Carnitine acetyltransferase: A review of its biology, enzymology, and bioorganic chemistry

The effects of L-carnitine supplementation on cardiovascular risk factors in participants with impaired glucose tolerance and diabetes: a systematic review and dose-response meta-analysis

AIMS

L-carnitine plays a role related to cardiometabolic factors, but its effectiveness and safety in CVD are still unknown. We aim to assess the effect of L-carnitine supplementation on CVD risk factors.

METHODS

A systematic literature search was conducted in PubMed, Web of Science, and Scopus until October 2022. The main outcomes were lipid profiles, anthropometric parameters, insulin resistance, serum glucose levels, leptin, blood pressure, and inflammatory markers. The pooled weighted mean difference (WMD) was calculated using a random-effects model.

RESULTS

We included the 21 RCTs (n = 2900) with 21 effect sizes in this study. L-carnitine supplementation had a significant effect on TG (WMD = - 13.50 mg/dl, p = 0.039), LDL (WMD = - 12.66 mg/dl, p < 0.001), FBG (WMD = - 6.24 mg/dl, p = 0.001), HbA1c (WMD = -0.37%, p = 0.013) HOMA-IR (WMD = -0.72, p = 0.038 (, CRP (WMD = - 0.07 mg/dl, P = 0.037), TNF-α (WMD = - 1.39 pg/ml, p = 0.033), weight (WMD = - 1.58 kg, p = 0.001 (, BMI (WMD = - 0.28 kg/m2, p = 0.017(, BFP (WMD = - 1.83, p < 0.001) and leptin (WMD = - 2.21 ng/ml, p = 0.003 (in intervention, compared to the placebo group, in the pooled analysis.

CONCLUSIONS

This meta-analysis demonstrated that administration of L-carnitine in diabetic and glucose intolerance patients can significantly reduce TG, LDL-C, FBG, HbA1c, HOMA-IR, CRP, TNF-α, weight, BMI, BFP, and leptin levels. PROSPERO registration code: CRD42022366992.

Neuropathic Pain in Aged People: An Unresolved Issue Open to Novel Drug Approaches, Focusing on Painful Diabetic Neuropathy

A majority of older patients suffer from neuropathic pain (NP) that significantly alters their daily activities and imposes a significant burden on health care. Multiple comorbidities and the risk of polypharmacy in the elderly make it challenging to determine the appropriate drug, dosage, and maintenance of therapy. Age-dependent processes play a contributing role in neuropathy given that diabetic neuropathy (DN) is the most common form of neuropathy. This narrative review is mainly focused on the drug treatment approach for neuropathy-associated pain in aged people including both drugs and dietary supplements, considering the latter as add-on mechanism-based treatments to increase the effectiveness of usual treatments by implementing their activity or activating other analgesic pathways. On one hand, the limited clinical studies assessing the effectiveness and the adverse effects of existing pain management options in this age segment of the population (> 65), on the other hand, the expanding global demographics of the elderly contribute to building up an unresolved pain management problem that needs the attention of healthcare providers, researchers, and health authorities as well as the expansion of the current therapeutic options.

Mitochondrial dysfunction, AMPK activation and peroxisomal metabolism: A coherent scenario for non-canonical 3-methylglutaconic acidurias

The occurrence of 3-methylglutaconic aciduria (3-MGA) is a well understood phenomenon in leucine oxidation and ketogenesis disorders (primary 3-MGAs). In contrast, its genesis in non-canonical (secondary) 3-MGAs, a growing-up group of disorders encompassing more than a dozen of inherited metabolic diseases, is a mystery still remaining unresolved for three decades. To puzzle out this anthologic problem of metabolism, three clues were considered: (i) the variety of disorders suggests a common cellular target at the cross-road of metabolic and signaling pathways, (ii) the response to leucine loading test only discriminative for primary but not secondary 3-MGAs suggests these latter are disorders of extramitochondrial HMG-CoA metabolism as also attested by their failure to increase 3-hydroxyisovalerate, a mitochondrial metabolite accumulating only in primary 3-MGAs, (iii) the peroxisome is an extramitochondrial site possessing its own pool and displaying metabolism of HMG-CoA, suggesting its possible involvement in producing extramitochondrial 3-methylglutaconate (3-MG). Following these clues provides a unifying common basis to non-canonical 3-MGAs: constitutive mitochondrial dysfunction induces AMPK activation which, by inhibiting early steps in cholesterol and fatty acid syntheses, pipelines cytoplasmic acetyl-CoA to peroxisomes where a rise in HMG-CoA followed by local dehydration and hydrolysis may lead to 3-MGA yield. Additional contributors are considered, notably for 3-MGAs associated with hyperammonemia, and to a lesser extent in CLPB deficiency. Metabolic and signaling itineraries followed by the proposed scenario are essentially sketched, being provided with compelling evidence from the literature coming in their support.

CMOS-compatible biosensor for L-carnitine detection

A CMOS-compatible ISFET with a Ta₂O₅ sensitive surface was developed. The structure was optimized for achieving high sensitivity using a subthreshold operation mode and by reducing the influence of the capacitances on the value of subthreshold swing. The developed ISFET was used as a basis for a biosensor for L-carnitine detection. To this end, carnitine acetyltransferase was immobilized on the ISFET sensitive surface. The immobilized enzyme was active (0.082 U/g _{model plate}). The complete microsystem, consisting of a packaged chip, an immobilized enzyme and a microfluidic channel, detected L-carnitine at a range of 0.2-100 μM with a LOD of 0.2 μM. The biosensor response was linear in the range of 0.2-50 μM of L-carnitine with sensitivity 18.0 ± 1.7 mV/μM. An experiment with artificial urine containing 1.3 μM L-carnitine showed that the proposed biosensor could be used on a real sample. Therefore, a new sensor specially optimized for biosensing CMOS-compatible ISFET structures and direct determination of L-carnitine with immobilized carnitine acetyltransferase was developed.

Reversible Product Release and Recapture by a Fungal Polyketide Synthase Using a Carnitine Acyltransferase Domain

Fungal polyketides have significant biological activities, yet the biosynthesis by highly reducing polyketide synthases (HRPKSs) remains enigmatic. An uncharacterized group of HRPKSs was found to contain a C-terminal domain with significant homology to carnitine O-acyltransferase (cAT). Characterization of one such HRPKS (Tv6-931) from Trichoderma virens showed that the cAT domain is capable of esterifying the polyketide product with polyalcohol nucleophiles. This process is readily reversible, as confirmed through the holo ACP-dependent transesterification of the released product. The methyltransferase (MT) domain of Tv6-931 can perform two consecutive α-methylation steps on the last β-keto intermediate to yield an α,α-gem-dimethyl product, a new programing feature among HRPKSs. Recapturing of the released product by cAT domain is suggested to facilitate complete gem-dimethylation by the MT.

In vivo longitudinal proton magnetic resonance spectroscopy on neonatal hypoxic-ischemic rat brain injury: Neuroprotective effects of acetyl-L-carnitine

PURPOSE

This study evaluated the longitudinal metabolic alterations after neonatal hypoxia-ischemia (HI) in rats and tested the neuroprotective effect of acetyl-L-carnitine (ALCAR) using in vivo proton short-TE Point-RESolved Spectroscopy method.

METHODS

Rice-Vannucci model was used on 7-day-old Sprague-Dawley rats. Data were acquired from contralateral and ipsilateral cortex and hippocampus, respectively at 4 time points (24-h, 72-h, 7-days, 28-days) post-HI. The effect of subcutaneous administration of ALCAR (100 mg/kg) immediately after HI, at 4-h, 24-h, and 48-h post-HI was determined.

RESULTS

Significant reductions in glutathione (P < 0.005), myo-inositol (P < 0.002), taurine (P < 0.001), and total creatine (P < 0.005) were observed at 24-h postinjury compared with the control group in the ipsilateral hippocampus of the HI rat pups. ALCAR-treated-HI rats had lower levels of lactate and maintained total creatine at 24-h and had smaller lesion size compared with the HI only rats.

CONCLUSION

Severe oxidative, osmotic stress, impaired phosphorylation, and a preference for anaerobic glycolysis were found in the ipsilateral hippocampus in the HI pups at 24-h postinjury. ALCAR appeared to have a neuroprotective effect if administered early after HI by serving as an energy substrate and promote oxidative cerebral energy producing and minimize anaerobic glycolysis.

Long-Term Effect of Acetyl-L-Carnitine for Antiretroviral Toxic Neuropathy

BACKGROUND

Nucleoside analogue reverse transcriptase inhibitors (NRTIs) used for the treatment of HIV can a cause distal symmetrical peripheral polyneuropathy by disruption of mitochondrial metabolism. Treatment with acetyl-L-carnitine (ALCAR) has shown short-term symptomatic and histological improvement. Long-term effects have not been investigated.

PURPOSE

To assess the subjective and objective degree of antiretroviral toxic neuropathy (ATN) during treatment with ALCAR.

METHOD

A cohort of 21 patients with ATN who commenced treatment with ALCAR between March 1999 and October 2001 was reviewed after a mean of 4.3 years using standardized questionnaires and neurological examination.

RESULTS

Of the 21 patients, 2 had died and 3 were lost to follow-up. 16 patients were assessed. 10 were still on potentially neurotoxic drugs. 13 were still taking ALCAR. 9 were pain free. The most common symptom was numbness (mild, moderate, and severe in 12, 3, and 0 patients, respectively), followed by paraesthesia (8, 2, 2), pain (4, 3, 0), and burning (5, 2, 0). There was mildly reduced sensation in the toes of 8 patients. 13 patients reported that ALCAR had improved their symptoms very much or moderately, 2 reported no change, and 1 reported a moderate worsening.

CONCLUSION

ALCAR led to long-term symptomatic improvement in most patients without the need to discontinue neurotoxic drugs. Although in this study there was no control group, this agent appears to be an effective pathogenesis-based treatment for ATN.

Substrate specificity of human carnitine acetyltransferase: Implications for fatty acid and branched-chain amino acid metabolism

Carnitine acyltransferases catalyze the reversible conversion of acyl-CoAs into acylcarnitine esters. This family includes the mitochondrial enzymes carnitine palmitoyltransferase 2 (CPT2) and carnitine acetyltransferase (CrAT). CPT2 is part of the carnitine shuttle that is necessary to import fatty acids into mitochondria and catalyzes the conversion of acylcarnitines into acyl-CoAs. In addition, when mitochondrial fatty acid β-oxidation is impaired, CPT2 is able to catalyze the reverse reaction and converts accumulating long- and medium-chain acyl-CoAs into acylcarnitines for export from the matrix to the cytosol. However, CPT2 is inactive with short-chain acyl-CoAs and intermediates of the branched-chain amino acid oxidation pathway (BCAAO). In order to explore the origin of short-chain and branched-chain acylcarnitines that may accumulate in various organic acidemias, we performed substrate specificity studies using purified recombinant human CrAT. Various saturated, unsaturated and branched-chain acyl-CoA esters were tested and the synthesized acylcarnitines were quantified by ESI-MS/MS. We show that CrAT converts short- and medium-chain acyl-CoAs (C2 to C10-CoA), whereas no activity was observed with long-chain species. Trans-2-enoyl-CoA intermediates were found to be poor substrates for this enzyme. Furthermore, CrAT turned out to be active towards some but not all the BCAAO intermediates tested and no activity was found with dicarboxylic acyl-CoA esters. This suggests the existence of another enzyme able to handle the acyl-CoAs that are not substrates for CrAT and CPT2, but for which the corresponding acylcarnitines are well recognized as diagnostic markers in inborn errors of metabolism.

Acetyl-l-carnitine increases nerve regeneration and target organ reinnervation - a morphological study

Peripheral nerve injury frequently results in functional morbidity since standard management fails to adequately address many of the neurobiological hurdles to optimal regeneration. Neuronal survival and regeneration are neurotrophin dependent and require increased aerobic capacity. Acetyl-l-carnitine (ALCAR) facilitates this need and prevents neuronal loss. ALCAR is clinically safe and is shown here to significantly improve nerve regeneration and target organ reinnervation. Two groups of five rats underwent sciatic nerve division followed by immediate repair. One group received parenteral ALCAR (50mg/kg/day) from time of operation until termination at 12 weeks. A 'sham treatment' group received normal saline. A third group was left unoperated and did not receive any treatment. A segment of nerve was harvested between 5mm proximal and 10mm distal to the repair in operated groups, and at the corresponding level in the unoperated group. Mean axonal count in normal, non-axotomised nerve was 14,720 (SD 2378). That of the saline group (17,217 SD 1808) was not significantly different from normal nerve (P=0.0985). Mean number of myelinated axons in the ALCAR group (24,460 SD 3750) was significantly greater than both sham group (P<0.01) and normal nerve (P=0.0012). Mean myelin thickness in the saline treated group (0.408 microm SD 0.067 microm) was less than normal nerve (0.770 microm SD 0.143 microm) (P<0.001). Mean myelin thickness in the ALCAR group (0.627 microm SD 0.052 microm) was greater than the sham (saline) group (P<0.01) and not statistically different from normal nerve (P=0.07). ALCAR increased dermal PGP9.5 staining by 210% compared to sham treatment (P<0.0001) and significantly reduced the mean percentage weight loss in gastrocnemius muscle (ALCAR group 0.203% vs. 0.312% in sham group P=0.015). ALCAR not only increases the number of regenerating nerve fibres but also morphologically improves the quality of regeneration and target organ reinnervation. Adjuvant ALCAR treatment may improve both sensory and motor outcomes and merits further investigation.

Pharmacokinetics of acetyl-L-carnitine given in single or multiple doses to HIV-1 Infected patients with toxic peripheral polyneuropathy

The use of nucleoside reverse transcriptase inhibitors in the treatment of HIV infection is associated with antiretroviral toxic polyneuropathy (ATN). Previous studies suggest that long term treatment with Acetyl-L-carnitine (ALCAR) 1.5 gram twice daily improves symptoms and promotes nerve regeneration. It is unknown whether the drug’s pharmacokinetic profile would allow for a once daily administration. Twenty three HIV-1 infected subjects taking ALCAR for ATN were enrolled in a cross over trial and switched from twice to once daily dosing. Their regimen was changed from 1.5g twice daily to 1g (4 patients), 2g (7), and 3g (12) once daily, respectively. Twelve healthy volunteers served as control. Plasma levels of ALCAR and its metabolite L-carnitine were measured. Patients receiving ALCAR had higher pre-dose levels than control subjects. Post dose levels were not significantly higher than pre dose levels in any treatment group. The pre / post dose ALCAR concentrations were 7.6 / 7.7, 7.1 / 6.8, 7.7 / 6.8, and 7.1 / 7.5 µmol/l for 1.5g twice daily, 1g once daily, 2g once daily, and 3g once daily, respectively. All values were significantly higher than the mean concentration in the control group (4.3 µmol/l). For ALCAR and L-carnitine, measurements for once daily regimens did not differ from the twice daily regimen. Once daily dosing of ALCAR can achieve similar plasma levels as twice daily dosing but intra-mitochondrial levels remain unknown. The pharmacokinetic profile of orally administered ALCAR is complex and likely to be highly affected by endogenous concentrations.

Acetyl-L-carnitine in HIV-associated antiretroviral toxic neuropathy

Nucleoside analogue reverse transcriptase inhibitors (NRTIs), used as part of highly active antiretroviral therapy for the treatment of HIV and AIDS, disrupt neuronal mitochondrial DNA synthesis, resulting in antiretroviral toxic neuropathy (ATN). Acetyl-L-carnitine (ALC) enhances neurotrophic support of sensory neurons, potentially causing symptom relief and nerve regeneration, and in addition has numerous other effects on metabolic function that might be of benefit in such patients.ALC has been given to HIV patients with symptomatic ATN in a number of clinical studies administered either twice daily intramuscularly or as oral sachets or tablets. It has been shown to significantly reduce a variety of validated pain ratings, and is generally safe and well tolerated. Using a measure of neuronal innervation in standardised skin biopsies of the affected area, cutaneous nerve density has been improved by the administration of ALC in subjects with symptomatic ATN and reduced epidermal and dermal innervation, associated with clinical improvement, which was maintained over a 4-year period. Improvements were seen in both the structure and function of small sensory fibres, which were sustained over time whilst subjects received ALC. Other open-label, non-randomised studies have shown similar benefits in patients with ATN in terms of pain reduction over the short term. Further placebo-controlled studies of both treatment and prophylaxis have been completed and are under analysis to characterise further the usefulness of this pathogenesis-based therapy for ATN.

A double-blind, parallel-group, placebo-controlled, multicentre study of acetyl l-carnitine in the symptomatic treatment of antiretroviral toxic neuropathy in patients with HIV-1 infection

BACKGROUND

Nucleoside reverse transcriptase inhibitors (NRTIs) disrupt neuronal mitochondrial DNA synthesis, resulting in antiretroviral toxic neuropathy (ATN). Acetyl-L-carnitine (ALCAR) enhances neurotrophic support of sensory neurones, potentially providing symptom relief and nerve regeneration.

OBJECTIVE

The objective of the study was to assess the safety and efficacy compared to placebo of intramuscular ALCAR in HIV-positive patients with symptomatic distal symmetrical polyneuropathy.

METHODS

Ninety patients were enrolled and randomized to receive ALCAR [500 mg twice a day (bid); n=43] or placebo (n=47) intramuscularly twice daily for 14 days followed by 42 days of oral ALCAR 1000 mg bid. Assessment of pain was obtained using the Visual Analogue Scale (VAS), Total Symptom Score (TSS), Clinical Global Impression of Change, McGill Pain Questionnaire (MPQ), and the need for rescue analgesics.

RESULTS

There was no statistically significant difference in changes in VAS over 14 days between groups for the intent-to-treat (ITT) population, but for the efficacy-evaluable (EE) population ALCAR treatment produced a significantly greater reduction in pain compared with placebo (P=0.022). The proportion of patients with an improvement in TSS over 14 days was greater in the ALCAR group compared with the placebo group, but the differences were not statistically significant. During the open-label phase, patients experienced an improvement in pain, as measured by the VAS, TSS and McGill Pain Questionnaire.

CONCLUSION

ALCAR, administered twice a day intramuscularly to HIV-1-infected patients with symptomatic ATN, significantly reduced weekly mean pain ratings on the VAS compared with placebo. Treatment with oral ALCAR improved symptoms for the patient group as a whole. Intramuscular and oral ALCAR was generally safe and well tolerated.

Delayed acetyl-l-carnitine administration and its effect on sensory neuronal rescue after peripheral nerve injury

Protection of sensory neurons after peripheral nerve injury is clinically crucial since inadequate sensory recovery is seriously affected by the death of up to 40% of sensory neurons. Immediate acetyl-L-carnitine (ALCAR) treatment eliminates this cell loss, but may not always be clinically feasible, hence we studied the effect of delaying the initiation of ALCAR treatment. Five groups of rats (n=5 per group) underwent unilateral sciatic nerve axotomy. ALCAR treatment (50 mg/kg/day) was initiated immediately, or after delays of 6 h, 24 h or 7 days after injury. A sham-treated group served as control. L4 and L5 dorsal root ganglia were harvested bilaterally 2 weeks after injury and stereological sensory neuron counts were obtained. Immediate sham treatment provided no neuroprotection (25% loss). Cell loss was eliminated when ALCAR was commenced within<or=24 h of axotomy. No statistically significant neuroprotective effect (18% loss) was evident compared to sham when ALCAR administration was initiated 7 days post-axotomy. When commenced within a clinically applicable time frame ALCAR treatment remains highly neuroprotective, potentially improving clinical outcome following peripheral nerve trauma.

Crystal structures of murine carnitine acetyltransferase in ternary complexes with its substrates

Carnitine acyltransferases catalyze the reversible exchange of acyl groups between coenzyme A (CoA) and carnitine. They have important roles in many cellular processes, especially the oxidation of long-chain fatty acids in the mitochondria for energy production, and are attractive targets for drug discovery against diabetes and obesity. To help define in molecular detail the catalytic mechanism of these enzymes, we report here the high resolution crystal structure of wild-type murine carnitine acetyltransferase (CrAT) in a ternary complex with its substrates acetyl-CoA and carnitine, and the structure of the S554A/M564G double mutant in a ternary complex with the substrates CoA and hexanoylcarnitine. Detailed analyses suggest that these structures may be good mimics for the Michaelis complexes for the forward and reverse reactions of the enzyme, representing the first time that such complexes of CrAT have been studied in molecular detail. The structural information provides significant new insights into the catalytic mechanism of CrAT and possibly carnitine acyltransferases in general.

HIV-associated neuropathic pain: epidemiology, pathophysiology and management

Peripheral neuropathy is associated with numerous systemic illnesses including HIV infection. Neuropathic pain constitutes approximately 25-50% of all pain clinic visits. Distal symmetrical polyneuropathy (DSP) is the most common form of peripheral neuropathy in individuals with HIV infection. DSP is distinguished from other forms of neuropathy on the basis of history and neurological examination. The pain associated with DSP can be debilitating. Therefore, it is important to diagnose HIV-associated DSP properly and treat the neuropathic pain in order to improve quality of life. We review the clinical manifestations, epidemiology, pathophysiology and management strategies for HIV-associated DSP.

Structure and function of carnitine acyltransferases

Carnitine acyltransferases catalyze the exchange of acyl groups between carnitine and coenzyme A (CoA). These enzymes include carnitine acetyltransferase (CrAT), carnitine octanoyltransferase (CrOT), and carnitine palmitoyltransferases (CPTs). CPT-I and CPT-II are crucial for the beta-oxidation of long-chain fatty acids in the mitochondria by enabling their transport across the mitochondrial membrane. The activity of CPT-I is inhibited by malonyl-CoA, a crucial regulatory mechanism for fatty acid oxidation. Mutation or dysregulation of the CPT enzymes has been linked to many serious, even fatal human diseases, and these enzymes are promising targets for the development of therapeutic agents against type 2 diabetes and obesity. We have determined the crystal structures of murine CrAT, alone and in complex with its substrate carnitine or CoA. The structure contains two domains. Surprisingly, these two domains share the same backbone fold, which is also similar to that of chloramphenicol acetyltransferase and dihydrolipoyl transacetylase. The active site is located at the interface between the two domains, in a tunnel that extends through the center of the enzyme. Carnitine and CoA are bound in this tunnel, on opposite sides of the catalytic His343 residue. The structural information provides a molecular basis for understanding the catalysis by carnitine acyltransferases and for designing their inhibitors. In addition, our structural information suggests that the substrate carnitine may assist the catalysis by stabilizing the oxyanion in the reaction intermediate.

Acetyl-l-carnitine: a pathogenesis based treatment for HIV-associated antiretroviral toxic neuropathy

BACKGROUND

Nucleoside analogue reverse transcriptase inhibitors (NRTI) disrupt neuronal mitochondrial DNA synthesis, impairing energy metabolism and resulting in a distal symmetrical polyneuropathy (DSP), an antiretroviral toxic neuropathy (ATN) that causes significant morbidity in HIV disease. Serum acetyl-l-carnitine (ALCAR) levels are decreased in neuropathy associated with NRTI therapy. ALCAR enhances neurotrophic support of sensory neurons and promotes energy metabolism, potentially causing nerve regeneration and symptom relief.

OBJECTIVE

To assess the efficacy of oral ALCAR (1500 mg twice daily) for up to 33 months in an open cohort of 21 HIV-positive patients with established ATN.

METHODS

Skin biopsies were excised from the leg before ALCAR treatment, at 6-12 month intervals thereafter and from HIV-negative non-neuropathic controls. Fibre types in epidermal, dermal and sweat gland innervation were quantified immunohistochemically.

RESULTS

After 6 month's treatment, mean immunostaining area for small sensory fibres increased (epidermis 100%, P = 0.006; dermis 133%, P < 0.05) by more than that for all fibre types (epidermis 16%, P = 0.04; dermis 49%, P < 0.05; sweat glands 60%, P < 0.001) or for sympathetic fibres (sweat glands 41%, P < 0.0003). Compared with controls, epidermal, dermal and sweat gland innervation reached 92%, 80% and 69%, respectively, after 6 month's treatment. Innervation improvements continued (epidermis and dermis) or stabilized (sweat glands) after 24 month's treatment. Neuropathic grade improved in 76% of patients and remained unchanged in 19%. HIV RNA load, CD4 and CD8 cell counts did not alter significantly throughout the study.

CONCLUSIONS

ALCAR treatment improves symptoms, causes peripheral nerve regeneration and is proposed as a pathogenesis-based treatment for DSP.

Redesign of carnitine acetyltransferase specificity by protein engineering

In eukaryotes, L-carnitine is involved in energy metabolism by facilitating beta-oxidation of fatty acids. Carnitine acetyltransferases (CrAT) catalyze the reversible conversion of acetyl-CoA and carnitine to acetylcarnitine and free CoA. To redesign the specificity of rat CrAT toward its substrates, we mutated Met564. The M564G mutated CrAT showed higher activity toward longer chain acyl-CoAs: activity toward myristoyl-CoA was 1250-fold higher than that of the wild-type CrAT, and lower activity toward its natural substrate, acetyl-CoA. Kinetic constants of the mutant CrAT showed modification in favor of longer acyl-CoAs as substrates. In the reverse case, mutation of the orthologous glycine (Gly553) to methionine in carnitine octanoyltransferase (COT) decreased activity toward its natural substrates, medium- and long-chain acyl-CoAs, and increased activity toward short-chain acyl-CoAs. Another CrAT mutant, M564A, was prepared and tested in the same way, with similar results. We conclude that Met564 blocks the entry of medium- and long-chain acyl-CoAs to the catalytic site of CrAT. Three-dimensional models of wild-type and mutated CrAT and COT support this hypothesis. We show for the first time that a single amino acid is able to determine the substrate specificity of CrAT and COT.

Structural and biochemical studies of the substrate selectivity of carnitine acetyltransferase

Carnitine acyltransferases catalyze the exchange of acyl groups between coenzyme A (CoA) and carnitine. They have important roles in many cellular processes, especially the oxidation of long-chain fatty acids, and are attractive targets for drug discovery against diabetes and obesity. These enzymes are classified based on their substrate selectivity for short-chain, medium-chain, or long-chain fatty acids. Structural information on carnitine acetyltransferase suggests that residues Met-564 and Phe-565 may be important determinants of substrate selectivity with the side chain of Met-564 located in the putative binding pocket for acyl groups. Both residues are replaced by glycine in carnitine palmitoyltransferases. To assess the functional relevance of this structural observation, we have replaced these two residues with small amino acids by mutagenesis, characterized the substrate preference of the mutants, and determined the crystal structures of two of these mutants. Kinetic studies confirm that the M564G or M564A mutation is sufficient to increase the activity of the enzyme toward medium-chain substrates with hexanoyl-CoA being the preferred substrate for the M564G mutant. The crystal structures of the M564G mutant, both alone and in complex with carnitine, reveal a deep binding pocket that can accommodate the larger acyl group. We have determined the crystal structure of the F565A mutant in a ternary complex with both the carnitine and CoA substrates at a 1.8-A resolution. The F565A mutation has minor effects on the structure or the substrate preference of the enzyme.

Primary sensory neuronal rescue with systemic acetyl-l-carnitine following peripheral axotomy. A dose-response analysis

The loss of a large proportion of primary sensory neurons after peripheral nerve axotomy is well documented. As a consequence of this loss, the innervation density attained on completion of regeneration will never be normal, regardless of how well the individual surviving neurons regenerate. Acetyl-L-carnitine (ALCAR), an endogenous peptide in man, has been demonstrated to protect sensory neurons, thereby avoiding loss after peripheral nerve injury. In this study we examined the dose-response effect of ALCAR on the primary sensory neurons in the rat dorsal root ganglia (DRG) 2 weeks after sciatic nerve axotomy. Six groups of adult rats (n=5) underwent unilateral sciatic nerve axotomy, without repair, followed by 2 weeks systemic treatment with one of five doses of ALCAR (range 0.5-50 mg/kg/day), or normal saline. L4 and L5 dorsal root ganglia were then harvested bilaterally and sensory neuronal cell counts obtained using the optical disector technique. ALCAR eliminated neuronal loss at higher doses (50 and 10 mg/kg/day), while lower doses did result in loss (12% at 5 mg/kg/day, p<0.05; 19% at 1 mg/kg/day, p<0.001; 23% at 0.5 mg/kg/day, p<0.001) compared to contralateral control ganglia. Treatment with normal saline resulted in a 25% (p<0.001) loss, demonstrating no protective effect in accordance with previous studies.ALCAR preserves the sensory neuronal cell population after axotomy in a dose-responsive manner and as such, has potential for improving the clinical outcome following peripheral nerve trauma when doses in excess of 10 mg/kg/day are employed.

Pharmacological enhancement of peripheral nerve regeneration in the rat by systemic acetyl-L-carnitine treatment

Peripheral nerve trauma remains a major cause of morbidity, largely due to the death of approximately 40% of innervating sensory neurons, and to slow regeneration after repair. Acetyl-L-carnitine (ALCAR) is a physiological peptide that virtually eliminates sensory neuronal death, and may improve regeneration after primary nerve repair. This study determines the effect of ALCAR upon regeneration after secondary nerve repair, thereby isolating its effect upon neuronal regenerative capacity. Two months after unilateral sciatic nerve division 1 cm nerve graft repairs were performed (n=5), and treatment with 50 mg/kg/day ALCAR was commenced for 6 weeks until harvest. Regeneration area and distance were determined by quantitative immunohistochemistry. ALCAR treatment significant increased immunostaining for both nerve fibres (total area 264% increase, P<0.001; percentage area 229% increase, P<0.001), and Schwann cells (total area 111% increase, P<0.05; percentage area 86% increase, P<0.05), when compared to no treatment. Regeneration into the distal stump was greatly enhanced (total area 2,242% increase, P=0.008; percentage area 3,034% increase, P=0.008). ALCAR significantly enhances the regenerative capacity of neurons that survive peripheral nerve trauma, in addition to its known neuroprotective effects.

Acetyl-L-carnitine physical-chemical, metabolic, and therapeutic properties: relevance for its mode of action in Alzheimer's disease and geriatric depression

Acetyl-L-carnitine (ALCAR) contains carnitine and acetyl moieties, both of which have neurobiological properties. Carnitine is important in the beta-oxidation of fatty acids and the acetyl moiety can be used to maintain acetyl-CoA levels. Other reported neurobiological effects of ALCAR include modulation of: (1) brain energy and phospholipid metabolism; (2) cellular macromolecules, including neurotrophic factors and neurohormones; (3) synaptic morphology; and (4) synaptic transmission of multiple neurotransmitters. Potential molecular mechanisms of ALCAR activity include: (1) acetylation of -NH2 and -OH functional groups in amino acids and N terminal amino acids in peptides and proteins resulting in modification of their structure, dynamics, function and turnover; and (2) acting as a molecular chaperone to larger molecules resulting in a change in the structure, molecular dynamics, and function of the larger molecule. ALCAR is reported in double-blind controlled studies to have beneficial effects in major depressive disorders and Alzheimer's disease (AD), both of which are highly prevalent in the geriatric population.

Pea chloroplast carnitine acetyltransferase

The purpose of this study was to resolve the controversy as to whether or not chloroplasts possess the enzyme carnitine acetyltransferase (CAT) and whether the activity of this enzyme is sufficient to support previously reported rates of fatty acid synthesis from acetylcarnitine. CAT catalyses the freely reversible reaction: carnitine + short-chain acylCoA <--> short-chain acylcarnitine + CoASH. CAT activity was detected in thc chloroplasts of Pisum sativum L. With membrane-impermeable acetyl CoA as a substrate. activity was only detected in ruptured chloroplasts and not with intact chloroplasts, indicating that the enzyme was located on the stromal side of the envelope. In crude preparations, CAT could only be detected using a sensitive radioenzymatic assay due to competing reactions from other enzymes using acetyl CoA and large amounts of ultraviolet-absorbing materials. After partial purification of the enzyme, CAT was detected in both the forward and reverse directions using spectrophotometric assays. Rates of 100 nmol of product formed per minute per milligram of protein were obtained, which is sufficient to support reported fatty acid synthesis rates from acetylcarnitine. Chloroplastic CAT showed optimal activity at pH 8.5 and had a high substrate specificity, handling C2-C4 acyl CoAs only. We believe that CAT has been satisfactorily demonstrated in pea chloroplasts.

Stereoselective synthesis of a conformationally defined cyclohexyl carnitine analogue that binds CPT-1 with high affinity

Carnitine (1, 3-hydroxy-4-trimethylammoniobutyrate) is important in mammalian tissue as a carrier of acyl groups. In order to explore the binding requirements of the carnitine acyltransferases for carnitine, we designed conformationally defined cyclohexyl carnitine analogues. These diastereomers contain the required gauche conformation between the trimethylammonium and hydroxy groups but vary the conformation between the hydroxy and carboxylic acid groups. Here we describe the synthesis and biological activity of the all-trans diastereomer (2), which was prepared by the ring opening of trans-methyl 2,3-epoxycylohexanecarboxylate with NaN3. Racemic 2 was a competitive inhibitor of neonatal rat cardiac myocyte CPT-1 (K(i) 0.5 mM for racemic 2; K(m) 0.2 mM for L-carnitine) and a noncompetitive inhibitor of neonatal rat cardiac myocyte CPT-2 (K(i) 0.67 mM). These results suggest that 2 represents the bound conformation of carnitine for CPT-1.

Reduction in the MK-801 binding sites of the NMDA sub-type of glutamate receptor in a mouse model of congenital hyperammonemia: prevention by acetyl-L-carnitine

Our earlier studies on the pharmacotherapeutic effects of acetyl-L-carnitine (ALCAR), in sparse-fur (spf) mutant mice with X linked ornithine transcarbamylase deficiency, have shown a restoration of cerebral ATP, depleted by congenital hyperammonemia and hyperglutaminemia. The reduced cortical glutamate and increased quinolinate may cause a down-regulation of the N-methyl-D-aspartate (NMDA) receptors, observed by us in adult spf mice. We have now studied the kinetics of [3H]-MK-801 binding to NMDA receptors in spf mice of different ages to see the effect of chronic hyperammonemia on the glutamate neurotransmission. We have also studied the Ca2+-dependent and independent (4-aminopyridine (AP) and veratridine-mediated) release of glutamate and the uptake of [3H]-glutamate in synaptosomes isolated from mutant spf mice and normal CD-1 controls. All these studies were done with and without ALCAR treatment (4 mmol/kg wt i.p. daily for 2 weeks), to see if its effect on ATP repletion could correct the glutamate neurotransmitter abnormalities. Our results indicate a normal MK-801 binding in 12-day-old spf mice but a significant reduction immediately after weaning (21 day), continuing into the adult stage. The Ca2+-independent release of endogenous glutamate from synaptosomes was significantly elevated at 35 days, while the uptake of glutamate into synaptosomes was significantly reduced in spf mice. ALCAR treatment significantly enhanced the MK-801 binding, neutralized the increased glutamate release and restored the glutamate uptake into synaptosomes of spf mice. These studies point out that: (a) the developmental abnormalities of the NMDA sub-type of glutamate receptor in spf mice could be due to the effect of sustained hyperammonemia, causing a persistent release of excess glutamate and inhibition of the ATP-dependent glutamate transport, (b) the modulatory effects of ALCAR on the NMDA binding sites could be through a repletion of ATP, required by the transporters to efficiently remove extracellular glutamate.

Carnitines increase plasma levels of adenosine and ATP in humans

In order to help to clarify the mode of action of carnitine derivatives, plasma levels of adenosine, ATP and inosine were evaluated following the infusion of 0.75, 0.50 and 0.25 mg/kg/min propionyl-L-carnitine (PLC) for 30 min in patients affected with peripheral arterial disease. Moreover, the effects of 0.75 mg/kg/min acetyl-L-carnitine (ALC) and L-carnitine (LC) were studied in the same conditions. Finally, the activity of 7.5 mg/kg/min PLC administered for 3 min was also evaluated. PLC and ALC produced a significant increase in plasma levels of adenosine and ATP, whereas LC induced less relevant changes. The administration of the compounds did not affect the adenosine/inosine ratio. Peak plasma levels of adenosine preceded in any case those of ATP. The possibility can be suggested that the pharmacological activity of PLC, ALC, and LC may be mediated, at least in part, by an interference with the endogenous purine system. Since these effects may be related to physiological mechanisms of tissue protection, new pharmacological perspectives for the compounds may arise.

Kinetic analysis of the catalytic mechanism of serotonin N-acetyltransferase (EC 2.3.1.87)

Serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AANAT, EC 2.3.1.87) is the penultimate enzyme in melatonin biosynthesis. This enzyme is of special biological interest because large changes in its activity drive the large night/day rhythm in circulating melatonin in vertebrates. In this study the kinetic mechanism of AANAT action was studied using bacterially expressed glutathione S-transferase (GST)-AANAT fusion protein. The enzymologic behavior of GST-AANAT and cleaved AANAT was essentially identical. Two-substrate kinetic analysis generated an intersecting line pattern characteristic of a ternary complex mechanism. The dead end inhibitor analog desulfo-CoA was competitive versus acetyl-CoA and noncompetitive versus tryptamine. Tryptophol was not an alternative substrate but was a dead end competitive inhibitor versus tryptamine and an uncompetitive inhibitor versus acetyl-CoA, indicative of an ordered binding mechanism requiring binding of acetyl-CoA first. N-Acetyltryptamine, a reaction product, was a noncompetitive inhibitor versus tryptamine and uncompetitive with respect to acetyl-CoA. Taken together these results support an ordered BiBi ternary complex (sequential) kinetic mechanism for AANAT and provide a framework for inhibitor design.

Acetyl-carnitine deficiency in AIDS patients with neurotoxicity on treatment with antiretroviral nucleoside analogues

OBJECTIVE

A severe dose limiting axonal peripheral neuropathy may develop in subjects on treatment with the nucleoside analogues didanosine (ddl), zalcitabine (ddC), and stavudine (d4T). The impairment of mitrochondrial DNA synthesis is crucial to the pathogenesis of this disorder although other mechanisms have not been ruled out. The depletion of acetyl-carnitine, which regulates the metabolism and function of peripheral nerves could contribute to the neurotoxicity of these compounds.

DESIGN

Non-randomized, cross-sectional study of selected patients.

METHODS

We measured the serum levels of acetyl- and total carnitine in 12 subjects with axonal peripheral neuropathy developed on treatment with different regimens of neurotoxic nucleoside analogues (ddl, ddC, d4T). Subjects who did not develop peripheral neuropathy while staying on treatment with ddl (n = 10) or zidovudine (n = 11) served as the control groups. HIV-negative subjects with axonal on demyelinating autoimmune neuropathies (n = 10) and healthy individuals (n = 13) were additional control groups.

RESULTS

Subjects experiencing axonal peripheral neuropathy on treatment with ddl, ddC and d4T had significantly reduced levels of acetyl-carnitine in comparison to the control groups. No difference was observed in the levels of total carnitine between study subjects and the control groups.

CONCLUSIONS

Our results demonstrate that subjects who developed peripheral neuropathy while staying on treatment with ddl, ddC and d4T had acetyl-carnitine deficiency. The normal levels of total carnitine in the study group appear to indicate the specificity of the defect and rule out coexisting relevant nutritional problems. The critical role of acetyl-carnitine for the metabolism and function of the peripheral nerves supports the view that the acetyl-carnitine deficiency found in these subjects may contribute to the neurotoxicity of ddl, ddC and d4T, even though the interference with mitochondrial DNA synthesis is regarded as the main cause of their toxicity.

Purification and properties of carnitine acetyltransferase from citric acid producing Aspergillus niger

Carnitine acetyltransferase was purified from the citric acid producing A. niger mycelium with a protein band showing a relative molecular weight of 77,000 and a pH optimum of 7.3. The K(m) values for the purified enzyme for acetyl-CoA and for carnitine were 0.1 mM and 1 mM, respectively. Carnitine acetyltransferase was located both in the mitochondria and in the cytosol. Both mitochondrial and cytosolic enzyme were purified using ammonium sulfate precipitation, Mono Q and Superose 12 separation. Regarding the localization, except for maximum velocity, there were no differences observed in substrate specificity and inhibition. Inhibition of the enzyme with micromolar concentrations of Cu2+ could contribute to a greater citric acid biosynthesis. Carnitine acetyltransferase can be considered as an enzyme necessary for the transport of acetyl groups through mitochondrial membrane in both directions.

Molecular cloning and sequence analysis of the rat liver carnitine octanoyltransferase cDNA, its natural gene and the gene promoter

The full-length cDNA and the natural gene for rat peroxisomal carnitine octanoyltransferase (COT) have been isolated and sequenced. The 2681 bp long cDNA contains an open reading frame for 613 amino acids, resulting in a protein with a deduced molecular weight of 70,301, and a C-terminal peroxisomal targeting sequence (Ala-His-Leu). The isolated COT cDNA has 51 bp of the 5' untranslated region (UTR), 791 bp of 3' UTR, two putative polyadenylation sites, and a poly(A19-23) tail. Screening of a rat genomic DNA library in the lambda phage with the COT cDNA probe resulted in the isolation of seven overlapping clones, together containing the complete COT gene with seventeen exons. All of the exon-intron boundary sequences conform to the GT-AG rule. The COT gene appears to spread over 40 to 60 kbp region of the rat genome. The transcription initiation site of the COT gene was determined through primer extension, and the promoter sequence up to the position -1140 was established. The promoter lacks the canonical TATA box and a promoter-reporter construct containing the sequence encompassing -1140 to +84 base positions and the firefly luciferase reporter cDNA yielded about 100-fold increase in promoter activity in transfected hepatoma cells. Some of the consensus sequences for putative cis elements present in the promoter sequence are: the two CCAAT motifs for CTF/NF1/CBP binding (at -284 and -93), two GC boxes for Sp1 binding (at -160 and -68), two AP2 sites (at -359 and -25), a half site (TGACCT) for the peroxisome proliferator activated receptor (PPAR) binding at -737 within a partial palindromic sequence region. Potential regulatory elements, such as several palindromes and repeat motifs for five different sequence segments, are also identified.

Molecular organization of hepatocyte peroxisomes

The matrix of peroxisomes has been considered to be homogeneous. However, a fine network of tubules is visible in electron micrographs at very high magnification. This substructure becomes more positive in a high-contrast photocopy and with an imaging-plate method. Clofibrate, bezafibrate, and aspirin increase peroxisomes. In proliferated peroxisomes, the density of matrix is low and the fine network is more visible. The effect of proliferators is more significant in males than in females. This sex difference may involve the action of estrogen, growth hormone, cytochrome P-450 and thyroxine. Mg-ATPase is localized on the limiting membrane of peroxisomes. Even on the membrane of irregular projections of proliferated peroxisomes, Mg-ATPase is evident cytochemically. Carnitine acetyltransferase is detectable in the matrix of proliferated peroxisomes. Withdrawal of proliferators results in a rapid decrease of peroxisomes. This may indicate the existence of peroxisome suppressors. Alternatively, dynamic transformation of vesicular to tubular types in peroxisome reticulum may occur. Such transformation has been described in lysosomes and mitochondria.

The presence of L-carnitine in ocular tissues of the rabbit

Carnitine plays an important role in the metabolism of fatty acids. Its presence is considerable in tissues that use fatty acids as an important source of energy, such as the heart and skeletal muscle. Free carnitine and acid soluble acylcarnitines are present in various tissues of the rabbit eye. The lowest concentration of carnitine was observed in the vitreous humor and the highest in the lens. The ratio, acid soluble acylcarnitine/free carnitine, was lower in the cornea, aqueous humor, vitreous humor and lens, than in iris, ciliary body and choroid-retina. The topical administration of carnitine increased both free carnitine and acetylcarnitine in cornea, and only free carnitine in aqueous humor and choroid retina. Only after intravenous administration, did the levels of free and acyl-carnitine increase in the iris and ciliary body. Neither of the two carnitine species was changed in vitreous humor. The determination of the activity of carnitine acetyltransferase in the eye showed that in the ciliary body the values of activity were three times higher than those in the iris and choroid-retina. The elevated ratio of acid soluble acylcarnitines with respect to free carnitine in iris, ciliary body, choroid-retina as well as the higher activity of carnitine acetyltransferase in the ciliary body, suggest that carnitine plays an important role in those tissues of the eye where cells of a muscular nature are present and may represent, after esterification, an important energy reserve.

Effects of C-methylated carnitine analogs on rates of mitochondrial fatty acid oxidation

Carnitine analogs containing one and/or two methyl substituents on the alpha-, beta-, or gamma-carbon were evaluated in isolated rat liver mitochondria for their effects on fatty acid oxidation. Their abilities to either support, in the absence of carnitine, or inhibit, in the presence of carnitine, carnitine-dependent fatty acid oxidation were determined by the conversion of radiolabeled [1-14C]palmitic acid to acid-soluble radiolabeled products. None of the methylcarnitine analogs were observed to be significant inhibitors of palmitate oxidation at concentrations (1.0 mM) up to ten times that for L-carnitine. The two diastereomers of D,L-4-methylcarnitine, however, were able to support palmitate oxidation in the absence of carnitine, and rates were roughly 40% of that obtained with equimolar (0.1 mM) L-carnitine.

Sites of action of carnitine and its derivatives on the cardiovascular system: interactions with membranes

Carnitine plays an essential role in the regulation of long-chain fatty acid metabolism in skeletal and cardiac muscle, a process that is mediated by well-characterized enzymatic mechanisms. Here, Irving Fritz and Edoardo Arrigoni-Martelli review the evidence that carnitine and its O-acyl derivatives also influence membrane fluidity, ion channel functions, smooth muscle contractility, membrane stability and cardiac functions. The authors present the view that direct interactions of carnitine derivatives with cell membranes are independent of reactions catalysed by carnitine acyltransferases. They propose that the novel actions discussed are implicated in the mechanisms by which carnitine and its derivatives protect perfused hearts subjected to ischaemia or to oxidative stress, and help people suffering from certain types of myocardial ischaemia or peripheral arterial disease.

Synthesis of L-carnitine by microorganisms and isolated enzymes

L-Carnitine, a quaternary ammonium compound, plays an important role in beta-oxidation of fatty acids in mammals. The increasing demand for this compound in medicine has led to the development of numerous procedures for L-carnitine production. This review discusses the possibilities of microbial and enzymatical synthesis of L-carnitine and gives an overview on the pathways of L-carnitine metabolism and related enzymes in microorganisms.

Carnitine acetyltransferase activity in the human brain and its microvessels is decreased in Alzheimer's disease

L-Carnitine and acetyl-L-carnitine facilitate mitochondrial beta-oxidation of fatty acids. In the brain, they may also have a role in acetylcholine synthesis. Carnitine acetyltransferase catalyzes the interchange between L-carnitine and acetyl-L-carnitine. Recently, acetyl-L-carnitine was reported to have a beneficial effect in patients with Alzheimer's disease. We therefore assessed carnitine acetyl-transferase activity in selected brain regions and in isolated cerebral microvessels obtained at autopsy from patients with Alzheimer's disease and from age-matched control subjects. We found a 25 to 40% decrease in carnitine acetyltransferase activity in patients with Alzheimer's disease, which attained statistical significance in most brain regions and in cerebral microvessels. These findings document another neurochemical abnormality in patients with Alzheimer's disease and provide a rationale for the use of acetyl-L-carnitine in the treatment of patients with Alzheimer's disease.

Inhibition of cell-cell adhesion and morphogenesis of Dictyostelium by carnitine

Carnitine (gamma-trimethylammonium beta-hydroxy-butyric acid) possesses the novel property of preventing cell aggregation elicited by clusterin or by fibrinogen (I.B. Fritz and K. Burdzy, J. Cell. Physiol., 140:18-28 [1989]). In investigations reported here, we show that carnitine also affects cell-cell adhesion in Dictyostelium discoideum, a cellular slime mold whose cells interact in specific and complex manners during discrete stages of development. Two types of cell adhesion systems sequentially appear on the surface of developing Dictyostelium cells, involving the surface glycoprotein gp24 which mediates EDTA-sensitive binding sites, and the surface glycoprotein gp80 which mediates the EDTA-resistant binding sites. Addition of increasing concentrations of D(+)-carnitine and L(-)-carnitine resulted in a progressive inhibition of both the EDTA-sensitive binding sites and the EDTA-resistant binding sites of Dictyostelium cells at different stages of development. In contrast, comparable or higher concentrations of choline, acetyl-beta-methylcholine, or deoxycarnitine had no detectable effects on cell aggregation. Concentrations of carnitine required for 50% inhibition of EDTA-resistant adhesion sites were found to be dependent upon levels of gp80 expressed by Dictyostelium, with greatest inhibition by carnitine of reassociation of cells containing the lowest levels of gp80. Removal of carnitine from cells by washing resulted in the rapid restoration of the ability of Dictyostelium to form aggregates and to resume normal development. We discuss possible mechanisms by which carnitine inhibits the aggregation of cells.

Effect of carnitine acetyltransferase inhibition on rat hepatocyte metabolism

Carnitine acetyltransferase (CAT) catalyzes the reversible transfer of short chain (less than six carbons in length) acyl groups from acyl-CoA thioesters to form the corresponding acylcarnitines. This reaction has been suggested to be of importance in decreasing cellular content of acyl-CoA under conditions characterized by accumulation of poorly metabolized, potentially toxic acyl-CoAs. To study the importance of the CAT reaction, the effect of CAT inhibitors on rat hepatocyte metabolism in the presence of propionate was examined. Acetyl-DL-aminocarnitine inhibited [14C]propionylcarnitine accumulation by isolated hepatocytes incubated with [14C]propionate (1.0-10.0 mM). Inhibition of propionylcarnitine formation by acetyl-DL-aminocarnitine was concentration dependent and was not due to non-specific cellular toxicity as [14C]glucose formation from [14C]propionate, and [1-14C]pyruvate oxidation were unaffected by the CAT inhibitor. Inhibition of propionylcarnitine formation was increased by preincubating hepatocytes with acetyl-DL-aminocarnitine, suggesting competition for cellular uptake between carnitine and the inhibitor. Hemiacetylcartinium (HAC) and meso-2,6-bis(carboxymethyl)4,4-dimethylmorpholinium bromide (CMDM), potent inhibitors of CAT in broken cell systems, did not inhibit hepatocyte propionylcarnitine formation under the conditions evaluated. Propionate (5 mM) inhibited hepatocyte pyruvate (10 mM) oxidation, and this inhibition was partially reversed by 5 mM carnitine. Addition of 5.0 mM acetyl-DL-aminocarnitine abolished the stimulatory effect of carnitine on pyruvate oxidation in the presence of propionate. These studies establish that acetyl-DL-aminocarnitine inhibits intact hepatocyte CAT activity, and thus provide a useful probe of the role of CAT in cellular metabolism. CAT activity appears to be critical for carnitine-mediated reversal of propionate-induced inhibition of pyruvate oxidation.

Purification of heart and liver mitochondrial carnitine acetyltransferase

Heart and liver mitochondrial, as well as liver peroxisomal, carnitine acetyltransferase was purified to apparent homogeneity and some properties, primarily of heart mitochondrial carnitine acetyltransferase, were determined. Hill coefficients for propionyl-CoA are 1.0 for each of the enzymes. The molecular weight of heart mitochondrial carnitine acetyltransferase, determined by SDS-PAGE, is 62,000. It is monomeric in the presence of catalytic amounts of substrate. Polyclonal antibodies against purified rat liver peroxisomal carnitine acetyltransferase precipitate liver and heart mitochondrial and liver peroxisomal carnitine acetyltransferase, but not liver peroxisomal carnitine octanoyltransferase. Liver peroxisomes, mitochondria, and microsomes and heart mitochondria all give multiple bands on Western blotting with the antibody against carnitine acetyltransferase. Major protein bands occur at the molecular weight of carnitine acetyltransferase and at 33 to 35 kDa.