Sensitivity of inhibition of rat liver mitochondrial outer-membrane carnitine palmitoyltransferase by malonyl-CoA to chemical- and temperature-induced changes in membrane fluidity

Acidosis attenuates CPT-I-supported bioenergetics as a potential mechanism limiting lipid oxidation

Fuel interactions in contracting muscle represent a complex interplay between enzymes regulating carbohydrate and fatty acid catabolism, converging in the mitochondrial matrix. While increasing exercise intensity promotes carbohydrate use at the expense of fatty acid oxidation, the mechanisms underlying this effect remain poorly elucidated. As a potential explanation, we investigated whether exercise-induced reductions in intramuscular pH (acidosis) attenuate carnitine palmitoyltransferase-I (CPT-I)-supported bioenergetics, the rate-limiting step for fatty acid oxidation within mitochondria. Specifically, we assessed the effect of a physiologically relevant reduction in pH (pH 7.2 versus 6.8) on single and mixed substrate respiratory responses in murine skeletal muscle isolated mitochondria and permeabilized fibers. While pH did not influence oxidative phosphorylation stoichiometry (ADP/O ratios), coupling efficiency, oxygen affinity, or ADP respiratory responses, acidosis impaired lipid bioenergetics by attenuating respiration with L-carnitine and palmitoyl-CoA, while enhancing the inhibitory effect of malonyl-CoA on CPT-I. These acidotic effects were largely retained following a single bout of intense exercise. At rest, pyruvate and succinate-supported respiration were also impaired by acidosis. However, providing more pyruvate and ADP at pH 6.8 to model increases in glycolytic flux and ATP turnover with intense exercise overcame the acidotic attenuation of carbohydrate-linked oxidative phosphorylation. Importantly, this situation is fundamentally different from lipids where CPT-I substrate sensitivity and availability is impaired at higher power outputs suggesting lipid metabolism may be more susceptible to the effects of acidosis, possibly contributing to fuel shifts with increasing exercise intensity.

The multi-factor modulated biogenesis of the mitochondrial multi-span protein Om14

The mitochondrial outer membrane (MOM) harbors proteins that traverse the membrane via several helical segments and are called multi-span proteins. To obtain new insights into the biogenesis of these proteins, we utilized yeast mitochondria and the multi-span protein Om14. Testing different truncation variants, we show that while only the full-length protein contains all the information that assures perfect targeting specificity, shorter variants are targeted to mitochondria with compromised fidelity. Employing a specific insertion assay and various deletion strains, we show that proteins exposed to the cytosol do not contribute significantly to the biogenesis process. We further demonstrate that Mim1 and Porin support optimal membrane integration of Om14 but none of them are absolutely required. Unfolding of newly synthesized Om14, its optimal hydrophobicity, and higher fluidity of the membrane enhanced the import capacity of Om14. Collectively, these findings suggest that MOM multi-span proteins follow different biogenesis pathways in which proteinaceous elements and membrane behavior contribute to a variable extent to the combined efficiency.

Mitochondrial membranes in cardiac muscle from Antarctic notothenioid fishes vary in phospholipid composition and membrane fluidity

Antarctic notothenioid fishes are highly stenothermal, yet their tolerance for warming is species-dependent. Because a body of literature points to the loss of cardiac function as underlying thermal limits in ectothermic animals, we investigated potential relationships among properties of ventricular mitochondrial membranes in notothenioids with known differences in both cardiac mitochondrial metabolism and organismal thermal tolerance. Fluidity of mitochondrial membranes was quantified by fluorescence depolarization for the white-blooded Chaenocephalus aceratus and the red-blooded Notothenia coriiceps. In these same membranes, lipid compositions and products of lipid peroxidation, the latter of which can disrupt membrane order, were analyzed in both species and in a second icefish, Pseudochaenichthys georgianus. Mitochondrial membranes from C. aceratus were significantly more fluid than those of the more thermotolerant species N. coriiceps (P < .0001). Consistent with this, ratios of total phosphatidylethanolamine (PE) to total phosphatidylcholine (PC) were lower in membranes from both species of icefishes, compared to those of N. coriiceps (P < .05). However, membranes of N. coriiceps displayed a greater unsaturation index (P < .0001). No differences among species were found in membrane products of lipid peroxidation. With rising temperatures, greater contents of PC in mitochondrial membranes from ventricles of icefishes are likely to promote membrane hyperfluidization at a lower temperature than for cardiac mitochondrial membranes from the red-blooded notothenioid. We propose that physical and chemical properties of the mitochondrial membranes may contribute to some of the observed differences in thermal sensitivity of physiological function among these species.

Dietary silymarin supplementation promotes growth performance and improves lipid metabolism and health status in grass carp (Ctenopharyngodon idellus) fed diets with elevated lipid levels

This study was carried out to evaluate whether silymarin supplementation influences growth, lipid metabolism, and health status in grass carp fed elevated dietary lipid levels. The juvenile fish (27.43 ± 0.17 g/tail) were fed six isonitrogenous and isocaloric diets in a factorial design containing 0, 100, or 200 mg kg^-1 silymarin (SM0, SM100, SM200) associated with either 4 or 8 % lipid level (low lipid, LL, and high lipid, HL, respectively) for 82 days. The results showed that both dietary silymarin supplementation and high lipid level significantly enhanced growth performance (WG, SGR), protein efficiency ratio, and feed utilization. Silymarin supplementation significantly reduced the VSI, hepatic lipid content, and the total bilirubin concentration in the serum. The gallbladdersomatic index displayed higher in the SM100 groups than SM200 groups. Serum total cholesterol content exhibited lower in the SM100 groups than SM0 groups. Meanwhile, significant interactions were shown for hepatic gene expression of HSL and CPT1 by two factors, and SM100 group had higher hepatic gene expression of HSL and CPT1 in fish fed with the HL diets. The SM100 groups up-regulated hepatic gene expressions of HMGCR and CYP7A1 compared with the SM0 groups. Silymarin supplementation notably reduced the elevated serum MDA content induced by HL treatments. Thus, silymarin supplementation markedly promoted growth and protein efficiency, suppressed lipid accumulation, and improved health status in grass carp fed with high-lipid diets, which might be associated with its enhancement of lipolysis and β-oxidation, antioxidant capacity.

Hepatic β-oxidation and regulation of carnitine palmitoyltransferase (CPT) I in blunt snout bream Megalobrama amblycephala fed a high fat diet

High-fat diets may promote growth, partly through their protein-sparing effects. However, high-fat diets often lead to excessive fat deposition, which may have a negative impact on fish such as poor growth and suppressive immune. Therefore, this study investigated the effects of a fat-rich diet on the mechanisms of fat deposition in the liver. Three-hundred blunt snout bream (Megalobrama amblycephala) juveniles (initial mass 18.00±0.05 g) were fed with one of two diets (5% or 15% fat) for 8 weeks. β-Oxidation capacity and regulation of rate-limiting enzymes were assessed. Large fat droplets were present in hepatocytes of fish fed the high-fat diet. This observation is thought to be largely owing to the reduced capacity for mitochondrial and peroxisomal β-oxidation in the livers of fish fed the high-fat diet, as well as the decreased activities of carnitine palmitoyltransferase (CPT) I and acyl-CoA oxidase (ACO), which are enzymes involved in fatty-acid metabolism. Study of CPT I kinetics showed that CPT I had a low affinity for its substrates and a low catalytic efficiency in fish fed the high-fat diet. Expression of both CPT I and ACO was significantly down-regulated in fish fed the high-fat diet. Moreover, the fatty-acid composition of the mitochondrial membrane varied between the two groups. In conclusion, the attenuated β-oxidation capacity observed in fish fed a high-fat diet is proposed to be owing to decreased activity and/or catalytic efficiency of the rate-limiting enzymes CPT I and ACO, via both genetic and non-genetic mechanisms.

Regulation of carnitine palmitoyltransferase (CPT) I during fasting in rainbow trout (Oncorhynchus mykiss) promotes increased mitochondrial fatty acid oxidation

Periods of fasting, in most animals, are fueled principally by fatty acids, and changes in the regulation of fatty acid oxidation must exist to meet this change in metabolic substrate use. We examined the regulation of carnitine palmitoyltransferase (CPT) I, to help explain changes in mitochondrial fatty acid oxidation with fasting. After fasting rainbow trout (Oncorhynchus mykiss) for 5 wk, the mitochondria were isolated from red muscle and liver to determine (1) mitochondrial fatty acid oxidation rate, (2) CPT I activity and the concentration of malonyl-CoA needed to inhibit this activity by 50% (IC(50)), (3) mitochondrial membrane fluidity, and (4) CPT I (all five known isoforms) and peroxisome proliferator-activated receptor (PPARα and PPARβ) mRNA levels. Fatty acid oxidation in isolated mitochondria increased during fasting by 2.5- and 1.75-fold in liver and red muscle, respectively. Fasting also decreased sensitivity of CPT I to malonyl-CoA (increased IC(50)), by two and eight times in red muscle and liver, respectively, suggesting it facilitates the rate of fatty acid oxidation. In the liver, there was also a significant increase CPT I activity per milligram mitochondrial protein and in whole-tissue PPARα and PPARβ mRNA levels. However, there were no changes in mitochondrial membrane fluidity in either tissue, indicating that the decrease in CPT I sensitivity to malonyl-CoA is not due to bulk fluidity changes in the membrane. However, there were significant differences in CPT I mRNA levels during fasting. Overall, these data indicate some important changes in the regulation of CPT I that promote the increased mitochondrial fatty acid oxidation that occurs during fasting in trout.

An environment-dependent structural switch underlies the regulation of carnitine palmitoyltransferase 1A

The enzyme carnitine palmitoyltransferase 1 (CPT1), which is anchored in the outer mitochondrial membrane (OMM), controls the rate-limiting step in fatty acid β-oxidation in mammalian tissues. It is inhibited by malonyl-CoA, the first intermediate of fatty acid synthesis, and it responds to OMM curvature and lipid characteristics, which reflect long term nutrient/hormone availability. Here, we show that the N-terminal regulatory domain (N) of CPT1A can adopt two complex amphiphilic structural states, termed Nα and Nβ, that interchange in a switch-like manner in response to offered binding surface curvature. Structure-based site-directed mutageneses of native CPT1A suggest Nα to be inhibitory and Nβ to be noninhibitory, with the relative Nα/Nβ ratio setting the prevalent malonyl-CoA sensitivity of the enzyme. Based on the amphiphilic nature of N and molecular modeling, we propose malonyl-CoA sensitivity to be coupled to the properties of the OMM by Nα-OMM associations that alter the Nα/Nβ ratio. For enzymes residing at the membrane-water interface, this constitutes an integrative regulatory mechanism of exceptional sophistication.

Packing of transmembrane domain 2 of carnitine palmitoyltransferase-1A affects oligomerization and malonyl-CoA sensitivity of the mitochondrial outer membrane protein

The purpose of this study was to investigate the sequence-dependence of oligomerization of transmembrane domain 2 (TM2) of rat carnitine palmitoyltransferase 1A (rCPT1A), to elucidate the role of this domain in the function of the full-length enzyme. Oligomerization of TM2 was studied qualitatively using complementary genetic assays that facilitate measurement of helix-helix interactions in the Escherichia coli inner membrane, and multiple quantitative biophysical methods. The effects of TM2-mutations on oligomerization and malonyl-CoA inhibition of the full-length enzyme (expressed in the yeast Pichia pastoris) were quantified. Changes designed to disrupt close-packing of the GXXXG(A) motifs reduced the oligomeric state of the corresponding TM2 peptides from hexamer to trimer (or lower), a reduction also observed on mutation of the TM2 sequence in the full-length enzyme. Disruption of these GXXXG(A) motifs had a parallel effect on the malonyl-CoA sensitivity of rCPT1A, reducing the IC(50) from 30.3 ± 5.0 to 3.0 ± 0.6 μM. For all measurements, wild-type rCPT1A was used as a control alongside various appropriate (e.g., molecular mass) standards. Our results suggest that sequence-determined, TM2-mediated oligomerization is likely to be involved in the modulation of malonyl-CoA inhibition of CPT1A in response to short- and long-term changes in protein-protein and protein-lipid interactions that occur in vivo.

Membrane microenvironment regulation of carnitine palmitoyltranferases I and II

CPT (carnitine palmitoyltransferase) 1 and CPT2 regulate fatty acid oxidation. Recombinant rat CPT2 was isolated from the soluble fractions of bacterial extracts and expressed in Escherichia coli. The acyl-CoA chain-length-specificity of the recombinant CPT2 was identical with that of the purified enzyme from rat liver mitochondrial inner membranes. The Km for carnitine for both the mitochondrial preparation and the recombinant enzyme was identical. In isolated mitochondrial outer membranes, cardiolipin (diphosphatidylglycerol) increased CPT1 activity 4-fold and the Km for carnitine 6-fold. It decreased the Ki for malonyl-CoA inhibition 60-fold, but had no effect on the apparent Km for myristoyl-CoA. Cardiolipin also activated recombinant CPT2 almost 4-fold, whereas phosphatidylglycerol, phosphatidylserine and phosphatidylcholine activated the enzyme 3-, 2- and 2-fold respectively. Most of the recombinant CPT2 was found to have substantial interaction with cardiolipin. A model is proposed whereby cardiolipin may hold the fatty-acid-oxidizing enzymes in the active functional conformation between the mitochondrial inner and outer membranes in conjunction with the translocase and the acyl-CoA synthetase, thus combining all four enzymes into a functional unit.

Carnitine palmitoyl transferase activity and whole muscle oxidation rates vary with fatty acid substrate in avian flight muscles

Birds primarily fuel migratory flights with fat, and the composition of that fat has the potential to affect overall lipid oxidation rates. We measured the whole muscle lipid oxidation rates in extensor digitorum communis muscles from white-throated sparrows (Zonotrichia albicollis Gmelin) incubated for 20 min at 20°C with radiolabeled stearate (18:0), oleate (18:1ω9), or linoleate (18:2ω6). Lipid oxidation rates were ~40% higher with linoleate than oleate (oleate: 36 ± 8.54 μmol CO(2) g(-1) h(-1)), and ~75% lower with stearate compared with oleate, indicating that maximal lipid oxidation rates can indeed be affected by the type of fatty acid supplied to the muscle. Additionally, we investigated the activity of the mitochondrial fatty acid transport-associated enzyme carnitine palmitoyl transferase (CPT) in pectoralis muscles of 5 bird species (Zonotrichia albicollis, Philomachus pugnax, Sturnus vulgaris, Taeniopygia guttata, Passer domesticus). Activity was measured in homogenized samples using various fatty acyl-CoA substrates (16:0, 16:1, 18:0, 18:1ω9, 18:2ω6, 18:3ω3, 18:3ω6, 20:0, 20:4ω6, 22:6ω3) in a spectrophotometric assay. CPT activity increased with the degree of unsaturation and decreased with chain length. CPT activity did not differ between ω3 and ω6 isomers of 18:3, nor was the pattern of CPT substrate preference different between captive white-throated sparrows in a migratory (i.e., displaying Zugunruhe) or non-migratory state. These findings can explain previously observed differences in peak performance induced by dietary fat composition and suggest that lipid supply is limiting to maximal exercise performance in birds.

Carnitine palmitoyltransferase I control of acetogenesis, the major pathway of fatty acid {beta}-oxidation in liver of neonatal swine

To examine the regulation of hepatic acetogenesis in neonatal swine, carnitine palmitoyltransferase I (CPT I) activity was measured in the presence of varying palmitoyl-CoA (substrate) and malonyl-CoA (inhibitor) concentrations, and [1-(14)C]-palmitate oxidation was simultaneously measured. Accumulation rates of (14)C-labeled acetate, ketone bodies, and citric acid cycle intermediates within the acid-soluble products were determined using radio-HPLC. Measurements were conducted in mitochondria isolated from newborn, 24-h (fed or fasted), and 5-mo-old pigs. Acetate rather than ketone bodies was the predominant radiolabeled product, and its production increased twofold with increasing fatty acid oxidation during the first 24-h suckling period. The rate of acetogenesis was directly proportional to CPT I activity. The high activity of CPT I in 24-h-suckling piglets was not attributable to an increase in CPT I gene expression, but rather to a large decrease in the sensitivity of CPT I to malonyl-CoA inhibition, which offset a developmental decrease in affinity of CPT I for palmitoyl-CoA. Specifically, the IC(50) for malonyl-CoA inhibition and K(m) value for palmitoyl-CoA measured in 24-h-suckling pigs were 1.8- and 2.7-fold higher than measured in newborn pigs. The addition of anaplerotic carbon from malate (10 mM) significantly reduced (14)C accumulation in acetate (P < 0.003); moreover, the reduction was much greater in newborn (80%) than in 24-h-fed (72%) and 5-mo-old pigs (55%). The results demonstrate that acetate is the primary product of hepatic mitochondrial beta-oxidation in Sus scrofa and that regulation during early development is mediated primarily via kinetic modulation of CPT I.

Enhanced susceptibility of Cpt1c knockout mice to glucose intolerance induced by a high-fat diet involves elevated hepatic gluconeogenesis and decreased skeletal muscle glucose uptake

AIMS/HYPOTHESIS

Carnitine palmitoyltransferase-1 (CPT1)c is a novel isoform in the CPT1 family and is found specifically in the brain. Cpt1c knockout (KO) mice are more susceptible to high-fat diet (HFD)-induced obesity. However, the underlying mechanism of this phenotype and the question of whether CPT1c is involved in the pathogenesis of diet-induced insulin resistance are unclear.

METHODS

To assess the potential role of CPT1c in the regulation of whole-body glucose homeostasis, we generated Cpt1c KO mice and challenged them with HFD or standard chow. Glucose homeostasis of each group was assessed weekly.

RESULTS

After 8 weeks of HFD feeding, Cpt1c KO mice developed a phenotype of more severe insulin resistance than that in wild-type controls. The increased susceptibility of Cpt1c KO mice to HFD-induced insulin resistance was independent of obesity. Impaired glucose tolerance in Cpt1c KO mice was attributable to elevated hepatic gluconeogenesis and decreased glucose uptake in skeletal muscle. These effects correlated with decreased hepatic and intramuscular fatty acid oxidation and expression of oxidative genes as well as with elevated triacylglycerol content in these tissues. Interestingly, Cpt1c deletion caused a specific elevation of hypothalamic CPT1a and CPT1b isoform expression and activity. We demonstrated that elevated plasma NEFA concentration is one mechanism via which this compensatory effect is induced.

CONCLUSIONS/INTERPRETATION

These results further establish the role of CPT1c in controlling whole-body glucose homeostasis and in the regulation of hypothalamic Cpt1 isoform expression. We identify changes in hepatic and skeletal muscle glucose metabolism as important mechanisms determining the phenotype of Cpt1c KO mice.

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.

Malonyl CoenzymeA decarboxylase regulates lipid and glucose metabolism in human skeletal muscle

OBJECTIVE

Malonyl coenzyme A (CoA) decarboxylase (MCD) is a key enzyme responsible for malonyl-CoA turnover and functions in the control of the balance between lipid and glucose metabolism. We utilized RNA interference (siRNA)-based gene silencing to determine the direct role of MCD on metabolic responses in primary human skeletal muscle.

RESEARCH DESIGN AND METHODS

We used siRNA to silence MCD gene expression in cultured human myotubes from healthy volunteers (seven male and seven female) with no known metabolic disorders. Thereafter, we determined lipid and glucose metabolism and signal transduction under basal and insulin-stimulated conditions.

RESULTS

RNA interference-based silencing of MCD expression (75% reduction) increased malonyl-CoA levels twofold and shifted substrate utilization from lipid to glucose oxidation. RNA interference-based depletion of MCD reduced basal palmitate oxidation. In parallel with this reduction, palmitate uptake was decreased under basal (40%) and insulin-stimulated (49%) conditions compared with myotubes transfected with a scrambled sequence. MCD silencing increased basal and insulin-mediated glucose oxidation 1.4- and 2.6-fold, respectively, compared with myotubes transfected with a scrambled sequence. In addition, glucose transport and cell-surface GLUT4 content was increased. In contrast, insulin action on IRS-1 tyrosine phosphorylation, tyrosine-associated phosphatidylinositol (PI) 3-kinase activity, Akt, and glycogen synthase kinase (GSK) phosphorylation was unaltered between myotubes transfected with siRNA against MCD versus a scrambled sequence.

CONCLUSIONS

These results provide evidence that MCD silencing suppresses lipid uptake and enhances glucose uptake in primary human myotubes. In conclusion, MCD expression plays a key reciprocal role in the balance between lipid and glucose metabolism.

Intertissue regulation of carnitine palmitoyltransferase I (CPTI): mitochondrial membrane properties and gene expression in rainbow trout (Oncorhynchus mykiss)

Carnitine palmitoyltransferase (CPT) I is regulated by several genetic and non-genetic factors including allosteric inhibition, mitochondrial membrane composition and/or fluidity and transcriptional regulation of enzyme content. To determine the intrinsic differences in these regulating factors that may result in differences between tissues in fatty acid oxidation ability, mitochondria were isolated from red, white and heart muscles and liver tissue from rainbow trout. Maximal activity (V(max)) for beta-oxidation enzymes and citrate synthase per mg tissue protein as well as CPT I in isolated mitochondria followed a pattern across tissues of red muscle>heart>white muscle>liver suggesting both quantitative and qualitative differences in mitochondria. CPT I inhibition showed a similar pattern with the highest malonyl-CoA concentration to inhibit activity by 50% (IC(50)) found in red muscle while liver had the lowest. Tissue malonyl-CoA content was highest in white muscle with no differences between the other tissues. Interestingly, the gene expression profiles did not follow the same pattern as the tissue enzyme activity. CPT I mRNA expression was greatest in heart>red muscle>white muscle>liver. In contrast, PPARalpha mRNA was greatest in the liver>red muscle>heart>white muscle. There were no significant differences in the mRNA expression of PPARbeta between tissues. As well, no significant differences were found in the mitochondrial membrane composition between tissues, however, there was a tendency for red muscle to exhibit higher proportions of PUFAs as well as a decreased PC:PE ratio, both of which would indicate increased membrane fluidity. In fact, there were significant correlations between IC(50) of CPT I for malonyl-CoA and indicators of membrane fluidity across tissues. This supports the notion that sensitivity of CPT I to its allosteric regulator could be modulated by changes in mitochondrial membrane composition and/or fluidity.

Prepartum Nutrition Alters Fatty Acid Composition in Plasma, Adipose Tissue, and Liver Lipids of Periparturient Dairy Cows

The fatty acyl profile of phospholipids (PL) determines the fluidity of cell membranes and affects cell function. The degree to which long-chain fatty acid (LCFA) composition of PL and triacylglycerols (TG) in liver and total lipids in adipose tissue can be altered by prepartum nutrition in peripartal dairy cows is unclear. Multiparous Holsteins (n = 25) were assigned to 1 of 4 prepartal diets: 1) CA, the control diet fed to meet 120% of energy requirements; 2) CR, a control diet fed to meet 80% of requirements; 3) S, a diet supplemented with mostly saturated free fatty acids (47% 16:0, 36% 18:0, 14% cis-18:1) and fed to meet 120% of requirements; or 4) U, a diet similar to S except that cows were abomasally infused with soybean oil so that the diet plus infused fat would meet 120% of requirements. Diets were fed for 40 d prepartum; all cows received a lactation diet postpartum. Groups CR and U had lower prepartum intakes of dry matter and net energy, but glucose concentrations in plasma were similar among treatments. Cows fed S, U, or CR had greater nonesterified fatty acids in plasma prepartum, but cows fed U had decreased beta-hydroxybutyrate postpartum. Postpartal concentrations of total lipids and glycogen in liver tissue were similar among treatments. Cows in group U had a greater percentage of 18:2 but less 16:0, 18:0, and 20:4 in plasma total lipids than cows fed S. Treatment U increased 18:2 and 18:3 and decreased 18:1 in subcutaneous adipose tissue at 1 d postpartum. Across diets, percentages of 16:0 and trans-18:1 were increased, and 18:0, 20:3, and 20:5 were decreased, in hepatic PL at d 1 postpartum. Significant treatment x time interactions indicated that treatment U increased 18:2 in hepatic PL at the expense of 18:1, 20:3, 20:4, 22:6, and 24:0 on d 1 postpartum, but changes were normalized by d 65 postpartum. The unsaturation index of hepatic PL was lower at d 1 than at d -45 or 65, which implies that hepatic membrane fluidity decreased around parturition. The unsaturation index at d 1 was greater for cows fed S than those fed CA or U. Percentages of 16:0, 18:1, and 22:0 were increased, and 18:0, 20:3, 20:4, 20:5, 24:0, and 26:0 were decreased, in hepatic TG at d 1. Prepartal feed restriction modestly affected tissue LCFA profiles. The LCFA profile of adipose tissue, liver PL, and liver TG can be altered by dietary LCFA supply prepartum; changes in liver are normalized by 65 d postpartum.

The mitochondrial intermembrane loop region of rat carnitine palmitoyltransferase 1A is a major determinant of its malonyl-CoA sensitivity

Carnitine palmitoyltransferase (CPT) 1A adopts a polytopic conformation within the mitochondrial outer membrane, having both the N- and C-terminal segments on the cytosolic aspect of the membrane and a loop region connecting the two transmembrane (TM) segments protruding into the inter membrane space. In this study we demonstrate that the loop exerts major effects on the sensitivity of the enzyme to its inhibitor, malonyl-CoA. Insertion of a 16-residue spacer between the C-terminal part of the loop sequence (i.e. between residues 100 and 101) and TM2 (which is predicted to start at residue 102) increased the sensitivity to malonyl-CoA inhibition of the resultant mutant protein by more than 10-fold. By contrast, the same insertion made between TM1 and the loop had no effects on the kinetic properties of the enzyme, indicating that effects on the catalytic C-terminal segment were specifically induced by loop-TM2 interactions. Enhanced sensitivity was also observed in all mutants in which the native TM2-loop pairing was disrupted either by making chimeras in which the loops and TM2 segments of CPT 1A and CPT 1B were exchanged or by deleting successive 9-residue segments from the loop sequence. The data suggest that the sequence spanning the loop-TM2 boundary determines the disposition of this TM in the membrane so as to alter the conformation of the C-terminal segment and thus affect its interaction with malonyl-CoA.

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.

Fat to the fire: the regulation of lipid oxidation with exercise and environmental stress

Lipids are an important fuel for submaximal aerobic exercise. The ways in which lipid oxidation is regulated during locomotion is an area of active investigation. Indeed, the integration between cellular regulation of lipid metabolism and whole-body exercise performance is a fascinating but often overlooked research area. Additionally, the interaction between environmental stress, exercise, and lipid oxidation has not been sufficiently examined. There are many functional and structural steps as fatty acids are mobilized, transported, and oxidized in working muscle, which may serve either as regulatory points for responding to acute or chronic stimuli or as raw material for natural selection. At the whole-animal level, the partitioning of lipids and carbohydrates across exercise intensities is remarkably similar among mammals, which suggests that there is conservation in regulatory mechanisms. Conversely, the proportions of circulatory and intramuscular fuels differ between species and across exercise intensities. Responses to acute and chronic environmental stress likely involve the interaction of genetic and nongenetic changes in the fatty acid pathway. Determining which of these factors help regulate the fatty acid pathway and what impact they have on whole-animal lipid oxidation and performance is an important area of future research. Using an integrative approach to complete the information loop from gene to physiological function provides the most powerful mode of analysis.

C75 [4-methylene-2-octyl-5-oxo-tetrahydro-furan-3-carboxylic acid] activates carnitine palmitoyltransferase-1 in isolated mitochondria and intact cells without displacement of bound malonyl CoA

Carnitine palmitoyltransferase 1beta (CPT-1beta) is a key regulator of the beta oxidation of long-chain fatty acids in skeletal muscle and therefore a potential therapeutic target for diseases associated with defects in lipid metabolism such as obesity and type 2 diabetes. C75 [4-methylene-2-octyl-5-oxo-tetrahydro-furan-3-carboxylic acid] is an alpha-methylene-butyrolactone that has been characterized as both an inhibitor of fatty acid synthase and more recently, an activator of CPT-1 (Thupari et al., 2002). Using human CPT-1beta expressed in the yeast Pichia pastoris, we demonstrate that C75 can activate the skeletal muscle isoform of CPT-1 and overcome inactivation of the enzyme by malonyl CoA, an important physiological repressor of CPT-1, and the malonyl CoA mimetic Ro25-0187 [{5-[2-(naphthalen-2-yloxy)-ethoxy]-thiophen-2-yl}-oxo-acetic acid]. We also show that C75 can activate CPT-1 in intact hepatocytes to levels similar to those achieved with inhibition of acetyl-CoA carboxylase, the enzyme that produces malonyl CoA. Finally, we demonstrate that concentrations of C75 sufficient for activation of CPT-1 do not displace bound malonyl CoA. We conclude that CPT-1 is an activator of human CPT-1beta and other CPT-1 isoforms but that it does not activate CPT-1 through antagonism of malonyl CoA binding.

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.

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.

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.

Fatty acid-induced apoptosis in neonatal cardiomyocytes: redox signaling

Exposure of neonatal rat cardiac myocytes to palmitate and glucose produces apoptosis as seen by cytochrome c release, caspase 3-like activation, DNA laddering, and poly(ADP-ribose) polymerase cleavage. The purpose of this study was to understand the role of reactive oxygen species in the initiation of programmed cell death by palmitate. We found that palmitate (but not oleate) produces inhibition of carnitine palmitoyltransferase I, accumulation of ceramide, and inhibition of electron transport complex III. These events are subsequent to cytochrome c release and loss of the mitochondrial membrane potential. No differences in H2O2 production or N-terminal c-Jun kinase phosphorylation were detected between myocytes incubated in palmitate and control myocytes (nonapoptotic) incubated in oleate. These results suggest that the palmitate-induced loss of the mitochondrial membrane potential is not associated with H2O2 synthesis and that a membrane potential is required to generate reactive oxygen species following ceramide inhibition of complex III.

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.

A metabolic role for mitochondria in palmitate-induced cardiac myocyte apoptosis

After cardiac ischemia, long-chain fatty acids, such as palmitate, increase in plasma and heart. Palmitate has previously been shown to cause apoptosis in cardiac myocytes. Cultured neonatal rat cardiac myocytes were studied to assess mitochondrial alterations during apoptosis. Phosphatidylserine translocation and caspase 3-like activity confirmed the apoptotic action of palmitate. Cytosolic cytochrome c was detected at 8 h and plateaued at 12 h. The mitochondrial membrane potential (DeltaPsi) in tetramethylrhodamine ethyl ester-loaded cardiac myocytes decreased significantly in individual mitochondria by 8 h. This loss was heterogeneous, with a few energized mitochondria per myocyte remaining at 24 h. Total ATP levels remained high at 16 h. The DeltaPsi loss was delayed by cyclosporin A, a mitochondrial permeability transition inhibitor. Mitochondrial swelling accompanied changes in DeltaPsi. Carnitine palmitoyltransferase I activity fell at 16 h; this decline was accompanied by ceramide increases that paralleled decreased complex III activity. We conclude that carnitine palmitoyltransferase I inhibition, ceramide accumulation, and complex III inhibition are downstream events in cardiac apoptosis mediated by palmitate and occur independent of events leading to caspase 3-like activation.

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.

Use of six chimeric proteins to investigate the role of intramolecular interactions in determining the kinetics of carnitine palmitoyltransferase I isoforms

The two isoforms of carnitine palmitoyltransferase I (CPT I; muscle (M)- and liver (L)-type) of the mitochondrial outer membrane have distinct kinetic characteristics with respect to their affinity for one of the substrates (l-carnitine) and the inhibitor malonyl-CoA. Moreover, they differ markedly in their hysteretic behavior with respect to malonyl-CoA and in their response to changes in the in vivo metabolic state. However, the two proteins are 62% identical and have the same overall structure. Using liver mitochondria, we have previously shown that the protein is polytopic within the outer membrane, comprising a 46-residue cytosolic N-terminal sequence, two transmembrane segments (TM1 and TM2) separated by a 27-residue loop, and a large catalytic domain (also cytosolic) (Fraser, F., Corstorphine, C. G., and Zammit, V. A. (1997) Biochem. J. 323, 711-718). We have now conducted a systematic study on six chimeric proteins constructed from combinations of three linear segments of rat L- and M-CPT I and on the two parental proteins to elucidate the effects of altered intramolecular interactions on the kinetics of CPT activity. The three segments were (i) the cytosolic N-terminal domain plus TM1, (ii) the loop plus TM2, and (iii) the cytosolic catalytic C-terminal domain. The kinetic properties of the chimeric proteins expressed in Pichia pastoris were studied. We found that alterations in the combinations of the N-terminal plus TM1 and C-terminal domains as well as in the N terminus plus TM1/TM2 pairings resulted in changes in the K(m) values for carnitine and palmitoyl-CoA and the sensitivity to malonyl-CoA of the L-type catalytic domain. The changes in affinity for malonyl-CoA and palmitoyl-CoA occurred independently of changes in the affinity for carnitine. The kinetic characteristics of the M-type catalytic domain and, in particular, its malonyl-CoA sensitivity were much less susceptible to influence by exchange of the other two segments of the protein. The marked difference in the response of the two catalytic domains to changes in the N-terminal domain and TM combinations explains the previously observed differences in the response of L- and M-CPT I to altered physiological state in intact mitochondria and to modulation of altered lipid molecular order of the mitochondrial outer membrane in vivo and in vitro.

The malonyl-CoA-long-chain acyl-CoA axis in the maintenance of mammalian cell function

Long-chain acyl-CoA esters have potent specific actions (e.g. on gene transcription, membrane trafficking) as well as non-specific ones (e.g. on phospholipid bilayers). They are synthesized on the cytosolic aspects of several intracellular membranes, to give rise to (a) cytosolic pool(s) to which a variety of enzymes and processes have access, including some localized in the nucleus. Their concentration in cells is highly regulated, interconversion with corresponding acylcarnitines being the most important mechanism involved. This reaction is catalysed by cytosol-accessible carnitine long-chain acyl (palmitoyl) transferase activities that are themselves located on multiple membrane systems. Regulation of these activities is through the inhibitory action of malonyl-CoA. Hence the existence of a potent malonyl-CoA-acyl-CoA axis through which many processes involved in the maintenance of mammalian cell function are regulated. The molecular, topographical and physiological interactions that make this possible are described and discussed.

Sequencing and functional expression of the malonyl-CoA-sensitive carnitine palmitoyltransferase from Drosophila melanogaster

Using expressed sequence tag data, we obtained a cDNA for a carnitine palmitoyltransferase I (CPT I)-like molecule from Drosophila melanogaster. The cDNA encodes a 782-residue protein that shows 49% and 48% sequence identity with the rat liver and skeletal-muscle isoforms of CPT I respectively. The sequence has two predicted membrane-spanning regions, suggesting that it adopts the same topology as its mammalian counterparts. The sequence contains all the residues that have been shown to be characteristic of carnitine acetyltransferases. Expression in the yeast Pichia pastoris confirmed that the cDNA does encode a CPT enzyme. The activity was found to be associated with a mitochondria-enriched fraction. Kinetic analysis revealed a K(m) for carnitine of 406 microM and a K(m) for palmitoyl-CoA of 105 microM. The CPT activity was very sensitive to inhibition by malonyl-CoA, with an IC(50) of 0.74 microM when the activity was assayed with 35 microM palmitoyl-CoA and 1% (w/v) albumin at pH 7.0. A histidine residue at position 140 in rat liver CPT I has been indicated to be important for inhibition by malonyl-CoA. The equivalent residue (position 136) in Drosophila CPT I is arginine, implying that any basic residue might be compatible with such sensitivity. Evidence is presented that, unlike in mammals, Drosophila has only a single CPT I gene. Sequences suggesting the existence of a splice variant in the 5' untranslated region were found; this was consistent with the existence of two promoters for the CPT I gene.

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.

Short-term regulation of carnitine palmitoyltransferase I in cultured rat hepatocytes: spontaneous inactivation and reactivation by fatty acids

Liver carnitine palmitoyltransferase I (CPT I), the rate-limiting enzyme of mitochondrial beta-oxidation, rapidly loses its activity when hepatocytes are put in culture. 3-Thia fatty acids reactivate the enzyme and can increase its activity 3-4-fold in 5-10 min. Normal fatty acids are also able to stimulate CPT I, but to a limited extent, compared to 3-thia fatty acid. This activation does not affect malonyl-CoA sensitivity. CPT I in hepatocytes from both fasted and fasted-carbohydrate refed rats is inactivated and reactivated to a similar extent. Free dodecylthioacetic acid (DTA) is at least as efficient as DTA-CoA as activator. CPT I activity in isolated mitochondria is not influenced by incubation with DTA, suggesting that the regulation of CPT I depends on an extramitochondrial component(s) in the cell. It is concluded that fatty acids activate pre-existing, inactive CPT I without involvement of gene transcription and independently of malonyl-CoA.

Roles of the N- and C-terminal domains of carnitine palmitoyltransferase I isoforms in malonyl-CoA sensitivity of the enzymes: insights from expression of chimaeric proteins and mutation of conserved histidine residues

The mitochondrial outer membrane enzyme carnitine palmitoyltransferase I (CPT I) plays a major role in the regulation of fatty acid entry into the mitochondrial matrix for beta-oxidation by virtue of its inhibition by malonyl-CoA. Two isoforms of CPT I, the liver type (L) and muscle type (M), have been identified, the latter being 100 times more sensitive to malonyl-CoA and having a much higher Km for the substrate carnitine. Here we have examined the roles of different regions of the CPT I molecules in their response to malonyl-CoA, etomoxir (an irreversible inhibitor) and carnitine. To this end, we analysed the properties of engineered rat CPT I constructs in which (a) the N-terminal domain of L-CPT I was deleted, (b) the N-terminal domains of L- and M-CPT I were switched, or (c) each of three conserved histidine residues located towards the N-terminus in L-CPT I was mutated. Several novel points emerged: (1) whereas the N-terminal domain is critical for a normal malonyl-CoA response, it does not itself account for the widely disparate sensitivities of the liver and muscle enzymes to the inhibitor; (2) His-5 and/or His-140 probably play a direct role in the malonyl-CoA response, but His-133 does not; (3) the truncated, chimaeric and point- mutant variants of the enzyme all bound the covalent, active-site- directed ligand, etomoxir; and (4) only the most radical alteration of L-CPT I, i.e. deletion of the N-terminal 82 residues, affected the response to carnitine. We conclude that the N-terminal domain of CPT I plays an essential, but permissive, role in the inhibition of the enzyme by malonyl-CoA. By contrast, the larger C-terminal region dictates the degree of sensitivity to malonyl-CoA, as well as the response to carnitine; it is also sufficient for etomoxir binding. Additionally, further weight is added to the notion that one or more histidine residues may be involved in the CPT I-malonyl-CoA interaction.

Lipid molecular order in liver mitochondrial outer membranes, and sensitivity of carnitine palmitoyltransferase I to malonyl-CoA

Mitochondrial outer membranes were prepared from livers of rats that were in the normal fed state, starved for 48 h, or made diabetic by injection of streptozotocin. Membranes were also prepared from starved late-pregnant rats. The latter three conditions have previously been shown to induce varying degrees of desensitization of mitochondrial overt carnitine palmitoyltransferase (CPT I) to malonyl-CoA inhibition. We measured the fluorescence polarization anisotropy of two probes, 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene-p-toluenes ulfonate (TMA-DPH) which, when incorporated into membranes, report on the hydrophobic core and on the peripheral regions of the bilayer, respectively. The corresponding polarization indices (rDPH and rTMA-DPH) were calculated. In membranes of all three conditions characterized by CPT I desensitization to malonyl-CoA, rDPH was decreased, whereas there was no change in rTMA-DPH, indicating that CPT I is sensitive to changes in membrane core, rather than peripheral, lipid order. The major lipid components of the membranes were analyzed. Although significant changes with physiological state were observed, there was no consistent pattern of changes in gross lipid composition accompanying the changes to membrane fluidity and CPT I sensitivity to malonyl-CoA. We conclude that CPT I kinetic characteristics are sensitive to changes in lipid composition that are localized to specific membrane microdomains.

Acetyl-CoA carboxylase control of fatty acid oxidation in hearts from hibernating Richardson's ground squirrels

Although mammalian hibernators rely on stored body fat as a source of energy, direct measurement of energy substrate preference in heart tissue during hibernation, as well as potential mechanisms controlling fatty acid oxidation has not been examined. In order to determine whether an increase in fatty acid utilization occurs during hibernation, glucose and palmitate oxidation were measured in isolated working hearts from hibernating and non-hibernating Richardson's ground Squirrels. Hearts were perfused at either 37 degrees or 5 degrees C with perfusate containing 11 mM [U-14C]glucose and 1.2 mM [9,10-3H]palmitate, which allowed for direct measurement of both glucose oxidation (14CO2 production) and fatty acid oxidation (3H2O production). The contribution of fatty acid oxidation as a source of citric acid cycle acetyl-CoA was significantly greater in hearts from hibernating animals, compared to hearts from non-hibernating animals. Since acetyl-CoA carboxylase (ACC) regulates cardiac fatty acid oxidation (producing malonyl-CoA, a potent inhibitor of mitochondrial fatty acid uptake), we measured the activity and expression of ACC in these hearts. ACC activity was significantly decreased in hibernating ground squirrels, regardless of whether ACC was assayed at 37 degrees or 5 degrees C. This decrease in activity could not be explained by a change in the activity of 5'AMP-activated protein kinase, which can phosphorylate and inhibit ACC. Rather, the expression of the 280 kDa isoform of ACC (which predominates in cardiac muscle) was decreased in hearts from hibernating squirrel hearts. This suggests that a down regulation of ACC expression occurs as an adaptation for the increased utilization of fatty acid in hearts of hibernating ground squirrels.

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

The submitochondrial distribution of the overt and latent carnitine palmitoyltransferases (CPT I and II respectively) of rat liver mitochondria were studied. Separation of outer and inner membranes, as well as of a fraction of intermediate density consisting of contact sites between the two membranes, was achieved, as judged by the distribution of marker enzymes. Both CPT I and CPT II were found to be enriched within the contact- site fraction of mitochondria. These data show that the two carnitine acyltransferases are distributed non-uniformly within their respective membranes, and that subpopulations of the two enzymes occur in close proximity within the mitochondrial membrane structure, while retaining their different accessibilities to cytosolic and matrix pools of metabolites. As the number of contact sites is known to vary with changes in the energy status of mitochondria, the possibility that such changes may acutely affect the proportion of CPT I within the distinctive lipid environment of the contact sites, and thus its overall kinetic characteristics, is discussed.

Dietary fatty acids influence the activity and metabolic control of mitochondrial carnitine palmitoyltransferase I in rat heart and skeletal muscle

The fatty acid composition of the diet has been found to influence the activity and sensitivity of mitochondrial carnitine palmitoyltransferase I (CPT I; EC 2.3.1.21) to inhibition by malonyl CoA in rat heart and skeletal muscle. The nutritional state of rats has been shown to have less influence on the activity and metabolic control of mitochondrial CPT I in heart and skeletal muscle tissue than in the liver, a tissue in which CPT I activity and sensitivity to inhibition by malonyl CoA can be shown to be regulated acutely under different nutritional conditions. However, because manipulation of the nutritional state in these previous studies was restricted mainly to examining the effect of starvation, this study was undertaken to determine whether, as in liver, the fatty acid content and composition of the diet can regulate the activity and metabolic control of CPT I in heart and skeletal muscle. Rats were fed for up to 10 wk either a nonpurified low fat diet (30 g fat/kg) or a high fat diet (200 g fat/kg) containing one of the following five oil types: hydrogenated coconut oil (HCO), olive oil (OO), safflower oil (SO), evening primrose oil (EPO) or menhaden (fish) oil (MO). Feeding a diet enriched in MO had the most pronounced effect. Rats fed MO had a significantly greater skeletal muscle CPT I specific activity and tissue capacity, and a lower sensitivity of CPT I to malonyl CoA inhibition compared with rats fed a low fat diet, but the duration of feeding required to modulate this sensitivity was longer than that observed previously for the liver enzyme. Progressively greater sensitivity of heart CPT I to malonyl CoA occurred with feeding duration in all groups. These studies indicate that the fatty acid composition of the diet is involved in the regulation of mitochondrial CPT I activity in heart and skeletal muscle.

Regulation of mitochondrial outer-membrane carnitine palmitoyltransferase (CPT I): role of membrane-topology

The topology of the outer membrane carnitine palmitoyltransferase (CPT I) of rat liver mitochondria was studied systematically using several experimental approaches. Studies with immobilized malonyl-CoA and octanoyl-CoA showed that functionally the active and regulatory sites of CPT I are exposed on the outer (cytosolic) surface of the mitochondrial outer membrane. Anti-peptide antibodies generated against three linear peptide sequences that occur in between and on either side of two hydrophobic, putative transmembrane domains were used to (a) ascertain which were bound by intact mitochondria and mitochondria in which the outer membrane was permeabilized to proteins; and (b) to determine the size of fragments generated by limited proteolysis (by trypsin or proteinase K) of CPT I in intact or outer membrane-ruptured mitochondria. The sizes and immunoreactivity of the proteolytic fragments generated were correlated with the effects of the proteases on CPT I activity and malonyl-CoA sensitivity. The results of all the different approaches suggested the following: (i) CPT I has two transmembrane domains; (ii) both the N- and C-termini are exposed on the cytosolic side of the membrane; (iii) the linker region between the two transmembrane domains protrudes into the intermembrane space; (iv) both the active site and the malonyl-CoA-binding site are exposed on the cytosolic side of the membrane; (v) the amino-terminus of the protein interacts with the C-terminal domain of the protein to maintain the optimal conformation required for activity of the enzyme and its sensitivity to malonyl-CoA.

Topology of carnitine palmitoyltransferase I in the mitochondrial outer membrane

The topology of carnitine palmitoyltransferase I (CPT I) in the outer membrane of rat liver mitochondria was studied using several approaches. 1. The accessibility of the active site and malonyl-CoA-binding site of the enzyme from the cytosolic aspect of the membrane was investigated using preparations of octanoyl-CoA and malonyl-CoA immobilized on to agarose beads to render them impermeant through the outer membrane. Both immobilized ligands were fully able to interact effectively with CPT I. 2. The effects of proteinase K and trypsin on the activity and malonyl-CoA sensitivity of CPT I were studied using preparations of mitochondria that were either intact or had their outer membranes ruptured by hypo-osmotic swelling (OMRM). Proteinase K had a marked but similar effect on CPT I activity irrespective of whether only the cytosolic or both sides of the membrane were exposed to it. However, it affected sensitivity more rapidly in OMRM. By contrast, trypsin only reduced CPT I activity when incubated with OMRM. The sensitivity of the residual CPT I activity was unaffected by trypsin. 3. The proteolytic fragments generated by these treatments were studied by Western blotting using three anti-peptide antibodies raised against linear epitopes of CPT I. These showed that a proteinase K-sensitive site close to the N-terminus was accessible from the cytosolic side of the membrane. No trypsin-sensitive sites were accessible in intact mitochondria. In OMRM, both proteinase K and trypsin acted from the inter-membrane space side of the membrane. 4. The ability of intact mitochondria and OMRM to bind to each of the three anti-peptide antibodies was used to study the accessibility of the respective epitopes on the cytosolic and inter-membrane space sides of the membrane. 5. The results of all these approaches indicate that CPT I adopts a bitopic topology within the mitochondrial outer membrane; it has two transmembrane domains, and both the N- and C-termini are exposed on the cytosolic side of the membrane, whereas the linker region between the transmembrane domains protrudes into the intermembrane space.

Effect of membrane environment on the activity and inhibitability by malonyl-CoA of the carnitine acyltransferase of hepatic microsomal membranes

We have investigated the extent to which membrane environment affects the catalytic properties of the malonyl-CoA-sensitive carnitine acyltransferase of liver microsomal membranes. Arrhenius-type plots of activity were linear in the absence and presence of malonyl-CoA (2.5 microM). Sensitivity to malonyl-CoA increased with decreasing assay temperature. Partly purified enzyme displayed an increased K0.5 (substrate concentration supporting half the maximal reaction rate) for myristoyl-CoA and a reduced sensitivity to malonyl-CoA compared with the enzyme in situ in membranes. Reconstitution with liposomes of a range of compositions restored the K0.5 for myristoyl-CoA to values similar to that seen in native membranes. The lipid requirements for restoration of sensitivity to malonyl-CoA were more stringent. When animals were starved for 24 h the specific activity of carnitine acyltransferase in microsomal membrane residues was increased 3.3-fold, whereas sensitivity to malonyl-CoA was decreased to 1/2.8. When enzymes partly purified from fed and starved animals were reconstituted into crude soybean phosphatidylcholine liposomes there was no difference in sensitivity to malonyl-CoA. When partly purified enzyme from fed rats was reconstituted into liposomes prepared from microsomal membrane lipids from fed animals it was 2.2-fold more sensitive to malonyl-CoA than when reconstituted with liposomes prepared from microsomal membrane lipids from starved animals. This suggests that the physiological changes in sensitivity to malonyl-CoA are mediated via changes in membrane lipid composition rather than via modification of the enzyme protein itself. The increased specific actvity of acyltransferase observed on starvation could not be attributed to changes in membrane lipid composition.

Mammalian mitochondrial beta-oxidation

The enzymic stages of mammalian mitochondrial beta-oxidation were elucidated some 30-40 years ago. However, the discovery of a membrane-associated multifunctional enzyme of beta-oxidation, a membrane-associated acyl-CoA dehydrogenase and characterization of the carnitine palmitoyl transferase system at the protein and at the genetic level has demonstrated that the enzymes of the system itself are incompletely understood. Deficiencies of many of the enzymes have been recognized as important causes of disease. In addition, the study of these disorders has led to a greater understanding of the molecular mechanism of beta-oxidation and the import, processing and assembly of the beta-oxidation enzymes within the mitochondrion. The tissue-specific regulation, intramitochondrial control and supramolecular organization of the pathway is becoming better understood as sensitive analytical and molecular techniques are applied. This review aims to cover enzymological and organizational aspects of mitochondrial beta-oxidation together with the biochemical aspects of inherited disorders of beta-oxidation and the intrinsic control of beta-oxidation.

The role of changes in the sensitivity of hepatic mitochondrial overt carnitine palmitoyltransferase in determining the onset of the ketosis of starvation in the rat

The relationships between the increase in blood ketone-body concentrations and several parameters that can potentially influence the rate of hepatic fatty acid oxidation were studied during progressive starvation (up to 24 h) in the rat in order to discover whether the sensitivity of mitochondrial overt carnitine palmitoyltransferase (CPT I) to malonyl-CoA plays an important part in determining the intrahepatic potential for fatty acid oxidation during the onset of ketogenic conditions. A rapid increase in blood ketone-body concentration occurred between 12 and 16 h of starvation, several hours after the marked fall in hepatic malonyl-CoA and in serum insulin concentrations and doubling of plasma non-esterfied fatty acid (NEFA) concentration. Consequently, both the changes in hepatic malonyl-CoA and serum NEFA preceded the increase in blood ketone-body concentration by several hours. The maximal activity of CPT I increased gradually throughout the 24 h period of starvation, but the increases did not become significant before 18 h of starvation. By contrast, the sensitivity of CPT I to malonyl-CoA and the increase in blood ketone-body concentration followed an identical time course, demonstrating the central importance of this parameter in determining the ketogenic response of the liver to the onset of the starved state.