Hepatic mitochondrial inner membrane properties and carnitine palmitoyltransferase A and B. Effect of diabetes and starvation

Curative role of natural PPARγ agonist in non-alcoholic fatty liver disease (NAFLD)

NAFLD is a condition that develops when the liver accumulates excess fat without alcohol consumption. This chronic liver ailment progresses along with insulin resistant and is typically not diagnosed until the patients have cirrhosis. Nuclear hormone receptor superfamily PPARs are essential for metabolism of fatty acids and glucose. In liver, lipid metabolism is regulated by nuclear receptors and PPARα, and PPARβ/δ encourages fatty acid β-oxidation. PPAR-γ, an energy-balanced receptor is a crucial regulator in NAFLD. The partial activation of PPAR-γ could lead to increased level of adiponectin and insulin sensitivity, thus improved NAFLD. Because of less side effects, natural compounds are emerged as potential therapeutic agents for NAFLD by PPARγ agonists. Although the results from preclinical studies are promising, further research is needed to determine the potential dosing and efficacy of mentioned compounds in human subjects. In this review, we summarize the effect of natural PPARγ agonist in the NAFLD.

Adaptation of Oxidative Phosphorylation Machinery Compensates for Hepatic Lipotoxicity in Early Stages of MAFLD

Alterations in mitochondrial function are an important control variable in the progression of metabolic dysfunction-associated fatty liver disease (MAFLD), while also noted by increased de novo lipogenesis (DNL) and hepatic insulin resistance. We hypothesized that the organization and function of a mitochondrial electron transport chain (ETC) in this pathologic condition is a consequence of shifted substrate availability. We addressed this question using a transgenic mouse model with increased hepatic insulin resistance and DNL due to constitutively active human SREBP-1c. The abundance of ETC complex subunits and components of key metabolic pathways are regulated in the liver of these animals. Further omics approaches combined with functional assays in isolated liver mitochondria and primary hepatocytes revealed that the SREBP-1c-forced fatty liver induced a substrate limitation for oxidative phosphorylation, inducing enhanced complex II activity. The observed increased expression of mitochondrial genes may have indicated a counteraction. In conclusion, a shift of available substrates directed toward activated DNL results in increased electron flows, mainly through complex II, to compensate for the increased energy demand of the cell. The reorganization of key compounds in energy metabolism observed in the SREBP-1c animal model might explain the initial increase in mitochondrial function observed in the early stages of human MAFLD.

Chronic and low exposure to a pharmaceutical cocktail induces mitochondrial dysfunction in liver and hyperglycemia: Differential responses between lean and obese mice

Pharmaceuticals are found in the environment but the impact of this contamination on human and animal health is poorly known. The liver could be particularly targeted since a significant number of these drugs are hepatotoxic, in particular via oxidative stress and mitochondrial dysfunction. Notably, the latter events can also be observed in liver diseases linked to obesity, so that the obese liver might be more sensitive to drug toxicity. In this study, we determined the effects of a chronic exposure to low doses of pharmaceuticals in wild-type and obese mice, with a particular focus on mitochondrial function. To this end, wild-type and ob/ob mice were exposed for 4 months to a cocktail of 11 pharmaceuticals provided in drinking water containing 0.01, 0.1, or 1 mg/L of each drug. At the end of the treatment, liver mitochondria were isolated and different parameters were measured. Chronic exposure to the pharmaceuticals reduced mitochondrial respiration driven by succinate and palmitoyl-l-carnitine in wild-type mice and increased antimycin-induced ROS production in ob/ob mice. Hyperglycemia and hepatic histological abnormalities were also observed in treated ob/ob mice. Investigations were also carried out in isolated liver mitochondria incubated with the mixture, or with each individual drug. The mitochondrial effects of the mixture were different from those observed in treated mice and could not be predicted from the results obtained with each drug. Because some of the 11 drugs included in our cocktail can be found in water at relatively high concentrations, our data could be relevant in environmental toxicology. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 1375-1389, 2017.

Even a Chronic Mild Hyperglycemia Affects Membrane Fluidity and Lipoperoxidation in Placental Mitochondria in Wistar Rats

It is known the deleterious effects of diabetes on embryos, but the effects of diabetes on placenta and its mitochondria are still not well known. In this work we generated a mild hyperglycemia model in female wistar rats by intraperitoneal injection of streptozotocin in 48 hours-old rats. The sexual maturity onset of the female rats was delayed around 6–7 weeks and at 16 weeks-old they were mated, and sacrificed at day 19th of pregnancy. In placental total tissue and isolated mitochondria, the fatty acids composition was analyzed by gas chromatography, and lipoperoxidation was measured by thiobarbituric acid reactive substances. Membrane fluidity in mitochondria was measured with the excimer forming probe dipyrenylpropane and mitochondrial function was measured with a Clark-type electrode. The results show that even a chronic mild hyperglycemia increases lipoperoxidation and decreases mitochondrial function in placenta. Simultaneously, placental fatty acids metabolism in total tissue is modified but in a different way than in placental mitochondria. Whereas the chronic mild hyperglycemia induced a decrease in unsaturated to saturated fatty acids ratio (U/S) in placental total tissue, the ratio increased in placental mitochondria. The measurements of membrane fluidity showed that fluidity of placenta mitochondrial membranes increased with hyperglycemia, showing consistency with the fatty acids composition through the U/S index. The thermotropic characteristics of mitochondrial membranes were changed, showing lower transition temperature and activation energies. All of these data together demonstrate that even a chronic mild hyperglycemia during pregnancy of early reproductive Wistar rats, generates an increment of lipoperoxidation, an increase of placental mitochondrial membrane fluidity apparently derived from changes in fatty acids composition and consequently, mitochondrial malfunction.

Dietary sphingolipids ameliorate disorders of lipid metabolism in Zucker fatty rats

Dietary sphingolipids (SL) inhibit colon carcinogenesis, reduce serum cholesterol, and improve skin barrier function and are considered to be "functional lipids". For comparative determination of the effects of SL with different chemical compositions on lipid metabolism and its related hepatic gene expression, Zucker fatty rats were fed pure sphingomyelin (SM) of animal origin and glucosylceramide (GC) of plant origin. After 45 days, the SM and GC diets led to significant reductions in hepatic lipid and plasma non-HDL cholesterol. Both SM and GC diets decreased plasma insulin levels, whereas only the GC diet increased the plasma adiponectin level. Hepatic gene expression analysis revealed increased expression of adiponectin receptor 2 (Adipor2), peroxisome proliferator-activated receptor alpha (PPARalpha), and pyruvate dehydrogenase kinase 4 (Pdk4). However, expression of stearoyl CoA desaturase (Scd1) was significantly decreased. These results suggest that dietary SL, even of different origins and chemical compositions, may prevent fatty liver and hypercholesterolemia through improvement of adiponectin signaling and consequent increases in insulin sensitivity.

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.

Phosphorylation of rat liver mitochondrial carnitine palmitoyltransferase-I: effect on the kinetic properties of the enzyme

Hepatic carnitine palmitoyltransferase-I (CPT-IL) isolated from mitochondrial outer membranes obtained in the presence of protein phosphatase inhibitors is readily recognized by phosphoamino acid antibodies. Mass spectrometric analysis of CPT-IL tryptic digests revealed the presence of three phosphopeptides including one with a protein kinase CKII (CKII) consensus site. Incubation of dephosphorylated outer membranes with protein kinases and [gamma-32P]ATP resulted in radiolabeling of CPT-I only by CKII. Using mass spectrometry, only one region of phosphorylation was detected in CPT-I isolated from CKII-treated mitochondria. The sequence of the peptide and position of phosphorylated amino acids have been determined unequivocally as FpSSPETDpSHRFGK (residues 740-752). Furthermore, incubation of dephosphorylated outer membranes with CKII and unlabeled ATP led to increased catalytic activity and rendered malonyl-CoA inhibition of CPT-I from competitive to uncompetitive. These observations identify a new mechanism for regulation of hepatic CPT-I by phosphorylation.

Expression of mitochondrial tricarboxylate carrier TCC mRNA and protein in the rat brain

The tricarboxylate carrier protein catalyzes an electroneutral exchange across the mitochondrial inner membrane of tricarboxylate, dicarboxylate or phosphoenolpyruvate. We examined expression and localization of mitochondrial tricarboxylate carrier TCC mRNA and protein in the rat brain. TCC mRNA was ubiquitously expressed in all rat tissues examined and was abundant in brain, liver and kidney. TCC protein as well as mRNA was widely expressed in brain, and the protein expression was strong in neuronal cells in the hippocampus, the olfactory bulb, the corpus mamillare and the cerebellum. Our results suggest that this tricarboxylate carrier protein may contribute to biosynthesis and bioenergetics in neuronal cells in brain.

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.

Influence of diet on the kinetic behavior of hepatic carnitine palmitoyltransferase I toward different acyl CoA esters

The influence of diet on the kinetics of the overt form of rat liver mitochondrial carnitine palmitoyltransferase (CPT I; EC 2.3.1.21) was studied using rats fed either a low-fat diet (3% w/w fat), or diets which were supplemented with either olive oil (OO), safflower oil (SO) or menhaden (fish) oil (MO) to 20% w/w of fat (high fat diets). When animals were fed each of these four diets for 10 days, the order of the apparent maximal activity (Vmax) of CPT I toward various individual fatty acyl CoA, when measured under a fixed molar ratio of acyl CoA/albumin, was 16:1 n-7 > 18:1 n-9 > 18:2 n-6 > 16:0 > 22:6 n-3, and was thus not affected by the fat composition of the diet. However, in all but one case, the SO and MO diets elicited a higher Vmax for each substrate than either the LF diet or the high fat OO diet. The apparent K0.5 for the different acyl CoA esters was generally lowest in LF-fed animals, and highest in those fed the high-fat SO diet. Moreover, when compared with the situation of animals fed high-fat diets, the K0.5 values of CPT I in LF-fed animals for palmitoyl CoA and oleoyl CoA were low. This possession by CPT I of a high "affinity" toward these nonessential fatty acyl CoAs, but a lower "affinity" toward linoleoyl CoA, the ester of an essential fatty acid, may enable this latter fatty acid to be spared from oxidation when its concentration in the diet is low. The data also emphasize that palmitoleoyl CoA, if available in the diet, is likely to be utilized by CPT I at a high rate.

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.

No correlation between changes in fatty acid-binding protein content and fatty acid oxidation capacity of rat tissues in experimental diabetes

Fatty acid-binding protein is considered to play an important role in fatty acid oxidation. Since diabetes mellitus causes marked changes of this latter metabolic process, we compared the effect of this pathological condition on both parameters in a comparative investigation of different rat tissues. Palmitate oxidation capacity and content of fatty acid-binding protein were determined in liver, heart and quadriceps muscle from rats with 2-week streptozotocin-induced diabetes mellitus and controls. In liver homogenates fatty acid oxidation capacity increased by 90%, but their content of fatty acid-binding protein decreased by 35%. Fatty acid oxidation capacity of heart and quadriceps muscle and fatty acid-binding protein content of quadriceps muscle did not change, but fatty acid-binding protein content of heart muscle doubled. Long-term diabetes (8 months) had a similar effect on content of this protein. In summary, changes of fatty acid oxidation capacity do not appear to correlate with fatty acid-binding protein content during the development of diabetes. This does not preclude other functions of fatty acid-binding proteins in regulation of lipid metabolism and processes in which fatty acids play a modulatory role.

Cardiolipins and biomembrane function

Evidence is discussed for roles of cardiolipins in oxidative phosphorylation mechanisms that regulate State 4 respiration by returning ejected protons across and over bacterial and mitochondrial membrane phospholipids, and that regulate State 3 respiration through the relative contributions of proteins that transport protons, electrons and/or metabolites. The barrier properties of phospholipid bilayers support and regulate the slow proton leak that is the basis for State 4 respiration. Proton permeability is in the range 10(-3)-10(-4) cm s-1 in mitochondria and in protein-free membranes formed from extracted mitochondrial phospholipids or from stable synthetic phosphatidylcholines or phosphatidylethanolamines. The roles of cardiolipins in proton conductance in model phospholipid membrane systems need to be assessed in view of new findings by Hübner et al. [313]: saturated cardiolipins form bilayers whilst natural highly unsaturated cardiolipins form nonlamellar phases. Mitochondrial cardiolipins apparently participate in bilayers formed by phosphatidylcholines and phosphatidylethanolamines. It is not yet clear if cardiolipins themselves conduct protons back across the membrane according to their degree of fatty acyl saturation, and/or modulate proton conductance by phosphatidylcholines and phosphatidylethanolamines. Mitochondrial cardiolipins, especially those with high 18:2 acyl contents, strongly bind many carrier and enzyme proteins that are involved in oxidative phosphorylation, some of which contribute to regulation of State 3 respiration. The role of cardiolipins in biomembrane protein function has been examined by measuring retained phospholipids and phospholipid binding in purified proteins, and by reconstituting delipidated proteins. The reconstitution criterion for the significance of cardiolipin-protein interactions has been catalytical activity; proton-pumping and multiprotein interactions have yet to be correlated. Some proteins, e.g., cytochrome c oxidase are catalytically active when dimyristoylphosphatidylcholine replaces retained cardiolipins. Cardiolipin-protein interactions orient membrane proteins, matrix proteins, and on the outerface receptors, enzymes, and some leader peptides for import; activate enzymes or keep them inactive unless the inner membrane is disrupted; and modulate formation of nonbilayer HII-phases. The capacity of the proton-exchanging uncoupling protein to accelerate thermogenic respiration in brown adipose tissue mitochondria of cold-adapted animals is not apparently affected by the increased cardiolipin unsaturation; this protein seems to take over the protonophoric role of cardiolipins in other mitochondria. Many in vivo influences that affect proton leakage and carrier rates selectively alter cardiolipins in amount per mitochondrial phospholipids, in fatty acyl composition and perhaps in sidedness; other mitochondrial membrane phospholipids respond less or not at all.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of mitochondrial carnitine palmitoyl transferases from liver and extrahepatic tissues

Developments in our understanding of the complex CPT enzyme system over the past ten years have been reviewed. Liver CPT1, which is probably distinct from that in several extrahepatic tissues, is subject to up- or down-regulation of its activity and kinetic properties with changing physiological state. Evidence is now accumulating to support the notion that the catalytic and malonyl-CoA-binding entities of CPT1 are separate polypeptides.

Effect of insulin on the properties of liver carnitine palmitoyltransferase in the starved rat: assessment by the euglycemic hyperinsulinemic clamp

The effect of insulin on the properties of liver carnitine palmitoyltransferase I (CPT I) was assessed in conscious starved rats with the euglycemic hyperinsulinemic clamp. A 24-hour clamp was necessary to fully reverse the effect of starvation on liver malonyl-CoA concentration, CPT I maximal activity, and apparent km and Ki for malonyl-CoA. Since glucagon was not decreased during the clamp, insulin is the major factor involved in the regulation of CPT I.

A study of properties and abundance of the components of liver carnitine palmitoyltransferases in mitochondrial inner and outer membranes. Effects of hypothyroidism, fasting and a ketotic diabetic state

1. Liver mitochondrial outer and inner membranes were isolated from normal, 48 h-fasted, streptozotocin-diabetic and hypothyroid rats. 2. Relative to membrane protein, fasting and diabetes substantially increased the activity of carnitine palmitoyltransferase (CPT) in outer membranes. Inner-membrane CPT specific activity was only slightly altered, being increased in diabetes and decreased in hypothyroidism. Abundance of an inner-membrane Mr-68,000 polypeptide that cross-reacted with an anti-CPT serum was significantly increased in diabetes and hypothyroidism. Relative to inner-membrane CPT activity, this cross-reactivity was increased by 37% in diabetes and by 400% in hypothyroidism, suggesting modification of the intrinsic activity of the CPT in these states. 3. CPT in outer membranes was inhibitable by malonyl-CoA, whereas inner-membrane CPT was insensitive to malonyl-CoA. Fasting and diabetes increased the IC50 (concentration of malonyl-CoA causing 50% inhibition) for outer-membrane CPT, whereas the IC50 was decreased in hypothyroidism. 4. Binding of [14C]malonyl-CoA was observed with both outer and inner membranes and was fitted to two-site models in each case. Fasting, diabetes and hypothyroidism changed the KD for binding at the higher-affinity site in outer membranes in a manner that correlated closely with changes in IC50 for inhibition of outer-membrane CPT by malonyl-CoA. Fasting and diabetes increased the abundance of this outer-membrane high-affinity malonyl-CoA-binding site, whereas hypothyroidism decreased its abundance.

Effects of cholesterol loading of mouse macrophages on carnitine palmitoyltransferase activity and sensitivity to inhibition by malonyl-CoA

Scavenger receptor-mediated uptake of acetylated low density lipoprotein-derived cholesterol by peritoneal mouse macrophages resulted in decreased activity of the malonyl-CoA inhibitable carnitine palmitoyltransferase and a decrease in the sensitivity of this enzyme to inhibition by malonyl-CoA.

Properties of the mitochondrial membrane and carnitine palmitoyltransferase in the periportal and the perivenous zone of the liver. Effects of chronic ethanol feeding

Rats were fed for 35 days a high-fat diet containing either 36% of total calories as ethanol (ethanol group) or an isocaloric amount of carbohydrate (control group). Then, mitochondria were isolated from the periportal and the perivenous zone of the liver in order to study the acinar heterogeneity of the effects of prolonged ethanol administration upon the properties of carnitine palmitoyltransferase I (CPT-I) and its membrane environment. Chronic ethanol ingestion selectively decreased CPT-I activity in periportal hepatocytes but equally increased enzyme sensitivity to malonyl-CoA and enzyme energy of activation in the two zones of the liver. In control animals, mitochondrial membrane showed higher fluidity and lower degree of saturation of phospholipid fatty acyl moieties in periportal than in perivenous hepatocytes. Prolonged ethanol feeding (i) decreased mitochondrial membrane fluidity; (ii) increased the proportion of palmitic acid and decreased that of arachidonic acid in mitochondrial phosphatidylcholine and phosphatidylethanolamine, whereas it drastically reduced the content of linoleic acid and concomitantly increased that of saturated and monoenoic fatty acids in cardiolipin; (iii) suppressed the disordering effects of the addition of ethanol to mitochondrial suspensions. All these ethanol-induced alterations of membrane fluidity and fatty acyl composition were not significantly different between periportal and perivenous mitochondria. In conclusion, chronic ethanol feeding changes the activity of CPT-I in a zone-selective manner but modifies both the regulatory properties of the enzyme and the properties of its lipid environment in a non-zone-selective manner. Hence factors in addition to the properties of the mitochondrial membrane seem to be involved in the ethanol-induced alterations of the CPT-I enzyme.

The effect of insulin supplementation on diabetes-induced alterations in the extractable levels of functional mitochondrial anion transport proteins

The effect of insulin supplementation on diabetes-induced alterations in the levels of functional mitochondrial anion transport proteins has been determined. The experimental approach consisted of the extraction of the pyruvate, dicarboxylate, and citrate transport proteins from the mitochondrial inner membrane with Triton X-114 using rat liver mitoplasts (prepared from control, diabetic, or insulin-supplemented diabetic animals) as the starting material, followed by the reconstitution of the function of each transporter in a proteoliposomal system. This experimental strategy permitted the quantification of the functional levels of these three transporters in the absence of the complications that arise when such measurements are carried out with intact mitochondria (or mitoplasts). We found that treatment of diabetic rats (i.e., animals that were injected with streptozotocin 3 weeks earlier) on a daily basis with insulin for 3 weeks resulted in a reversal of the diabetes-induced (a) increase in the extractable and reconstitutable total (and specific) transport activities of the pyruvate and dicarboxylate transporters and (b) decrease in the activity of the citrate transporter. These findings indicate that diabetes-induced alterations in the functional levels of mitochondrial anion transport proteins are a direct consequence of the insulin insufficiency that characterizes this disease. Furthermore, this study provides the first demonstration that insulin participates in the regulation of the functional levels of liver mitochondrial anion transport proteins.

Cholate separates the catalytic and malonyl-CoA-binding components of carnitine palmitoyltransferase from liver outer mitochondrial membranes

Sodium cholate was used as an anionic detergent to discriminate the two components of liver overt carnitine palmitoyltransferase (CPT1); namely a catalytic entity and a regulatory component that bound malonyl-CoA. Cholate solubilized approx. 40% of the malonyl-CoA binding entity from mitochondrial outer membranes without appreciable solubilization of CPT1 activity. Cholate did not interfere with binding of [14C]malonyl-CoA to outer membranes or to crude total mitochondrial membrane fractions. By contrast, the non-ionic detergent Tween-20 was ineffective in solubilizing the malonyl-CoA binding entity and also substantially interfered with the binding of [14C]malonyl-CoA. Both detergents appeared to cause total disengagement of the malonyl-CoA binding entity from the catalytic entity of CPT1 only when some inner membrane material was present. 'Reconstitution' experiments were performed in which a malonyl-CoA sensitivity conferring factor in cholate extracts from outer membranes was associated with CPT derived from inner membranes (CPT2). The IC50 for inhibition of CPT2 by malonyl-CoA in this artificial system was similar to that observed with CPT1 in situ in outer membranes. Extracts containing malonyl-CoA sensitivity conferring factor derived from outer membranes of fed or 48 h fasted rats were associated with CPT2 derived from fed rats. The outer membrane extracts from fasted animals conferred a lower maximum responsiveness to malonyl-CoA, but appeared to have a higher affinity for CPT2 than the extracts from fed rats. These results suggest that physiological state can alter the intrinsic properties of the malonyl-CoA sensitivity confering factor.

Characterization of overt carnitine palmitoyltransferase in rat platelets; involvement of insulin on its regulation

Saponin-permeabilization (30 micrograms/ml) of the platelet plasma membrane, which enables access of added compounds to mitochondrial overt carnitine palmitoyltransferase (CPT I), was applied to allow the rapid determination of CPT I activity in situ. The effects of diabetes and short-term incubation with insulin in vitro on the kinetic parameters and malonyl-CoA sensitivity of CPT I were also studied in rat platelets. CPT I exhibited ordinary Michaelis-Menten kinetics when platelets were incubated with palmitoyl-CoA. Malonyl-CoA showed an I50 (concentration giving 50% inhibition of CPT activity) of 0.92 +/- 0.11 microM in permeabilized platelets. Platelets obtained from diabetic rats (induced by streptozotocin injection) exhibited an increased Vmax and I50 for malonyl-CoA, and an unaltered Km for palmitoyl-CoA. In contrast, preincubation of platelets prepared from both fed control rats and diabetic rats with insulin (100 and 150 microU/ml) led to a decrease in enzyme activity when assayed with 75 microM palmitoyl-CoA and 0.5 mM L-carnitine as substrates. These in vivo and in vitro results suggested that insulin directly modulated rat platelet CPT I activity, as it does in the liver.

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

We have tested the possibility that alterations in the fluidity of the outer membrane of rat liver mitochondria could result in changes in the sensitivity of overt carnitine palmitoyltransferase (CPT I) to malonyl-CoA [Zammit (1986) Biochem. Soc. Trans. 14. 676-679]. The sensitivity of CPT I to malonyl-CoA inhibition was measured by using highly purified mitochondrial outer membranes prepared from fed or 48 h-starved rats in the presence and absence of agents that increase membrane fluidity by perturbing membrane lipid order [benzyl alcohol, isoamyl alcohol (3-methylbutan-l-ol) and 2-(2-methoxyethoxy)ethyl-8-(cis-2-n-octylpropyl)octanoate (A2C)]. All these agents resulted in marked decreases in the ability of malonyl-CoA to inhibit CPT I. This effect was accompanied by a modest increase in the absolute activity of CPT I in the absence of malonyl-CoA when the short-chain alcohols were used, but not when A2C was used, suggesting that the effect of increased membrane fluidity to decrease the malonyl-CoA sensitivity of CPT I may occur independently from other actions that may affect more directly the active site of the enzyme. In confirmation of the potential importance of fluidity changes, we showed that a marked increase in sensitivity of CPT I to malonyl-CoA could be produced when assays were performed at lower temperatures than those normally employed. These observations are discussed in the context of the slowness of the changes in CPT I sensitivity to malonyl-CoA inhibition that are induced by physiological perturbations.

Evidence that the sensitivity of carnitine palmitoyltransferase I to inhibition by malonyl-CoA is an important site of regulation of hepatic fatty acid oxidation in the fetal and newborn rabbit. Perinatal development and effects of pancreatic hormones in cultured rabbit hepatocytes

The temporal changes in oleate oxidation, lipogenesis, malonyl-CoA concentration and sensitivity of carnitine palmitoyltransferase I (CPT 1) to malonyl-CoA inhibition were studied in isolated rabbit hepatocytes and mitochondria as a function of time after birth of the animal or time in culture after exposure to glucagon, cyclic AMP or insulin. (1) Oleate oxidation was very low during the first 6 h after birth, whereas lipogenesis rate and malonyl-CoA concentration decreased rapidly during this period to reach levels as low as those found in 24-h-old newborns that show active oleate oxidation. (2) The changes in the activity of CPT I and the IC50 (concn. causing 50% inhibition) for malonyl-CoA paralleled those of oleate oxidation. (3) In cultured fetal hepatocytes, the addition of glucagon or cyclic AMP reproduced the changes that occur spontaneously after birth. A 12 h exposure to glucagon or cyclic AMP was sufficient to inhibit lipogenesis totally and to cause a decrease in malonyl-CoA concentration, but a 24 h exposure was required to induce oleate oxidation. (4) The induction of oleate oxidation by glucagon or cyclic AMP is triggered by the fall in the malonyl-CoA sensitivity of CPT I. (5) In cultured hepatocytes from 24 h-old newborns, the addition of insulin inhibits no more than 30% of the high oleate oxidation, whereas it stimulates lipogenesis and increases malonyl-CoA concentration by 4-fold more than in fetal cells (no oleate oxidation). This poor effect of insulin on oleate oxidation seems to be due to the inability of the hormone to increase the sensitivity of CPT I sufficiently. Altogether, these results suggest that the malonyl-CoA sensitivity of CPT I is the major site of regulation during the induction of fatty acid oxidation in the fetal rabbit liver.

Streptozotocin-induced alterations in the levels of functional mitochondrial anion transport proteins

The effect of streptozotocin-induced diabetes on the levels of functional mitochondrial anion transport proteins has been determined. The experimental approach utilized for these studies consisted of the extraction of each of four mitochondrial anion transport proteins from rat liver mitoplasts (isolated from diabetic and control animals) with the nonionic detergent Triton X-114, followed by the functional reconstitution of each transporter in a liposomal system via the freeze-thaw-sonication technique. This approach permitted the quantification of transporter function without the complications that occur when such measurements are carried out with intact mitochondria (or mitoplasts). We found that experimental diabetes caused an increase in the extractable and reconstitutable specific (and total) transport activities of the pyruvate and dicarboxylate transporters, a decrease in the activity of the citrate transporter, and no significant change in the activity of the phosphate transporter relative to control values. An examination of the time course of the appearance of changes in the reconstitutable activities of the pyruvate and citrate transporters following the injection of streptozotocin revealed differences. Thus, whereas the activity of the pyruvate transporter displayed the most pronounced increase (193%) 1 week following streptozotocin injection and then subsequently declined from this peak and plateaued at later times (99% and 96% increases at 3 and 8 weeks, respectively), the activity of the citrate transporter progressively decreased with time (31-51% decreases at 1-8 weeks). We suggest that the observed diabetes-induced changes in mitochondrial anion transporter function are predictable on the basis of diabetes-induced alterations in the activities of enzymes that constitute metabolic pathways to which these transporters either supply substrate or remove product. Furthermore, we speculate that mitochondrial anion transport proteins may be regulated in coordination with the enzymes of such associated metabolic pathways.

Turnover of carnitine palmitoyltransferase mRNA and protein in H4IIE cells. Effect of cyclic AMP and insulin

Regulation of the 68 kDa carnitine palmitoyltransferase (CPT) synthesis by (chlorophenylthio) cyclic AMP (cAMP) and insulin was studied in H4IIE cells in culture. Addition of 0.1 mM- or 1.0 mM-(chlorophenylthio) cAMP induced CPT mRNA and rate of transcription 2-4-fold by 15 min, reaching a plateau at 4-6-fold by 30 min. Addition of 5-15 nM-insulin plus 1.0 mM-cAMP suppressed the increases in transcription rate and mRNA levels occurring with cAMP alone. The t1/2 for CPT mRNA was 70-80 min and was not affected by cAMP. The t1/2 for CPT protein was 70 min, and was increased to 240 min in the presence of cAMP. The rate of CPT synthesis was also increased in the presence of cAMP. The data indicate that CPT synthesis is increased by cAMP via induction of transcription and subsequent increase in the CPT mRNA. Insulin acts to depress transcription and CPT mRNA. In addition, cAMP prolongs the t1/2 of CPT.

Co-ordinate induction of hepatic mitochondrial and peroxisomal carnitine acyltransferase synthesis by diet and drugs

The present studies examined the effect of agents that induce peroxisomal and mitochondrial beta-oxidation on hepatic mitochondrial carnitine palmitoyltransferase (CPT) and peroxisomal carnitine acyltransferase [CPTs of Ramsay (1988) Biochem. J. 249, 239-245; COT of Farrell & Bieber (1983) Arch. Biochem. Biophys. 222, 123-132 and Miyazawa, Ozasa, Osumi & Hashimoto (1983) J. Biochem. 94, 529-542]. In the first studies, high fat diets containing corn oil or fish oil were used to induce peroxisomal and mitochondrial enzymes. Rats were fed one of three diets for 4 weeks: (1) low fat, with corn oil as 11% of energy (kJ); (2) high fat, with corn oil as 45% of kJ; (3) high fat, with fish oil as 45% of kJ. At the end of 4 weeks, both mitochondrial CPT and peroxisomal CPTs exhibited increases in activity, immunoreactive protein, mRNA levels and transcription rates in livers of rats fed either high-fat diet compared to the low fat diet. Riboflavin deficiency or starvation for 48 h also increased the peroxisomal CPTs mRNA. A second set of studies used the plasticizer 2-(diethylhexyl)phthalate (DEHP), 0.5% clofibrate or 1% acetylsalicylic acid (fed for 3 weeks) to alter peroxisomal and mitochondrial fatty acid oxidation. With DEHP, the mitochondrial CPT and peroxisomal CPTs activity, immunoreactive protein, mRNA levels and and transcription rate were all increased by 3-5-fold. The peroxisomal CPTs activity, immunoreactive protein, mRNA levels and transcription rate were increased 2-3-fold by clofibrate and acetylsalicylic acid, again similar to mitochondrial CPT. The results of the combined studies using both diet and drugs to cause enzyme induction suggest that the synthesis of the carnitine acyltransferases (mitochondrial CPT and peroxisomal CPTs) may be co-ordinated with each other; however, the co-ordinate regulatory factors have not yet been identified.

Regulation of carnitine palmitoyltransferase in vivo by glucagon and insulin

Carnitine palmitoyltransferase (CPT total) activity and synthesis increase in states where the insulin/glucagon ratio is low, such as starvation and diabetes [Brady & Brady (1987) Biochem. J. 246, 641-646]. However, the effect of glucagon and insulin on CPT synthesis is unknown. The present experiments were designed to determine the effect of glucagon, cAMP [8-(chlorophenylthio) cyclic AMP], and insulin + cAMP on CPT transcription and mRNA amounts over time after injection. The CPT protein that was purified, used to generate antibody, and cloned in these studies was the 68 kDa mitochondrial protein described previously [Brady & Brady (1987) Biochem. J. 246, 641-646; Brady, Feng & Brady (1988) J. Nutr. 118, 1128-1136; Brady & Brady (1989) Diabetes 38, in the press]. Saline-injected control rats exhibited a 2-fold increase in hepatic CPT transcription rate and CPT mRNA over the 5 h experiment from 09:00 to 14:00 h. The effect was most probably due to the fasting state of the rats during the day. Glucagon injection caused an 8-fold increase in transcription rate by 90 min and a 4-fold increase in CPT mRNA by 90-120 min. The cAMP effect had reached a peak by the first time point taken (15 min). Transcription rate was increased 4-fold and CPT mRNA was increased 3-fold at this time. The combination of cAMP + insulin injection did not produce any significant increase in transcription rate or CPT mRNA over the saline-injected controls. CPT mRNA and transcription rate showed a clear dose-response to glucagon injection from 0 to 150 micrograms/100 g body wt. Total CPT activity and immunoreactive CPT were not increased during these experiments. The data indicate that glucagon and insulin interact in control of transcription rate and amount of CPT mRNA, but that increases in CPT immunoreactive protein and activity are temporally delayed. This lag probably relates to the half-life of the CPT protein in vivo, which has been estimated as 2-7 days.

Hemipalmitoylcarnitinium, a strong competitive inhibitor of purified hepatic carnitine palmitoyltransferase

We have synthesized (2S,6R:2R,6S)-6-carboxymethyl-2-hydroxy-2-pentadecyl-4,4-dimethylmorp holinium bromide (hemipalmitoylcarnitinium, HPC) which is a conformationally restricted analog inhibitor of carnitine palmitoyltransferase (CPT; EC 2.3.1.21). rac-HPC inhibits catalytic activity in purified rat liver CPT. In the forward reaction, HPC competes with both (R)-carnitine (Ki(app) = 5.1 +/- 0.7 microM) and palmitoyl-CoA (Ki(app) = 21.5 +/- 4.9 microM). In the reverse reaction, inhibition by HPC is competitive with palmitoyl-(R)-carnitine (Ki(app) = 1.6 +/- 0.6 microM), but inhibition is uncompetitive with CoA. The forward reaction is also competitively inhibited by its product, palmitoyl-(R)-carnitine, Ki(app)'s 14.2 +/- 2.1 microM relative to (R)-carnitine and 8.7 +/- 2.6 microM relative to palmitoyl-CoA. rac-HPC is the most potent synthetic reversible inhibitor of purified CPT. HPC fails to inhibit carnitine acetyltransferase (CAT; EC 2.3.1.7). Palmitoylcholine also inhibits CPT in the forward reaction, competing with (R)-carnitine (Ki(app) = 18.6 +/- 4.5 microM) and with palmitoyl CoA (Ki(app) = 10.4 +/- 2.5 microM). Choline is not an effective CPT inhibitor. We have shown [R.D. Gandour et al. (1986) Biochem. Biophys. Res. Commun. 138, 735-741] that hemiacetylcarnitinium inhibits CAT but not CPT. The combined data demonstrate further differences between the carnitine recognition sites in CPT and CAT.

Carnitine palmitoyltransferase: effects of diabetes, fasting, and pH on the reaction that generates acyl CoA

Although carnitine palmitoyltransferase (CPT) has received considerable attention, particularly its regulation by malonyl CoA, most studies have monitored the forward reaction, ie, the formation of acylcarnitine. We examined the opposite or reverse reaction, in which palmitoyl CoA is generated, in osmotically-disrupted rat hepatic mitochondria. Specifically, the effects of pH, fasting, and untreated recent-onset diabetes were investigated. As with the forward (f) reaction, the CPT reverse (r) velocity v pH curve was somewhat parabolic with a pH maximum at approximately 7.2 (except the CPT that was from the diabetic rats). However, as the pH rose, the CPT reverse and forward curves diverged due to a precipitous decline in the forward reaction. This discordance in rates in the alkaline range was apparent in all three groups of CPT but was most prominent in the diabetic preparation (for example, as the pH increased from 7.3 to 8.8, the respective declines in the f and r velocities were 74% and 2%). In addition, under our assay conditions the CPTr from diabetic rats not only had a higher velocity (55.4 +/- 1.4 nmol/min/mg protein) than that from the fed (32.1 +/- 3.1) or fasted (43.1 +/- 3.4) animals, but also the Vmax was found to be twofold greater, even though there was no difference in the Km for palmitoylcarnitine. In summary, diabetes affects the kinetics of the reverse reaction and, regardless of the animal's premortem condition, but more so in the diabetes, this reaction is less attenuated than the forward one as the pH rises.

Fatty acid metabolism in hepatocytes isolated from rats adapted to high-fat diets containing long- or medium-chain triacylglycerols

Fatty acid oxidation and synthesis were studied in isolated hepatocytes from adult rats adapted for 44 days on low-fat, high-carbohydrate (LF), diet or high-fat diets, composed of long-chain (LCT) or medium-chain (MCT) triacylglycerols. The rates of [1-14C]octanoate oxidation were almost similar in each group studied, whereas the oxidation of [1-14C]oleate was 50% lower in the LF group than in animals adapted to high-fat diets. The rates of oleate oxidation are inversely correlated with the rates of lipogenesis. However, it seems unlikely that [malonyl-CoA] itself represents the sole mechanism involved in the regulation of oleate oxidation during long-term LCT or MCT feeding, since: (1) despite a 3-fold higher concentration of malonyl-CoA in MCT-fed rats than in LCT-fed ones, the rates of oleate oxidation are similar; (2) when malonyl-CoA concentration is increased after stimulation of lipogenesis (by adding lactate + pyruvate) in MCT-fed rats, to a level comparable with that of the LF group, the rate of oleate oxidation remains 55% higher than that measured under similar conditions in the LF-fed rats; (3) in the LF group, the 90% decrease in malonyl-CoA concentration [by 5-(tetradecyloxy)-2-furoic acid] is not associated with a stimulation of oleate oxidation. By contrast, the sensitivity of carnitine palmitoyltransferase I (CPT I) to malonyl-CoA is markedly decreased in the LCT- and MCT-fed rats, by 90% and 70% respectively. The relevance of this decrease in the sensitivity of CPT I is discussed.

Role of carnitine palmitoyltransferase I in the regulation of hepatic ketogenesis during the onset and reversal of chronic diabetes

1. The kinetic properties of overt carnitine palmitoyltransferase (CPT I, EC 2.3.1.21) were studied in rat liver mitochondria isolated from untreated, diabetic and insulin-treated diabetic animals. A comparison was made of the time courses required for the changes in these properties of CPT I to occur and for the development of ketosis during the induction of chronic diabetes and its reversal by insulin treatment. 2. The development of hyperketonaemia over the first 5 days of insulin withdrawal from streptozotocin-treated rats was accompanied by parallel increases in the activity of CPT I and in the I0.5 (concentration required to produce 50% inhibition) of the enzyme for malonyl-CoA. 3. The rapid reversal of the ketotic state by treatment of chronically diabetic rats with 6 units of regular insulin was not accompanied by any change in the properties of CPT I over the first 4 h. Higher doses of insulin (15 units), delivered throughout a 4 h period, resulted in an increase in the affinity of CPT I for malonyl-CoA, but the sensitivity of the enzyme to the inhibitor was still significantly lower than in mitochondria from normal animals. 4. Conversely, when insulin treatment was continued over a 24 h period, full restoration of the sensitivity of the enzyme to malonyl-CoA was achieved. However, the activity of the enzyme was only decreased marginally. 5. These results are discussed in terms of the possibility that the major regulatory sites of the rate of hepatic oxidation may vary in different phases of the induction and reversal of chronic diabetes.

The relationship between fat synthesis and oxidation in the liver after re-feeding and its regulation by thyroid hormone

The administration of glucose to 48 h-starved euthyroid or hyperthyroid rats led to decreased blood concentrations of fatty acids and ketone bodies in both groups, but fatty acid concentrations were higher and ketone-body concentrations lower in the latter group. Decreased ketonaemia was not due to increased ketone-body clearance. Flux through carnitine palmitoyltransferase 1 was increased, consistent with the effects of hyperthyroidism on enzyme activity demonstrated in vitro. Correlations between the concentrations of ketone bodies and long-chain acylcarnitine measured in freeze-clamped liver samples indicated that a lower proportion of the product of beta-oxidation was used for ketone-body synthesis. Citrate concentrations were unaffected by hyperthyroidism, but lipogenesis was increased. The results are discussed in relation to the factors controlling hepatic carbon flux and energy requirements after re-feeding.

Hepatic and cardiac carnitine palmitoyltransferase activity. Effects of adriamycin and galactosamine

Carnitine palmitoyltransferase (CPT) activity is located on both the outer and inner sides of the mitochondrial inner membrane and is influenced by the surrounding lipids of the inner mitochondrial membrane. Both adriamycin and galactosamine interact with mitochondrial lipids as a part of their mechanism of toxicity, and thus these agents might be expected to affect CPT activity. Addition of adriamycin to both intact rat liver and heart mitochondria (CPT-A, outer CPT) and inverted submitochondrial vesicles (CPT-B, inner CPT) depressed CPT in the forward direction of reaction (palmitoyl-l-carnitine formation), but the CPT-B activity was more sensitive to the inhibitor. Adriamycin depressed the CPT-A reverse reaction (palmitoyl-CoA formation) to 40% of control, but it had no effect on the CPT-B reverse reaction. In vivo galactosamine administration depressed CPT-A and CPT-B 20-30% and did not affect subsequent action of in vitro adriamycin. Addition of cardiolipin (0.25 to 1.0 mg/assay) increased activity of the CPT-A forward reaction of both control and galactosamine-treated rats, but it did not affect CPT-B activity. The results suggest that CPT-A and CPT-B may be influenced differently by perturbants that affect lipids of the membrane.

Hepatic carnitine palmitoyltransferase turnover and translation rates in fed, starved, streptozotocin-diabetic and diethylhexyl phthalate-treated rats

Hepatic carnitine palmitoyltransferase (CPT) turnover was studied in control and in non-ketotic hyperglycaemic streptozotocin-diabetic rats. The degradation constant (kd) and half-life (t1/2) did not appear to be altered by mild diabetes. The hepatic CPT (micrograms/g of liver) was not increased by the mild, non-ketotic, diabetes. However, the total hepatic CPT (micrograms/liver) was 37% greater in the diabetic animals, owing to the increased liver weight. This resulted from a 40% increase in the synthesis constant (ks). Hepatic CPT activity (total detergent-solubilized) and translation rates were measured in fed, starved (48 h), non-ketotic diabetic, ketotic diabetic and diethylhexyl phthalate (DEHP)-treated rats. CPT activity (m units/mg of mitochondrial protein) was not significantly increased with non-ketotic diabetes (44% increase, but non-significant), but was increased approx. 2-fold with starvation and ketotic diabetes, and 3.5-fold with DEHP treatment. CPT expressed as units/liver was increased non-significantly (23%) in non-ketotic and starved rats, similar to the turnover study, but was significantly increased with ketotic diabetes and with DEHP treatment. mRNA-translation activity for CPT was elevated in all states to a somewhat greater extent than was activity. It was concluded that protein synthesis as a product of increased CPT-mRNA translation activity is a major means of long-term regulation.

Studies on the activation in vitro of carnitine palmitoyltransferase I in liver mitochondria from normal, diabetic and glucagon-treated rats

The activation of overt carnitine palmitoyltransferase activity that occurs when rat liver mitochondria are incubated at near-physiological temperatures and ionic strengths was studied for mitochondria obtained from animals in different physiological states. In all instances, it was found to be due exclusively to an increase in the catalytic capacity of the enzyme and not to an increase in affinity of the enzyme for palmitoyl-CoA. The enzyme in mitochondria from fed animals always showed a larger degree of activation than that in mitochondria from starved animals. This was the case even for mitochondria (e.g. from fed diabetic animals) in which the kinetic characteristics of carnitine palmitoyltransferase were more similar to those for the enzyme in mitochondria from starved rats. Glucagon treatment of rats before isolation of the mitochondria did not affect the characteristics either of the kinetic parameters of overt carnitine palmitoyltransferase or of its activation in vitro.

Effect of methylglyoxal bis(guanylhydrazone) on hepatic, heart and skeletal muscle mitochondrial carnitine palmitoyltransferase and beta-oxidation of fatty acids

Methylglyoxal bis(guanylhydrazone) (MGBG) is an antileukemic agent and a structural polyamine analogue which inhibits S-adenosyl methionine decarboxylase. However, MGBG also produces profound mitochondrial structural damage and inhibition of fatty acid oxidation. Carnitine palmitoyltransferase-A (CPT-A) is located on the outer surface of the inner mitochondrial membrane and is the putative rate-controlling enzyme for mitochondrial long-chain fatty acid oxidation. The present experiments were designed to determine if MGBG inhibits CPT-A. Liver, heart and skeletal muscle mitochondria were isolated from rats following 24 hr of starvation. Measuring the reaction in the direction of palmitoylcarnitine plus CoA formation from palmitoyl-CoA plus carnitine ("forward reaction"), MGBG was competitive with l-carnitine. The MGBG CPT-A Ki values were (mM): liver, 5.0 +/- 0.6 (N = 15); heart 3.2 +/- 1.2 (N = 3); and skeletal muscle, 2.8 +/- 1.0 (N = 3). Lysis of hepatic mitochondria with Triton X-100 yielded a Ki of 4.0 +/- 2.0, which was not significantly different from intact mitochondria or inverted vesicles (4.9 mM). Purified hepatic CPT had a Ki of 4.2 mM. MGBG did not inhibit purified CPT in the "reverse reaction" (palmitoyl-CoA plus carnitine formation from palmitoylcarnitine plus CoA). Spermine and spermidine, which are structurally similar to MGBG, did not inhibit either CPT activity or acid-soluble product formation from 1-[14C]palmitoyl-CoA. MGBG inhibited mitochondrial state 3 oxidation rates of palmitoyl-CoA and palmitoylcarnitine, as well as of glutamate. However, the fatty acid substrates were considerably more sensitive than glutamate to MGBG inhibition. MGBG also increased hepatic mitochondrial aggregation which was reversed by l-carnitine. Fluorescence polarization, using 1,6-diphenyl-1,3,5-hexatriene (DPH) as a probe, indicated that MGBG increased membrane rigidity in a dose-dependent manner. This effect was not altered by l-carnitine. MGBG also inhibited purified pigeon breast carnitine acetyltransferase (CAT; Ki = 1.6 mM). While MGBG appeared to be competitive with l-carnitine for both CPT and CAT, MGBG also exhibits a number of effects which may be mediated through membrane interaction and which are not reversed by carnitine.

Characterization of hepatic carnitine palmitoyltransferase. Use of bromoacyl derivatives and antibodies

Carnitine palmitoyltransferase (CPT) is a mitochondrial-inner-membrane enzyme, with activities located on both the outer and inner sides of the membrane. The inhibition of CPT by bromopalmitate derivatives was studied in intact hepatic mitochondria (representing CPT-A activity, the outer enzyme), in inverted submitochondrial vesicles (representing CPT-B, the inner enzyme), and in purified hepatic CPT. Bromopalmitoyl-CoA had an I50 (concentration giving 50% inhibition of CPT activity) of 0.63 +/- 0.08 microM in intact mitochondria and 2.44 +/- 0.86 microM in inverted vesicles. Preincubation of mitochondria with bromopalmitoyl-CoA decreased V max. for both CPT-A and CPT-B. Sonication decreased sensitivity to bromopalmitoyl-CoA, and solubilization with Triton abolished sensitivity at the concentrations used (0-10 microM). Purified CPT had a bromopalmitoyl-CoA I50 of 353 microM in aqueous buffer, 67 microM in 20% dimethyl sulphoxide, 45 microM in phosphatidylcholine liposomes and 26 microM in cardiolipin liposomes. Increasing [carnitine] at constant bromopalmitoyl-CoA concentrations or increasing [bromopalmitoyl-CoA] in the preincubation resulted in increased inhibition of purified CPT. 2-Tetradecylglycidyl-CoA and malonyl-CoA did not offer measurable protection against bromopalmitoyl-CoA inhibition of the purified CPT, suggesting a different site of interaction of bromopalmitoyl-CoA with CPT. The data suggest that the sensitivity of CPT to bromopalmitoyl-CoA may be modulated by membrane environment and assay conditions.

Restoration of the properties of carnitine palmitoyltransferase I in liver mitochondria during re-feeding of starved rats

The recovery of the parameters of the kinetic properties of carnitine palmitoyltransferase (CPT) I in liver mitochondria of starved rats was studied after re-feeding animals for various periods of time. There were no significant changes either in the activity of the enzyme at high palmitoyl-CoA concentrations or in the affinity of the enzyme for palmitoyl-CoA, or in the sensitivity of CPT I to malonyl-CoA inhibition after 3 h or 6 h re-feeding. After 24 h re-feeding, both the affinity of the enzyme for palmitoyl-CoA and the activity of the enzyme were still not significantly different from those for the enzyme in mitochondria from 24 h-starved animals. By contrast, the sensitivity of CPT I to malonyl-CoA inhibition was largely, but not fully, restored to that observed in mitochondria from fed rats.

Action in vivo and in vitro of 2-tetradecylglycidic acid, 2-tetradecylglycidyl-CoA and 2-tetradecylglycidylcarnitine on hepatic carnitine palmitoyltransferase

The effects of 2-tetradecylglycidic acid (TDGA), TDGA-CoA and TDGA-carnitine were examined in purified hepatic CPT (carnitine palmitoyltransferase) and in hepatic mitochondria and inverted submitochondrial vesicles derived from Sprague-Dawley rats. Since TDGA has been reported as a specific inhibitor of carnitine palmitoyltransferase-A (CPT-A), the focus was on kinetics and inhibition of CPT-A, and the relationship of this key enzyme to beta-oxidation. After administration of TDGA in vivo to overnight-starved rats, the Vmax. of CPT in intact mitochondria and in inverted vesicles (CPT-B) was depressed by 66%. The S0.5 for palmitoyl-CoA and Km for carnitine were unchanged. The I50 (concn. giving 50% inhibition) for malonyl-CoA was significantly increased from 20 to 141 microM in intact mitochondria, but unchanged (199 versus 268 microM) in inverted vesicles. The addition in vitro of TDGA-CoA (0-1.0 microM) gave I50 values of 0.29 and 0.27 microM (S.E.M. = 0.19) in intact mitochondria from fed and 48 h-starved rats, and 0.81 and 1.57 microM (S.E.M. = 0.29) for inverted vesicles derived from fed and starved rats. Addition in vitro of TDGA-carnitine to mitochondria from starved rats yielded an I50 value of 27.7 mM (S.E.M. = 12.2) for L-[methyl-14C]carnitine release from palmitoyl-L-[methyl-14C]carnitine and 0.64 mM (S.E.M. = 0.07) for palmitoyl-L-[methyl-14C]carnitine formation from L-[methyl-14C]carnitine in intact mitochondria. Inverted vesicles were not measurably sensitive to TDGA-carnitine up to 500 microM for the assay of L-[methyl-14C]carnitine release, but were as sensitive as intact mitochondria when inhibition was determined in the direction of palmitoyl-L-[methyl-14C]carnitine formation (I50 = 0.54 +/- 0.07 microM). When TDGA-CoA was added to intact mitochondria, then incubated for 5 min at room temperature and subsequently washed out, Vmax. of CPT decreased from 5.8 to 3.5 (S.E.M. = 0.6) in intact mitochondria, and from 17.2 to 6.3 (S.E.M. = 4.8) in inverted vesicles. The Km for L-carnitine and the S0.5 for palmitoyl-CoA increased 2-fold with TDGA-CoA pretreatment in both intact mitochondria and inverted vesicles. Detergent solubilization (0.05% Triton X-100) resulted in a complete loss of TDGA-CoA sensitivity (up to 1.0 microM measured). Sonicated mitochondria exhibited an I50 of 0.72 +/- 0.03 microM.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacologic action of L-carnitine on hypertriglyceridemia in obese Zucker rats

Administration of pharmacologic amounts of L-carnitine was studied in the hypertriglyceridemic Zucker rat. When administered subcutaneously, doses from 250 to 2,000 mg/kg/d significantly decreased plasma triglycerides in obese rats over eight to 12 weeks, with no effect on plasma triglycerides in lean rats. Oral doses at the same high levels were not effective in decreasing plasma triglycerides. Triglyceride secretion rate was reduced from 367 micrograms/min to 168 micrograms/min in treated obese rats. Concurrently, liver lipid was increased twofold in obese treated rats, and the livers of these rats showed significant fatty infiltration. The mechanism of action of carnitine in decreasing plasma triglycerides appeared to be via decreased secretion of triglycerides by the liver of obese rats. There was no effect of L-carnitine in lean or obese rats on the following variables: carnitine palmitoyltransferase-A kinetics or malonyl CoA inhibition, mitochondrial or peroxisomal oxidative capacity, lipoprotein lipase in heart, muscle, and adipose, or fecal lipids. The effect of pharmacologic L-carnitine thus appears to be an inhibition of triglyceride synthesis and/or secretion by the liver.