Hepatic mitochondrial inner-membrane properties, beta-oxidation and carnitine palmitoyltransferases A and B. Effects of genetic obesity and starvation

The antitumorigenic roles of BRCA1-BARD1 in DNA repair and replication

The tumour suppressor breast cancer type 1 susceptibility protein (BRCA1) promotes DNA double-strand break (DSB) repair by homologous recombination and protects DNA replication forks from attrition. BRCA1 partners with BRCA1-associated RING domain protein 1 (BARD1) and other tumour suppressor proteins to mediate the initial nucleolytic resection of DNA lesions and the recruitment and regulation of the recombinase RAD51. The discovery of the opposing functions of BRCA1 and the p53-binding protein 1 (53BP1)-associated complex in DNA resection sheds light on how BRCA1 influences the choice of homologous recombination over non-homologous end joining and potentially other mutagenic pathways of DSB repair. Understanding the functional crosstalk between BRCA1-BARD1 and its cofactors and antagonists will illuminate the molecular basis of cancers that arise from a deficiency or misregulation of chromosome damage repair and replication fork maintenance. Such knowledge will also be valuable for understanding acquired tumour resistance to poly(ADP-ribose) polymerase (PARP) inhibitors and other therapeutics and for the development of new treatments. In this Review, we discuss recent advances in elucidating the mechanisms by which BRCA1-BARD1 functions in DNA repair, replication fork maintenance and tumour suppression, and its therapeutic relevance.

Structure of the Fanconi anaemia monoubiquitin ligase complex

The Fanconi anaemia (FA) pathway repairs DNA damage caused by endogenous and chemotherapy-induced DNA crosslinks, and responds to replication stress1,2. Genetic inactivation of this pathway by mutation of genes encoding FA complementation group (FANC) proteins impairs development, prevents blood production and promotes cancer1,3. The key molecular step in the FA pathway is the monoubiquitination of a pseudosymmetric heterodimer of FANCD2-FANCI4,5 by the FA core complex-a megadalton multiprotein E3 ubiquitin ligase6,7. Monoubiquitinated FANCD2 then recruits additional protein factors to remove the DNA crosslink or to stabilize the stalled replication fork. A molecular structure of the FA core complex would explain how it acts to maintain genome stability. Here we reconstituted an active, recombinant FA core complex, and used cryo-electron microscopy and mass spectrometry to determine its structure. The FA core complex comprises two central dimers of the FANCB and FA-associated protein of 100 kDa (FAAP100) subunits, flanked by two copies of the RING finger subunit, FANCL. These two heterotrimers act as a scaffold to assemble the remaining five subunits, resulting in an extended asymmetric structure. Destabilization of the scaffold would disrupt the entire complex, resulting in a non-functional FA pathway. Thus, the structure provides a mechanistic basis for the low numbers of patients with mutations in FANCB, FANCL and FAAP100. Despite a lack of sequence homology, FANCB and FAAP100 adopt similar structures. The two FANCL subunits are in different conformations at opposite ends of the complex, suggesting that each FANCL has a distinct role. This structural and functional asymmetry of dimeric RING finger domains may be a general feature of E3 ligases. The cryo-electron microscopy structure of the FA core complex provides a foundation for a detailed understanding of its E3 ubiquitin ligase activity and DNA interstrand crosslink repair.

The effect of carnitine on ketogenesis in perfused livers from juvenile visceral steatosis mice with systemic carnitine deficiency

Juvenile visceral steatosis (JVS) mice have been reported to have systemic carnitine deficiency, and the carnitine concentration in the liver of JVS mice was markedly lower than that of controls (11.6 +/- 2.6 versus 393.5 +/- 56.4 nmol/g of wet liver). To evaluate the role of carnitine in mitochondrial beta-oxidation in liver, we examined the effects of carnitine on ketogenesis in perfused liver from control and JVS mice. In control mice, ketogenesis was increased by the infusion of 0.3 mM oleate, but not by L-carnitine. In contrast, although ketogenesis in JVS mice was not increased by the infusion of oleate, it was increased 2.5-fold by the addition of 1000 microM L-carnitine. Addition of 50, 100, and 200 microM L-carnitine increased ketogenesis in a dose-dependent manner. The infusion of 0.3 mM octanoate or butyrate increased ketogenesis in a carnitine-independent fashion in both control and JVS mice. These findings suggest that endogenous long chain fatty acids from accumulated triglycerides may be used as substrates in the presence of carnitine in JVS mice. The relationship between ketogenesis and free carnitine concentration was examined in livers from JVS mice. Ketogenesis increased as free carnitine levels increased until concentrations exceeded about 100 nmol/g of wet liver (340 microM). The free carnitine concentration required for half-maximal ketone body production in liver of JVS mice was 45 microM (13 nmol/g of wet liver), which corresponds to a K(m) value of carnitine palmitoyltransferase I. We conclude that carnitine is a rate-limiting factor for beta-oxidation in liver only when the carnitine level in liver is very low.

Cholate extracts of mitochondrial outer membranes increase inhibition by malonyl-CoA of carnitine palmitoyltransferase-I by a mechanism involving phospholipids

It has been reported that sodium cholate can separate the catalytic component of carnitine palmitoyltransferase-I (CPT-I) from a putative malonyl-CoA-binding regulatory protein capable of conferring sensitivity to malonyl-CoA on CPT-II. We found that cholate preferentially extracted a contaminating malonyl-CoA-sensitive CPT from mitochondrial inner membranes. When cholate extracts of outer membranes were incubated either with cholate extracts of inner membranes or with osmotically swollen mitochondria, inhibition of CPT by malonyl-CoA was increased. Treatment of intact mitochondria with subtilisin abolished the increased inhibition by malonyl-CoA, suggesting that the outer-membrane CPT-I was responsible for the increased inhibition. Incubation of cholate extracts with proteinase K did not prevent the increased inhibition. Fractionation of the cholate extract indicated the presence of phospholipids. Addition of cardiolipin or phosphatidylglycerol to osmotically swollen mitochondria increased sensitivity of CPT to malonyl-CoA, but several other phospholipids did not. When cardiolipin was added to intact mitochondria from either starved or fed rats, there were large increases in inhibition by malonyl-CoA; sensitivity in mitochondria from starved rats increased to that normally observed with mitochondria from fed rats. These results suggest that phospholipids are responsible for the increased inhibition of CPT by malonyl-CoA with added cholate extracts and that changes in membrane composition may be involved in the physiological regulation of CPT-I.

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.

New insights into the mitochondrial carnitine palmitoyltransferase enzyme system

Dissection of the mitochondrial carnitine palmitoyltransferase (CPT) enzyme system in terms of its structure/function relationships has proved to be a formidable task. Although no one formulation has gained universal agreement we believe that the weight of evidence supports a model with the following features: a) in any given tissue CPT I and CPT II are distinct proteins; b) CPT I, unlike CPT II, is detergent labile; c) within a species CPT II is expressed body wide, whereas CPT I exists as tissue specific isoforms; d) malonyl-CoA and other CPT I inhibitors probably interact at the catalytic center of the enzyme, not with a regulatory subunit. The amino acid sequences of rat and human CPT II (deduced from cDNA clones) show them to be similar proteins (greater than 80% identity) but encoded by mRNAs of significantly different sizes. Efforts to clone and sequence the cDNA for rat liver CPT I are presently underway.

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.

Abnormal fatty acid utilization by cultured cardiac cells from 7-day-old obese Zucker rats

Fatty acid utilization by muscle and nonmuscle heart cells in culture has been investigated in the 7-day-old Zucker rat to determine if this tissue could contribute to the lower energy expenditure reported in obese rats at the onset of obesity. The partitioning of oleate to oxidation and esterification products and the effect of genotype on this partitioning according to cell types were studied. Results showed that the fatty acid beta-oxidation and its esterification in neutral lipid was decreased by 30% in beating muscle cells from obese animals when compared with those from lean animals. In contrast, nonmuscle cells exhibited a decreased beta-oxidation alone. A similar fatty acid composition of the phospholipids was found in non-muscle cells of obese animals and their lean litter mates. In muscle cultures, palmitic and oleic acids are lower in cells of obese rats than in those of lean rats. The present study indicates that a defect in energy metabolism could be found in heart cells at the onset of obesity, suggesting that this defect is determined by intrinisic factor(s).

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.

Effects of dietary fish oil on human mammary carcinoma and on lipid-metabolizing enzymes

The growth rate of a human mammary carcinoma, MX-1, was significantly reduced in athymic "nude" mice fed fish oil. Tumors from the fish oil-fed animals also showed a greater sensitivity to two anti-neoplastic agents, mitomycin C and doxorubicin. Mitochondria were isolated from control livers, host livers and tumors from fish oil- and corn oil-fed animals, and increased levels of 20:5n-3 and 22:6n-3 were found in mitochondrial lipids in all three tissues from the fish oil-fed animals. To investigate the effect of dietary n-3 fatty acids on lipid metabolism, the activity of the acyl-CoA:carnitine acyltransferase and three acyl-CoA desaturases were measured. Carnitine acyltransferase activity toward all four acyl-CoA substrates tested was markedly increased in mitochondria from liver by feeding fish oil. In mitochondria from tumors, feeding fish oil resulted in an increased activity toward only 18:3n-3. These data suggest that fish oil may induce an increase in the oxidation of fatty acids. The delta 9-desaturase activity was decreased in microsomes from liver and tumor from fish oil-fed animals. However, both the delta 6 and delta 5 desaturases were increased in tumor and in control liver as a result of feeding fish oil. The delta 5 desaturase was not altered in microsomes from the host animals. The effect of fish oil on the delta 5 and delta 6 desaturases may involve alterations to metabolism of specific polyunsaturated fatty acids especially in the tumor tissue.

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.

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.

Purification and properties of the soluble carnitine palmitoyltransferase from bovine liver mitochondria

A new carnitine palmitoyltransferase (CPT) was purified to homogeneity from bovine liver mitochondria which were 96% free of peroxisomal contamination, as judged by catalase and glutamate dehydrogenase activities. The enzyme is easily removed from mitochondria, without the use of detergent. It is monomeric (Mr 63,500), unlike other preparations of CPT from mitochondria, and is most active with myristoyl-CoA and palmitoyl-CoA. The Km values are between 0.8 and 4 microM for a range of substrates from hexanoyl-CoA to stearoyl-CoA; these are much lower than values reported for other purified CPT preparations. The Km for L-carnitine is 185 microM measured with palmitoyl-CoA, and does not vary greatly with the chain length. This is also lower than the values reported for other CPT preparations, but higher than those cited for the medium-chain transferases. Kinetic and inhibitor studies were consistent with a rapid-equilibrium random-order mechanism. 2-Bromopalmitoyl-CoA, which is an inhibitor of the outer CPT, inhibited the enzyme competitively with palmitoyl-CoA as the variable substrate, when added without preincubation. If the enzyme was preincubated with 2-bromopalmitoyl-CoA and carnitine, the activity did not reappear after gel filtration of the protein. The inhibitor was bound in a 1:1 stoichiometry per subunit of enzyme.

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.

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)