential for correct calculation of CPTI activity. In fibroblasts of CPTI-deficient patients, CPTI activity was not detectable and this assay can be used for the diagnosis of CPTI-deficiency.

Stability and Association with the Cytomatrix of Mitochondrial DNA in Spontaneously Immortalized Mouse Embryo Fibroblasts Containing or Lacking the Intermediate Filament Protein Vimentin

To extend previous observations demonstrating differences in number, morphology, and activity of mitochondria in spontaneously immortalized vim(+) and vim(-) fibroblasts derived from wild-type and vimentin knockout mice, some structural and functional aspects of mitochondrial genome performance and integrity in both types of cells were investigated. Primary Vim(+/+) and Vim(-/-) fibroblasts, which escaped terminal differentiation by immortalization were characterized by an almost twofold lower mtDNA content in comparison to that of their primary precursor cells, whereby the average mtDNA copy number in two clones of vim(+) cells was lower by a factor of 0.6 than that in four clones of vim(-) cells. However, during serial subcultivation up to high passage numbers, the vim(+) and vim() fibroblasts increased their mtDNA copy number 1.5- and 2.5-fold, respectively. While early-passage cells of the vim(+) and vim(-) fibroblast clones differed only slightly in the ratio between mtDNA content and mitochondrial mass represented by mtHSP70 protein, after ca. 300 population doublings the average mtDNA/mtmass ratio in the vim(+) and vim() cells was increased by a factor of 2 and 4.5, respectively. During subcultivation, both types of cells acquired the fully transformed phenotype. These findings suggest that cytoskeletal vimentin filaments exert a strong influence on the mechanisms controlling mtDNA copy number during serial subcultivation of immortalized mouse embryo fibroblasts, and that vimentin deficiency causes a disproportionately enhanced mtDNA content in high-passage vim(-) fibroblasts. Such a role of vimentin filaments was supported by the stronger retention potential for mtDNA and mtDNA polymerase (gamma) detected in vim(+) fibroblasts by Triton X-100 extraction of mitochondria and agaroseembedded cells. Moreover, although the vim(+) and vim(-) fibroblasts were equally active in generating free radicals, the vim(-) cells exhibited higher levels of immunologically detectable 8-oxoG and mismatch repair proteins MSH2 and MLH1 in their mitochondria. Because in vim(-) fibroblasts only one point mutation was detected in the mtDNA D-loop control region, these cells are apparently able to efficiently remove oxidatively damaged nucleobases. On the other hand, a number of large-scale mtDNA deletions were found in high-passage vim(-) fibroblasts, but not in low-passage vim(-) cells and vim(+) cells of both low and high passage. Large mtDNA deletions were also induced in young vim(-) fibroblasts by treatment with the DNA intercalator ethidium bromide, whereas no such deletions were found after treatment of vim(+) cells. These results indicate that in immortalized vim(-) fibroblasts the mitochondrial genome is prone to large-scale rearrangements, probably due to insufficient control of mtDNA repair and recombination processes in the absence of vimentin.

Demonstration of N- and C-terminal domain intramolecular interactions in rat liver carnitine palmitoyltransferase 1 that determine its degree of malonyl-CoA sensitivity

We have previously proposed that changes in malonyl-CoA sensitivity of rat L-CPT1 (liver carnitine palmitoyltransferase 1) might occur through modulation of interactions between its cytosolic N- and C-terminal domains. By using a cross-linking strategy based on the trypsin-resistant folded state of L-CPT1, we have now shown the existence of such N-C (N- and C-terminal domain) intramolecular interactions both in wild-type L-CPT1 expressed in Saccharomyces cerevisiae and in the native L-CPT1 in fed rat liver mitochondria. These N-C intramolecular interactions were found to be either totally (48-h starvation) or partially abolished (streptozotocin-induced diabetes) in mitochondria isolated from animals in which the enzyme displays decreased malonyl-CoA sensitivity. Moreover, increasing the outer membrane fluidity of fed rat liver mitochondria with benzyl alcohol in vitro, which induced malonyl-CoA desensitization, attenuated the N-C interactions. This indicates that the changes in malonyl-CoA sensitivity of L-CPT1 observed in mitochondria from starved and diabetic rats, previously shown to be associated with altered membrane composition in vivo, are partly due to the disruption of N-C interactions. Finally, we show that mutations in the regulatory regions of the N-terminal domain affect the ability of the N terminus to interact physically with the C-terminal domain, irrespective of whether they increased [S24A (Ser24-->Ala)/Q30A] or abrogated (E3A) malonyl-CoA sensitivity. Moreover, we have identified the region immediately N-terminal to transmembrane domain 1 (residues 40-47) as being involved in the chemical N-C cross-linking. These observations provide the first demonstration by a physico-chemical method that L-CPT1 adopts different conformational states that differ in their degree of proximity between the cytosolic N-terminal and the C-terminal domains, and that this determines its degree of malonyl-CoA sensitivity depending on the physiological state.

Pathways and control of ketone body metabolism: on the fringe of lipid biochemistry

Ketone bodies become major body fuels during fasting and consumption of a high-fat, low-carbohydrate (ketogenic) diet. Hyperketonemia is associated with potential health benefits. Ketone body synthesis (ketogenesis) is the last recognizable step of lipid energy metabolism, a pathway that links dietary lipids and adipose triglycerides to the Krebs cycle and respiratory chain and has three highly regulated control points: (1) adipocyte lipolysis, (2) mitochondrial fatty acids entry, controlled by the inhibition of carnitine palmityl transferase I by malonyl coenzyme A (CoA) and (3) mitochondrial 3-hydroxy-3-methylglutaryl CoA synthase, which catalyzes the irreversible first step of ketone body synthesis. Each step is suppressed by an elevated circulating insulin level or insulin/glucagon ratio. The utilization of ketone bodies (ketolysis) also determines circulating ketone body levels. Consideration of ketone body metabolism reveals the mechanisms underlying the extreme fragility of dietary ketosis to carbohydrate intake and highlights areas for further study.

Bcl-2 overexpression corrects mitochondrial defects and ameliorates inherited desmin null cardiomyopathy

One of the hallmarks of cardiomyopathy and heart failure is pronounced and progressive cardiomyocyte death. Understanding the mechanisms involved in cardiomyocyte cell death is a topic of great interest for treatment of cardiac disease. Mice null for desmin, the muscle-specific member of the intermediate filament gene family, develop cardiomyopathy characterized by extensive cardiomyocyte death, fibrosis, calcification, and eventual heart failure. The earliest ultrastructural defects are observed in mitochondria. In the present study, we have demonstrated that these mitochondrial abnormalities are the primary cause of the observed cardiomyopathy and that these defects can be ameliorated by overexpression of bcl-2 in desmin null heart. Overexpression of bcl-2 in the desmin null heart results in correction of mitochondrial defects, reduced occurrence of fibrotic lesions in the myocardium, prevention of cardiac hypertrophy, restoration of cardiomyocyte ultrastructure, and significant improvement of cardiac function. Furthermore, we have found that loss of desmin also diminishes the capacity of mitochondria to resist exposure to calcium, a defect that can be partially restored by bcl-2 overexpression. These results point to a unique function for desmin in protection of mitochondria from calcium exposure that can be partially rescued by overexpression of bcl-2. We show that bcl-2 cardiac overexpression has provided significant improvement of an inherited form of cardiomyopathy, revealing the potential for bcl-2, and perhaps other genes in the family, as therapeutic agents for heart disease of many types, including inherited forms.

The liver isoform of carnitine palmitoyltransferase 1 is not targeted to the endoplasmic reticulum

Liver microsomal fractions contain a malonyl-CoA-inhibitable carnitine acyltransferase (CAT) activity. It has been proposed [Fraser, Corstorphine, Price and Zammit (1999) FEBS Lett. 446, 69-74] that this microsomal CAT activity is due to the liver form of carnitine palmitoyltransferase 1 (L-CPT1) being targeted to the endoplasmic reticulum (ER) membrane as well as to mitochondria, possibly by an N-terminal signal sequence [Cohen, Guillerault, Girard and Prip-Buus (2001) J. Biol. Chem. 276, 5403-5411]. COS-1 cells were transiently transfected to express a fusion protein in which enhanced green fluorescent protein was fused to the C-terminus of L-CPT1. Confocal microscopy showed that this fusion protein was localized to mitochondria, and possibly to peroxisomes, but not to the ER. cDNAs corresponding to truncated (amino acids 1-328) or full-length L-CPT1 were transcribed and translated in the presence of canine pancreatic microsomes. However, there was no evidence of authentic insertion of CPT1 into the ER membrane. Rat liver microsomal fractions purified by sucrose-density-gradient centrifugation contained an 88 kDa protein (p88) which was recognized by an anti-L-CPT1 antibody and by 2,4-dinitrophenol-etomoxiryl-CoA, a covalent inhibitor of L-CPT1. Abundance of p88 and malonyl-CoA-inhibitable CAT activity were increased approx. 3-fold by starvation for 24 h. Deoxycholate solubilized p88 and malonyl-CoA-inhibitable CAT activity from microsomes to approximately the same extent. The microsomal fraction contained porin, which, relative to total protein, was as abundant as in crude mitochondrial outer membranes fractions. It is concluded that L-CPT1 is not targeted to the ER membrane and that malonyl-CoA CAT in microsomal fractions is L-CPT1 that is derived from mitochondria, possibly from membrane contact sites.

Impaired ketogenesis is a major mechanism for disturbed hepatic fatty acid metabolism in rats with long-term cholestasis and after relief of biliary obstruction

BACKGROUND/AIMS

Rats with long-term cholestasis have reduced ketosis of unknown origin.

METHODS

Fatty acid metabolism was studied in starved rats with biliary obstruction for 4 weeks (bile duct ligated rats = BDL rats), and 3, 7, 14, 28 and 84 days after reversal of biliary obstruction by Roux-en-Y anastomosis (RY rats), and in sham-operated control rats.

RESULTS

BDL rats had reduced beta-hydroxybutyrate concentrations in plasma (0.25 +/- 0.10 vs. 0.75 +/- 0.20 mmol/l) and liver (2.57 +/- 0.20 vs. 4.63 +/- 0.61 micromol/g) which increased after restoring bile flow. Hepatic expression and activity of carnitine palmitoyltransferase I (CPT I) or CPT II were unaffected or decreased in BDL rats, respectively, and increased after restoring bile flow. Oxidative metabolism of different substrates by isolated liver mitochondria and activation of palmitate were reduced in BDL rats and recovered 7-14 days after restoring bile flow. Ketogenesis was decreased in mitochondria from BDL rats and recovered 3 months after restoring bile flow. Both mRNA and protein expression of hydroxymethylglutaryl-coenzyme A synthase (HMG-CoA synthase), the rate-limiting enzyme of ketogenesis, was reduced in livers of BDL rats and increased after reversing biliary obstruction.

CONCLUSIONS

In BDL rats, impairment of hepatic fatty acid metabolism is multifactorial. After reversing biliary obstruction, reduced activity of HMG-CoA synthase is the major factor.

Gamma-linolenic acid alters the composition of mitochondrial membrane subfractions, decreases outer mitochondrial membrane binding of hexokinase and alters carnitine palmitoyltransferase I properties in the Walker 256 rat tumour

Gamma-linolenic acid (GLA) is known to be an inhibitor of Walker 256 tumour growth in vivo and causes changes in both mitochondrial structure and cellular metabolism. The aim of the present study was to investigate in greater detail the changes in energy metabolism and ultrastructure induced by GLA in this tumour model. A diet containing 5.5% GLA, which is sufficient to cause a 45% decrease in tumour growth, was found to almost double the triacylglycerol (TAG) content of the tumour and to increase the quantity of 20:3 n-6, 20:4 n-6, 22:4 n-6 and 22:5 n-6 in the TAG fraction as determined by gas chromatography-mass spectrometry (GCMS) analysis. Morphometric analysis of the tumour by electron microscopy confirmed this increase in TAG content, identifying a doubling of lipid droplet content in the GLA dietary group. The surface density of mitochondrial cristae was reduced, along with a reduction in the number of contact sites (CS) and matrix granules. These three parameters are likely indicators of a reduction in mitochondrial metabolic activity. Measurement of hexokinase activity identified that much of the total hexokinase activity was in the mitochondrially bound form (66.5%) in the control tumour and that GLA caused a decrease in the amount of enzyme in the bound form (39.3%). The fatty acyl chain composition of the tumour mitochondrial subfractions, outer membranes (OM), CSs and inner membranes (IM) was determined by GCMS. All subfractions showed considerable increases in 20:3 n-6 and decreases in 18:1 n-9, 18:2 n-6 and 22:6 n-3, when exposed to GLA diet. These changes were reflected in a large increase in the n-6/n-3 ratio in the GLA OM vs. the control OM, 21.299 vs. 6.747, respectively. The maximal activity of OM carnitine palmitoyltransferase I (CPT I) was found to be decreased by 61.6% in the GLA diet group. This was accompanied by a decrease in malonyl CoA sensitivity and a decrease in affinity for 16:0 CoA substrate. Such changes in CPT I may be the cause of cytoplasmic acyl CoA accumulation seen in this tumour model. These effects, together with previously reported increases in lipid peroxidation, lead to the conclusion that GLA may cause inhibition of tumour cell growth through separate but interlinked pathways, all of which eventually lead to apoptosis and a decrease in tumour development. The influence of mitochondrial OM fatty acyl chain composition upon two important enzymes of energy metabolism, hexokinase and CPT I, both of which have been linked to apoptosis, is of considerable importance for future studies on fatty acid-induced cell death.

Control of mitochondrial beta-oxidation flux

The control of mitochondrial beta-oxidation, including the delivery of acyl moieties from the plasma membrane to the mitochondrion, is reviewed. Control of beta-oxidation flux appears to be largely at the level of entry of acyl groups to mitochondria, but is also dependent on substrate supply. CPTI has much of the control of hepatic beta-oxidation flux, and probably exerts high control in intact muscle because of the high concentration of malonyl-CoA in vivo. beta-Oxidation flux can also be controlled by the redox state of NAD/NADH and ETF/ETFH(2). Control by [acetyl-CoA]/[CoASH] may also be significant, but it is probably via export of acyl groups by carnitine acylcarnitine translocase and CPT II rather than via accumulation of 3-ketoacyl-CoA esters. The sharing of control between CPTI and other enzymes allows for flexible regulation of metabolism and the ability to rapidly adapt beta-oxidation flux to differing requirements in different tissues.

Isolation of hepatic mitochondrial contact sites: previously unrecognized inner membrane components

An improved, fast, and relatively simple procedure for isolation of hepatic mitochondrial contact sites is described. These contact sites include conventional outer membrane, but the inner membrane component (which we term fusion patches) has a unique biochemical composition characterized by a clustering of three specific inner membrane proteins of 54, 52, and 31 kDa identified by proteomics, respectively, as the alpha and beta subunits of ATP synthase and the liver isoform of adenine nucleotide transferase. The contact site fraction was prepared using a discontinuous sucrose gradient from crude outer membranes derived from swollen/shrunk rat liver mitochondria. The resultant contact sites were analyzed using a continuous sucrose density gradient, revealing an apparent heterogeneity due to varying amounts of retained fusion patches in relation to the unvarying outer membrane component. By electron microscopy, contact sites consist of small vacuoles that contain one or several tiny vesicles, many of which are composed of multiple, closely packed lamellae. The contact site subfraction morphology is consistent with the biochemical variation. Thus, contact sites are not haphazard fusions of outer and inner membrane, but consist in part of regions of inner membrane of novel composition (fusion patches) and of conventional outer membrane.

Mitochondrial intermembrane junctional complexes and their involvement in cell death

Mitochondria establish contact sites between the inner and outer membranes. The contact sites are held together by junctional complexes of the adenine nucleotide translocase (ANT; inner membrane) and the voltage-dependent anion channel (VDAC; outer membrane). The junctional complexes act as multifunctional recruitment centres, binding a range of proteins according to the function to be executed. Some of these, involving kinases and enzymes of lipid transfer, are readily understood as ongoing functions in energy and lipid metabolism. But the roles of other proteins recruited to the junctional complexes are less well defined. Here, we focus on the complexes formed with Bax and with cyclophilin-D, and their possible roles in apoptotic and necrotic cell death. We have isolated both types of complexes using glutathione-S-transferase fusion proteins of Bax and of cyclophilin-D. The VDAC/ANT/cyclophilin-D complex reconstitutes Ca(2+)- and cyclosporin A-sensitive permeability transition pore activity when incorporated into proteoliposomes. The complex forms readily in the absence of factors required for pore opening in isolated mitochondria, suggesting that these factors act on the preexisting complex, rather than drive its assembly, and that the complex is a physiological entity in healthy cells.

Palmitate oxidation in rat hepatocytes is inhibited by foetal calf serum

The rate of oxidation of fatty acids in mammals is minimal prior to birth. In this study, we have shown that foetal calf serum (FCS) inhibits oxidation of palmitate while serum from newborn calves is almost without effect. Foetal calf serum was also found to increase fatty acid synthesis from acetate. Uptake of laurate in mitochondria is partially dependent upon the carnitine palmitoyltransferase (CPT) I/CPT II system, while octanoate transport occurs without its participation. Comparison of the effects of FCS on the oxidation of palmitate, laurate and octanoate supports the view that the observed actions of FCS result from regulation of CPT I activity. The material in FCS that affects fatty acid metabolism has a molecular weight <3 kDa, as determined by dialysis and ultra-filtration studies.

Rat liver mitochondrial contact sites and carnitine palmitoyltransferase-I

In hepatic mitochondria, the outer membrane enzyme, carnitine palmitoyltransferase-I (CPT-I), appears to colocalize with contact sites. We have prepared contact sites that are essentially devoid of noncontact site membranes. The contact site fraction has a high specific activity for CPT-I and contains a protein at 88 kDa that is recognized by antibodies directed at two different peptide epitopes on CPT-I. Similarly long-chain acyl-CoA synthetase (LCAS) specific activity is high in this fraction; a protein at 79 kDa is recognized by an antibody against LCAS. Although activity of carnitine palmitoyltransferase-II (CPT-II) is present, it is not enriched in the contact site fraction, and a protein of 68 kDa weakly reacted with anti-CPT-II antibody. Likewise, carnitine-acylcarnitine translocase (CACT) protein is present, but at a somewhat reduced level. Using an analytical continuous sucrose gradient, we demonstrate that the activities of CPT-I and LCAS and their associated immunoreactive proteins are present in a constant amount throughout the contact site subfractions. The enzymatic activity of CPT-II and its associated immunoreactive protein, as well as immunoreactive CACT, is absent in the lighter density gradient subfractions and is present in the higher density subfractions only in trace amounts. This heterogeneity of the contact site fraction is due to unvarying amounts of outer membrane and increasing amounts of attached inner membrane with increasing density of the subfractions.

Distinct kinetics of carnitine palmitoyltransferase i in contact sites and outer membranes of rat liver mitochondria

Carnitine palmitoyltransferase I (CPT I) of rat liver mitochondria is an integral, polytopic protein of the outer membrane that is enriched at contact sites. As CPT I kinetics are highly dependent on its membrane environment, we have measured the kinetic parameters of CPT I present in rat liver submitochondrial membrane fractions enriched in either outer membrane or contact sites. The K(m) for palmitoyl-CoA was 2.4-fold higher for CPT I in outer membranes than that for the enzyme in contact sites. In addition, whereas in contact sites malonyl-CoA behaved as a competitive inhibitor of CPT I with respect to palmitoyl-CoA, in outer membranes malonyl-CoA inhibition was non-competitive. As a result of the combination of these changes, the IC(50) for malonyl-CoA was severalfold higher for CPT I in contact sites than for the enzyme in bulk outer membrane. The K(i) for malonyl-CoA, the K(m) for carnitine, and the catalytic constant of the enzyme were all unaffected. It is concluded that the different membrane environments in outer membranes and contact sites result in an altered conformation of L-CPT I that specifically affects the long-chain acyl-CoA binding site. The accompanying changes in the kinetics of the enzyme provide an additional potent mechanism for the regulation of L-CPT I activity.

Mitochondrial dysfunction in cardiac disease: ischemia--reperfusion, aging, and heart failure

Mitochondria contribute to cardiac dysfunction and myocyte injury via a loss of metabolic capacity and by the production and release of toxic products. This article discusses aspects of mitochondrial structure and metabolism that are pertinent to the role of mitochondria in cardiac disease. Generalized mechanisms of mitochondrial-derived myocyte injury are also discussed, as are the strengths and weaknesses of experimental models used to study the contribution of mitochondria to cardiac injury. Finally, the involvement of mitochondria in the pathogenesis of specific cardiac disease states (ischemia, reperfusion, aging, ischemic preconditioning, and cardiomyopathy) is addressed.

Molecular enzymology of carnitine transfer and transport

Carnitine (L-3-hydroxy-4-N-trimethylaminobutyric acid) forms esters with a wide range of acyl groups and functions to transport and excrete these groups. It is found in most cells at millimolar levels after uptake via the sodium-dependent carrier, OCTN2. The acylation state of the mobile carnitine pool is linked to that of the limited and compartmentalised coenzyme A pools by the action of the family of carnitine acyltransferases and the mitochondrial membrane transporter, CACT. The genes and sequences of the carriers and the acyltransferases are reviewed along with mutations that affect activity. After summarising the accepted enzymatic background, recent molecular studies on the carnitine acyltransferases are described to provide a picture of the role and function of these freely reversible enzymes. The kinetic and chemical mechanisms are also discussed in relation to the different inhibitors under study for their potential to control diseases of lipid metabolism.

Identification of positive and negative determinants of malonyl-CoA sensitivity and carnitine affinity within the amino termini of rat liver- and muscle-type carnitine palmitoyltransferase I

The extreme amino terminus and, in particular, residue Glu-3 in rat liver (L) carnitine palmitoyltransferase I (CPT I) have previously been shown to be essential for the sensitivity of the enzyme to inhibition by malonyl-CoA. Using the Pichia pastoris expression system, we now observe that, although mutants E3A (Glu-3 --> Ala) or Delta(3-18) of L-CPT I have markedly lowered sensitivity to malonyl-CoA compared with the wild-type protein, the mutant Delta(1-82) generated an enzyme that had regained much of the sensitivity of wild-type CPT I. This suggests that a region antagonistic to malonyl-CoA sensitivity is present within residues 19-82 of the enzyme. This was confirmed in the construct Delta(19-30), which was found to be 50-fold more sensitive than wild-type L-CPT I. Indeed, this mutant was >4-fold more sensitive than even the native muscle (M)-CPT I isoform expressed and assayed under identical conditions. This behavior was dependent on the presence of Glu-3, with the mutant E3A-Delta(19-30) having kinetic characteristics similar to those of the E3A mutant. The increase in the sensitivity of the L-CPT I-Delta(19-30) mutant was not due to a change in the mechanism of inhibition with respect to palmitoyl-CoA, nor to any marked change of the K(0.5) for this substrate. Conversely, for M-CPT I, a decrease in malonyl-CoA sensitivity was invariably observed with increasing deletions from Delta(3-18) to Delta(1-80). However, deletion of residues 3-18 from M-CPT I affected the K(m) for carnitine of this isoform, but not of L-CPT I. These observations (i) provide the first evidence for negative determinants of malonyl-CoA sensitivity within the amino-terminal segment of L-CPT I and (ii) suggest a mechanism for the inverse relationship between affinity for malonyl-CoA and for carnitine of the two isoforms of the enzyme.

Genomics of the human carnitine acyltransferase genes

Five genes in the human genome are known to encode different active forms of related carnitine acyltransferases: CPT1A for liver-type carnitine palmitoyltransferase I, CPT1B for muscle-type carnitine palmitoyltransferase I, CPT2 for carnitine palmitoyltransferase II, CROT for carnitine octanoyltransferase, and CRAT for carnitine acetyltransferase. Only from two of these genes (CPT1B and CPT2) have full genomic structures been described. Data from the human genome sequencing efforts now reveal drafts of the genomic structure of CPT1A and CRAT, the latter not being known from any other mammal. Furthermore, cDNA sequences of human CROT were obtained recently, and database analysis revealed a completed bacterial artificial chromosome sequence that contains the entire CROT gene and several exons of the flanking genes P53TG and PGY3. The genomic location of CROT is at chromosome 7q21.1. There is a putative CPT1-like pseudogene in the carnitine/choline acyltransferase family at chromosome 19. Here we give a brief overview of the functional relations between the different carnitine acyltransferases and some of the common features of their genes. We will highlight the phylogenetics of the human carnitine acyltransferase genes in relation to the fungal genes YAT1 and CAT2, which encode cytosolic and mitochondrial/peroxisomal carnitine acetyltransferases, respectively.

Reconstitution of purified, active and malonyl-CoA-sensitive rat liver carnitine palmitoyltransferase I: relationship between membrane environment and malonyl-CoA sensitivity

Carnitine palmitoyltransferase I (CPT I) catalyses the initial step of fatty acid import into the mitochondrial matrix, the site of beta-oxidation, and its inhibition by malonyl-CoA is a primary control point for this process. The enzyme exists in at least two isoforms, denoted L-CPT I (liver type) and M-CPT I (skeletal-muscle type), which differ in their kinetic characteristics and tissue distributions. A property apparently unique to L-CPT I is that its sensitivity to malonyl-CoA decreases in vivo with fasting or experimentally induced diabetes. The mechanism of this important regulatory effect is unknown and has aroused much interest. CPT I is an integral outer-membrane protein and displays little activity after removal from the membrane by detergents, precluding direct purification of active protein by conventional means. Here we describe the expression of a 6 x His-tagged rat L-CPT I in Pichia pastoris and purification of the detergent-solubilized enzyme in milligram quantities. Reconstitution of the purified product into a liposomal environment yielded a 200--400-fold increase in enzymic activity and restored malonyl-CoA sensitivity. This is the first time that a CPT I protein has been available for study in a form that is both pure and active. Comparison of the kinetic properties of the reconstituted material with those of L-CPT I as it exists in mitochondria prepared from yeast over-expressing the enzyme and in livers from fed or fasted rats permitted novel insight into several aspects of the enzyme's behaviour. The malonyl-CoA response of the liposomal enzyme was found to be greater when the reconstitution procedure was carried out at 22 degrees C compared with 4 degrees C (IC(50) approximately 11 microM versus 30 microM, respectively). When the sensitivities of L-CPT I in each of the different environments were compared, they were found to decrease in the following order: fed liver>fasted liver approximately liposomes prepared at 22 degrees C approximately P. pastoris mitochondria>liposomes prepared at 4 degrees C. In addition, pre-treatment of L-CPT I liposomes with the membrane-fluidizing reagent benzyl alcohol caused densensitization to the inhibitor. In contrast with the variable response to malonyl-CoA, the liposomal L-CPT I displayed a pH profile and kinetics with regard to the carnitine and acyl-CoA substrates similar to those of the enzyme in fed or fasted liver mitochondria. However, despite a normal sensitivity to malonyl-CoA, L-CPT I in P. pastoris mitochondria displayed aberrant behaviour with regard to each of these other parameters. The kinetic data establish several novel points. First, even after stringent purification procedures in the presence of detergent, recombinant L-CPT I could be reconstituted in active, malonyl-CoA sensitive form. Second, the kinetics of the reconstituted, 6 x His-tagged L-CPT I with regard to substrate and pH responses were similar to what is observed with rat liver mitochondria (whereas in P. pastoris mitochondria the enzyme behaved anomalously), confirming that the purified preparation is a suitable model for studying the functional properties of the enzyme. Third, wide variation in the response to the inhibitor, malonyl-CoA, was observed depending only on the enzyme's membrane environment and independent of interaction with other proteins. In particular, the fluidity of the membrane had a direct influence on this parameter. These observations may help to explain the mechanism of the physiological changes in the properties of L-CPT I that occur in vivo and are consistent with the current topographical model of the enzyme.

Fatty acid import into mitochondria

The mitochondrial carnitine system plays an obligatory role in beta-oxidation of long-chain fatty acids by catalyzing their transport into the mitochondrial matrix. This transport system consists of the malonyl-CoA sensitive carnitine palmitoyltransferase I (CPT-I) localized in the mitochondrial outer membrane, the carnitine:acylcarnitine translocase, an integral inner membrane protein, and carnitine palmitoyltransferase II localized on the matrix side of the inner membrane. Carnitine palmitoyltransferase I is subject to regulation at the transcriptional level and to acute control by malonyl-CoA. The N-terminal domain of CPT-I is essential for malonyl-CoA inhibition. In liver CPT-I activity is also regulated by changes in the enzyme's sensitivity to malonyl-CoA. As fluctuations in tissue malonyl-CoA content are parallel with changes in acetyl-CoA carboxylase activity, which in turn is under the control of 5'-AMP-activated protein kinase, the CPT-I/malonyl-CoA system is part of a fuel sensing gauge, turning off and on fatty acid oxidation depending on the tissue's energy demand. Additional mechanism(s) of short-term control of CPT-I activity are emerging. One proposed mechanism involves phosphorylation/dephosphorylation dependent direct interaction of cytoskeletal components with the mitochondrial outer membrane or CPT-I. We have proposed that contact sites between the outer and inner mitochondrial membranes form a microenvironment which facilitates the carnitine transport system. In addition, this system includes the long-chain acyl-CoA synthetase and porin as components.

Characterization of L-carnitine transport into rat skeletal muscle plasma membrane vesicles

Transport of L-carnitine into skeletal muscle was investigated using rat sarcolemmal membrane vesicles. In the presence of an inwardly directed sodium chloride gradient, L-carnitine transport showed a clear overshoot. The uptake of L-carnitine was increased, when vesicles were preloaded with potassium. When sodium was replaced by lithium or cesium, and chloride by nitrate or thiocyanate, transport activities were not different from in the presence of sodium chloride. However, L-carnitine transport was clearly lower in the presence of sulfate or gluconate, suggesting potential-dependent transport. An osmolarity plot revealed a positive slope and a significant intercept, indicating transport of L-carnitine into the vesicle lumen and binding to the vesicle membrane. Displacement experiments revealed that approximately 30% of the L-carnitine associated with the vesicles was bound to the outer and 30% to the inner surface of the vesicle membrane, whereas 40% was unbound inside the vesicle. Saturable transport could be described by Michaelis-Menten kinetics with an apparent Km of 13.1 microM and a Vmax of 2.1 pmol.(mg protein-1).s-1. L-Carnitine transport could be trans-stimulated by preloading the vesicles with L-carnitine but not with the carnitine precursor butyrobetaine, and was cis-inhibited by L-palmitoylcarnitine, L-isovalerylcarnitine, and glycinebetaine. On comparing carnitine transport into rat kidney brush-border membrane vesicles and OCTN2, a sodium-dependent high-affinity human carnitine transporter, cloned recently from human kidney also expressed in muscle, the Km values are similar but driving forces, pattern of inhibition and stereospecificity are different. This suggests the existence of more than one carnitine carrier in skeletal muscle.

Cancer and anticancer therapy-induced modifications on metabolism mediated by carnitine system

An efficient regulation of fuel metabolism in response to internal and environmental stimuli is a vital task that requires an intact carnitine system. The carnitine system, comprehensive of carnitine, its derivatives, and proteins involved in its transformation and transport, is indispensable for glucose and lipid metabolism in cells. Two major functions have been identified for the carnitine system: (1) to facilitate entry of long-chain fatty acids into mitochondria for their utilization in energy-generating processes; (2) to facilitate removal from mitochondria of short-chain and medium-chain fatty acids that accumulate as a result of normal and abnormal metabolism. In cancer patients, abnormalities of tumor tissue as well as nontumor tissue metabolism have been observed. Such abnormalities are supposed to contribute to deterioration of clinical status of patients, or might induce cancerogenesis by themselves. The carnitine system appears abnormally expressed both in tumor tissue, in such a way as to greatly reduce fatty acid beta-oxidation, and in nontumor tissue. In this view, the study of the carnitine system represents a tool to understand the molecular basis underlying the metabolism in normal and cancer cells. Some important anticancer drugs contribute to dysfunction of the carnitine system in nontumor tissues, which is reversed by carnitine treatment, without affecting anticancer therapeutic efficacy. In conclusion, a more complex approach to mechanisms that underlie tumor growth, which takes into account the altered metabolic pathways in cancer disease, could represent a challenge for the future of cancer research.

Evaluation of the affinity and turnover number of both hepatic mitochondrial and microsomal carnitine acyltransferases: relevance to intracellular partitioning of acyl-CoAs

Mitochondrial carnitine palmitoyltransferase I (CPT I) and microsomal carnitine acyltransferase I (CAT I) regulate the entry of fatty acyl moieties into their respective organelles. Thus, CPT I and CAT I occupy prominent positions in the pathways responsible for energy generation in mitochondria and the assembly of VLDL in the endoplasmic reticulum, respectively. Previous attempts to determine the intrinsic kinetic properties of CPT I and CAT I have been hampered by the occurrence of sigmoidal velocity curves. This was overcome, in this study, by the inclusion of recombinant acyl-CoA binding protein in the assay medium. For the first time, we have determined the concentrations of total functional enzyme (E(t)) by specific radiolabeling of the active site, the dissociation constants (K(d)) and the turnover numbers of CPT I and CAT I toward the CoA esters of oleic acid (C18:1) and docosahexaenoic acid (C22:6). The data show that carnitine inhibits CAT I at physiological concentrations which are not inhibitory to CPT I. Thus, carnitine concentration is likely to be a significant factor in determining the partitioning of acyl-CoAs between mitochondria and microsomes, a role which has not been previously recognized. Moreover, the finding that CAT I elicits a lower turnover toward the CoA ester of C22:6 (25 s(-)(1)) than toward that of C18:1 (111 s(-)(1)), while having similar K(d) values, suggests the use of this polyunsaturated fatty acid to inhibit VLDL biosynthesis.

Expression and regulation of carnitine palmitoyltransferase-Ialpha and -Ibeta genes

Two genes control expression of mitochondrial carnitine palmitoyltransferase-I (CPT-I), the enzyme that catalyzes the primary rate-controlling step in fatty acid oxidation. Two CPT-I isoforms have been found--a "liver" isoform (CPT-Ialpha) expressed in most tissues, but not in skeletal muscles, and a "muscle" isoform (CPT-Ibeta) expressed in muscles and adipocytes. Liver CPT-Ialpha increases dramatically at birth, but heart CPT-Ialpha is abundant in the fetus and diminishes at birth. Insulin, thyroid hormone, and fatty acids regulate expression of CPT-Ialpha in liver, whereas electrical stimulation increases CPT-Ibeta and decreases CPT-Ialpha in cardiac myocytes. Both genes are TATA-less and contain Sp1 transcription factor binding sites upstream of the start site of transcription. Multiple transcripts of both CPT-Ialpha and CPT-Ibeta exist, some of which may have roles in regulating fatty acid oxidation.

Evidence that carnitine palmitoyltransferase I (CPT I) is expressed in microsomes and peroxisomes of rat liver. Distinct immunoreactivity of the N-terminal domain of the microsomal protein

Mitochondria, microsomes and peroxisomes all express overt (cytosol-facing) carnitine palmitoyltransferase activity that is inhibitable by malonyl-CoA. The overt carnitine palmitoyltransferase activity (CPTo) associated with the different fractions was measured. Mitochondria accounted for 65% of total cellular CPTo activity, with the microsomal and peroxisomal contributions accounting for the remaining 25% and 10%, respectively. In parallel experiments, rat livers were perfused in situ with medium containing dinitrophenyl (DNP)-etomoxir in order to inhibit quantitatively and label covalently (with DNP-etomoxiryl-CoA) the molecular species responsible for CPTo activity in each of the membrane systems under near-physiological conditions. In all three membrane fractions, a single protein with an identical molecular mass of approximately 88,000 kDa (p88) was labelled after DNP-etomoxir perfusion of the liver. The abundance of labelled p88 was quantitatively related to the respective specific activities of CPTo in each fraction. On Western blots the same protein was immunoreactive with three anti-peptide antibodies raised against linear epitopes of the cytosolic N- and C-domains and of the inter-membrane space loop (L) domain of the mitochondrial enzyme (L-CPT I). However, the reaction of the microsomal protein with the anti-N peptide antibody (raised against epitope Val-14-Lys-29 of CPT I) was an order of magnitude stronger than expected from either microsomal CPTo activity or its DNP-etomoxiryl-CoA labelling. This suggests that the N-terminal domain of the microsomal protein differs from that in the mitochondrial or peroxisomal protein. This conclusion was confirmed using antibody back-titration experiments, in which the binding of anti-N and anti-C antibodies by mitochondria and microsomes was quantified.

Submitochondrial and subcellular distributions of the carnitine-acylcarnitine carrier

The submitochondrial and subcellular distributions of the carnitine-acylcarnitine translocase (CAC) have been studied. CAC is enriched to a much lesser extent than the carnitine palmitoyltransferases within the contact sites of mitochondria. A high-abundance protein of identical molecular size as the mitochondrial CAC that is immunoreactive with an anti-peptide antibody raised against a linear epitope of mitochondrial CAC is present in peroxisomes but not in microsomes. This suggests that CAC is targeted to at least two different locations within the liver cell and that acylcarnitine transport into peroxisomes is CAC mediated.

Comparisons of flux control exerted by mitochondrial outer-membrane carnitine palmitoyltransferase over ketogenesis in hepatocytes and mitochondria isolated from suckling or adult rats

The primary aim of this paper was to calculate and report flux control coefficients for mitochondrial outer-membrane carnitine palmitoyltransferase (CPT I) over hepatic ketogenesis because its role in controlling this pathway during the neonatal period is of academic importance and immediate clinical relevance. Using hepatocytes isolated from suckling rats as our model system, we measured CPT I activity and carbon flux from palmitate to ketone bodies and to CO2 in the absence and presence of a range of concentrations of etomoxir. (This is converted in situ to etomoxir-CoA which is a specific inhibitor of the enzyme.) From these data we calculated the individual flux control coefficients for CPT I over ketogenesis, CO2 production and total carbon flux (0.51 +/- 0.03; -1.30 +/- 0.26; 0.55 +/- 0.07, respectively) and compared them with equivalent coefficients calculated by similar analyses [Drynan, L., Quant, P.A. & Zammit, V.A. (1996) Biochem. J. 317, 791-795] in hepatocytes isolated from adult rats (0.85 +/- 0.20; 0.23 +/- 0.06; 1.06 +/- 0.29). CPT I exerts significantly less control over ketogenesis in hepatocytes isolated from suckling rats than those from adult rats. In the suckling systems the flux control coefficients for CPT I over ketogenesis specifically and over total carbon flux (< 0.6) are not consistent with the enzyme being rate-limiting. Broadly similar results were obtained and conclusions drawn by reanalysis of previous data {from experiments in mitochondria isolated from suckling or adult rats [Krauss, S., Lascelles, C.V., Zammit, V.A. & Quant, P.A. (1996) Biochem. J. 319, 427-433]} using a different approach of control analysis, although it is not strictly valid to compare flux control coefficients from different systems. Our overall conclusion is that flux control coefficients for CPT I over oxidative fluxes from palmitate (or palmitoyl-CoA) differ markedly according to (a) the metabolic state, (b) the stage of development, (c) the specific pathway studied and (d) the model system.

The malonyl-CoA-sensitive form of carnitine palmitoyltransferase is not localized exclusively in the outer membrane of rat liver mitochondria

The data used to support the idea that malonyl-coenzyme A (CoA)-sensitive carnitine palmitoyltransferase (CPT-I) is localized on the outer mitochondrial membrane are based on harsh techniques that disrupt mitochondrial physiology. We have turned to the use of the French press, which produces a shearing force that denudes mitochondria of their outer membrane without the physiologically disruptive effects characteristic of phosphate swelling. Our results indicate that the mitoplasts contain just 15-19% of the outer membrane marker enzyme activity while retaining 85% of the total CPT activity and 50% of both CPT-I, as well as long-chain acyl-CoA synthase activity, the latter two supposed outer membrane enzymes. These mitoplasts were shown by electron microscopy to have the configuration of mitochondria that merely have been divested of their outer membranes. Carnitine-dependent fatty acid oxidation was retained in the mitoplasts, showing that they were physiologically intact. Moreover, protein immunoblotting analysis showed that CPT-I, as well as the inner CPT-II, was localized in the mitoplast fraction. The outer membrane fraction, which consisted of membrane "ghosts," contained most (50-60%) of marker enzyme activity, monoamine oxidase-B and porin proteins, but only about 27-29% CPT-I activity. Because CPT-I and long-chain acyl-CoA synthetase appear to be associated with both inner and outer membranes, we postulate that these enzymes reside in contact sites, which represent a melding of both limiting membranes.