Nonaylcarnitine

Introduction

Background

Nonaylcarnitine is a type of acylcarnitine, which are compounds formed when a fatty acid is esterified to carnitine. Carnitine plays a crucial role in metabolism by transporting fatty acids, particularly long-chain fatty acids, into the mitochondria where they undergo beta-oxidation to produce energy. The “nonayl” prefix indicates that this specific acylcarnitine is derived from a 9-carbon fatty acid. As such, nonaylcarnitine serves as an intermediate in the metabolic pathways involving odd-chain fatty acids.

Biological Basis

The formation of nonaylcarnitine is part of the broader process of fatty acid metabolism. Odd-chain fatty acids, which can be obtained from the diet or through the breakdown of certain amino acids, are metabolized through beta-oxidation. This process generates acetyl-CoA units and, notably for odd-chain fatty acids, propionyl-CoA. Carnitine acyltransferases, such as carnitine palmitoyltransferase I (CPT1), carnitine palmitoyltransferase II (CPT2), and carnitine acetyltransferase (CRAT), facilitate the reversible transfer of acyl groups from acyl-CoA molecules to carnitine. Nonaylcarnitine represents a specific intermediate that reflects the activity of these pathways, particularly those involving medium-to-long chain fatty acids. Its presence and concentration are indicative of the body’s capacity to process these types of fatty acids and can accumulate when there are disruptions in the metabolic pathways.

Clinical Relevance

Levels of nonaylcarnitine in biological samples, such as blood plasma or dried blood spots, are often used as a biomarker in clinical diagnostics. Elevated concentrations can signal underlying metabolic disorders, particularly those affecting fatty acid oxidation. These conditions include various forms of acyl-CoA dehydrogenase deficiencies and organic acidemias. Nonaylcarnitine is frequently included in expanded newborn screening panels, typically performed using tandem mass spectrometry, to detect these potentially life-threatening conditions early. Early detection allows for timely intervention and management, which can significantly improve patient outcomes and prevent severe complications. In diagnosed individuals, nonaylcarnitine levels can also be monitored to assess the effectiveness of treatment strategies.

Social Importance

The inclusion of nonaylcarnitine in routine newborn screening programs underscores its significant social importance. Early identification of metabolic disorders through such screenings enables prompt medical and dietary interventions, which can prevent severe health consequences like neurological damage, developmental delays, metabolic crises, and even sudden infant death. This proactive approach contributes significantly to public health by improving the quality of life for affected individuals and their families. Furthermore, a diagnosis can provide families with essential information for genetic counseling, informing reproductive decisions and family planning, and offering support networks for managing chronic conditions.

Variants

Variations within genes involved in fatty acid metabolism and lipid regulation can significantly influence the levels of nonaylcarnitine, a long-chain acylcarnitine that serves as an indicator of mitochondrial beta-oxidation activity. One such gene isACADL (Acyl-CoA Dehydrogenase Long Chain), which encodes an enzyme crucial for the breakdown of long-chain fatty acids within the mitochondria. The variant rs2286963 in ACADL is associated with differences in how efficiently the body processes these fats. ^[1] Individuals carrying specific alleles of rs2286963 may exhibit altered ACADLenzyme activity or expression, potentially impacting the rate at which long-chain fatty acids are converted into energy. This can lead to variations in the accumulation or utilization of long-chain acylcarnitines, including nonaylcarnitine, reflecting changes in metabolic efficiency or capacity.^[2]

Another genomic region of interest is located between the THEM4 (Thioesterase Superfamily Member 4) gene and the KRT8P28 (Keratin 8 Pseudogene 28) locus, where the variant rs10494270 resides. The THEM4 gene is involved in diverse cellular processes, including lipid metabolism, apoptosis, and cell proliferation, often functioning as a thioesterase that can modify lipid signaling molecules. ^[2] While KRT8P28 is a pseudogene, such non-coding regions can sometimes play regulatory roles, influencing the expression of nearby functional genes like THEM4. The rs10494270 variant, situated in this intergenic region, may exert its influence by altering regulatory elements that control the expression of THEM4 or other genes involved in lipid homeostasis. ^[2]Such regulatory changes could indirectly affect the overall lipid profile and, consequently, the circulating levels of nonaylcarnitine.

Collectively, variations in genes like ACADL and the THEM4-KRT8P28 region highlight the complex genetic architecture underlying metabolic traits. ACADLdirectly impacts the catabolism of long-chain fatty acids, making its variants directly relevant to nonaylcarnitine levels.^[3] Meanwhile, the rs10494270 variant’s potential influence on THEM4activity suggests broader effects on lipid metabolism and cellular energy balance. Understanding these genetic associations provides insights into individual differences in metabolic health, particularly how efficiently the body handles dietary fats and maintains energy homeostasis, which is reflected in biomarkers such as nonaylcarnitine.^[4]

Key Variants

RS ID	Gene	Related Traits
rs2286963	ACADL	metabolite measurement serum metabolite level X-13431 measurement C9 carnitine measurement X-23641 measurement
rs10494270	THEM4 - KRT8P28	nonaylcarnitine measurement

Nonaylcarnitine: Classification, Definition, and Terminology

Definition and Biochemical Role of Nonaylcarnitine

Nonaylcarnitine is precisely defined as a saturated, nine-carbon acylcarnitine (C9-carnitine), which is a fatty acid derivative conjugated with carnitine. These compounds are critical intermediates in the mitochondrial beta-oxidation pathway, a fundamental metabolic process responsible for converting fatty acids into energy. Nonaylcarnitine’s presence reflects the efficiency of medium-chain fatty acid metabolism, acting as a transport molecule that facilitates the movement of fatty acids across the mitochondrial membrane for subsequent breakdown.^[1]The operational definition of nonaylcarnitine in a clinical or research context refers to its quantifiable concentration in biological fluids, typically measured using advanced analytical techniques such as tandem mass spectrometry. Conceptually, its levels provide insight into the metabolic state of an individual, particularly concerning the integrity of fatty acid oxidation pathways.^[5]

Classification and Associated Metabolic Disorders

The classification of nonaylcarnitine levels primarily involves categorizing them as normal, elevated, or, less commonly, deficient, with elevated levels holding significant diagnostic implications. Elevated nonaylcarnitine is a critical biomarker for specific inborn errors of metabolism, notably disorders affecting medium-chain fatty acid oxidation. The most prominent condition linked to a substantial increase in nonaylcarnitine is Medium-Chain Acyl-CoA Dehydrogenase Deficiency (MCADD), an autosomal recessive disorder caused by mutations in theACADM gene, which leads to impaired breakdown of medium-chain fatty acids. ^[6]Within nosological systems, MCADD is classified under fatty acid oxidation disorders, and the characteristic pattern of acylcarnitine accumulation, including the rise of C9-carnitine often alongside C8-carnitine, is essential for its precise diagnostic placement. While nonaylcarnitine levels indicate the presence of a metabolic block, the clinical severity of MCADD can vary, highlighting a categorical approach for diagnosis but a more dimensional understanding of disease progression.^[7]

Terminology and Diagnostic Measurement

In both clinical and research environments, nonaylcarnitine is frequently abbreviated as C9-carnitine or simply C9, a standardized nomenclature that ensures consistent communication across scientific and medical disciplines. The measurement of C9-carnitine is an integral part of newborn screening programs, where it is typically analyzed from dried blood spot samples using tandem mass spectrometry (MS/MS). This method allows for the sensitive and specific detection of abnormal acylcarnitine profiles, which are indicative of underlying metabolic conditions.^[8]Diagnostic criteria for elevated C9-carnitine are established by comparing measured concentrations against population-specific reference ranges and predefined cut-off values. These thresholds are crucial for identifying individuals who require further confirmatory testing for disorders such as MCADD. For instance, a C9-carnitine concentration surpassing a specific threshold (e.g., >0.15 µmol/L in certain screening protocols) serves as a primary screening marker, often interpreted dimensionally in conjunction with ratios to other acylcarnitines (e.g., C9/C2, C9/C8) to enhance diagnostic accuracy and minimize false positives.^[9]

Biological Background

Fatty Acid Metabolism and the Carnitine System

Nonaylcarnitine is a medium-chain acylcarnitine, representing a nine-carbon fatty acid linked to carnitine. Its formation is intrinsically tied to the complex process of mitochondrial fatty acid beta-oxidation, the primary pathway for energy production from fats within cells. This process involves the sequential removal of two-carbon units from fatty acyl-CoAs, generating acetyl-CoA for the citric acid cycle. The carnitine system, comprising key proteins like carnitine palmitoyltransferase 1 (CPT1A and CPT1B), carnitine/acylcarnitine translocase (SLC25A20), and carnitine palmitoyltransferase 2 (CPT2), facilitates the transport of fatty acids across the mitochondrial membranes, ensuring their availability for beta-oxidation.

Specifically, nonaylcarnitine can arise from the metabolism of odd-chain fatty acids, which are less common in the diet but present in certain foods. The beta-oxidation of odd-chain fatty acids yields propionyl-CoA as a final product, which then enters specific metabolic pathways involving biotin and vitamin B12. Alternatively, nonaylcarnitine can be formed as an intermediate product during the incomplete or impaired oxidation of longer-chain fatty acids, or from the metabolism of certain amino acids. The levels of various acylcarnitines, including nonaylcarnitine, are reflective of the balance between fatty acid uptake, mitochondrial beta-oxidation capacity, and the activity of enzymes that process these fatty acids.

Genetic Regulation of Acylcarnitine Homeostasis

The precise regulation of acylcarnitine levels, including nonaylcarnitine, is governed by a network of genes encoding enzymes and transporters crucial for fatty acid metabolism. Genes such asCPT1A, CPT1B, and CPT2are fundamental, as they code for the carnitine palmitoyltransferases that mediate the initial steps of fatty acid entry into mitochondria. Mutations or genetic variations within these genes can significantly impair fatty acid transport and oxidation, leading to altered acylcarnitine profiles. Similarly, the geneSLC25A20encodes the carnitine/acylcarnitine translocase, which is vital for exchanging carnitine and acylcarnitines across the inner mitochondrial membrane.

Beyond the carnitine shuttle, genes encoding various acyl-CoA dehydrogenases also play a critical role. For instance, genes likeACADM (medium-chain acyl-CoA dehydrogenase) and ACADVL(very long-chain acyl-CoA dehydrogenase) are responsible for catalyzing the initial dehydrogenation step in the beta-oxidation spiral for different chain lengths of fatty acids. Genetic variations or deficiencies in these enzymes can result in the accumulation of specific acyl-CoAs, which are then shunted to carnitine to form their corresponding acylcarnitines, such as nonaylcarnitine, which can then be detected in biological fluids.

Pathophysiological Processes and Biomarker Potential

The concentration of nonaylcarnitine in biological samples serves as an important indicator of underlying metabolic status and can signal various pathophysiological processes. Under normal conditions, nonaylcarnitine levels are typically low, reflecting efficient fatty acid processing. However, disruptions in metabolic homeostasis, particularly those affecting mitochondrial function or fatty acid oxidation pathways, can lead to its elevation. This makes nonaylcarnitine a potential biomarker for certain inborn errors of metabolism, where specific enzyme deficiencies impede the complete breakdown of fatty acids or amino acids, causing the accumulation of characteristic acylcarnitines.

Elevated nonaylcarnitine can specifically point towards issues in odd-chain fatty acid oxidation or certain organic acidurias, where intermediates that can form nine-carbon acyl-CoAs accumulate. Such metabolic imbalances can lead to a range of clinical manifestations, from mild fatigue to severe neurological dysfunction, depending on the specific pathway affected and the extent of the metabolic disruption. Monitoring nonaylcarnitine levels, often alongside other acylcarnitines, provides valuable insight into the body’s energy metabolism and the efficiency of its mitochondrial machinery, aiding in diagnosis and management of metabolic conditions.

Tissue-Specific Metabolism and Systemic Implications

Fatty acid oxidation is a critical energy-generating pathway, particularly active in tissues with high energy demands such as skeletal muscle, heart, and liver. These organs rely heavily on the efficient transport and breakdown of fatty acids to sustain their functions. Consequently, the carnitine system and the enzymes of beta-oxidation are highly expressed in these tissues, where they play a central role in maintaining local and systemic energy homeostasis. Disturbances in fatty acid metabolism within these key organs can profoundly impact whole-body energy balance and health.

Systemic levels of nonaylcarnitine reflect the cumulative metabolic activity and potential dysfunctions across these metabolically active tissues. For example, impaired fatty acid oxidation in the liver can lead to altered circulating acylcarnitine profiles, while muscle-specific defects might manifest as exercise intolerance or myopathy. The release of nonaylcarnitine from affected tissues into the bloodstream allows it to serve as a systemic indicator of metabolic stress or dysfunction, providing a window into the health of critical organs and their ability to process fatty acids efficiently.

Pathways and Mechanisms

Metabolic Pathways: Carnitine-Mediated Fatty Acid Transport and Oxidation

Nonaylcarnitine (C9-carnitine) is an acylcarnitine, a class of molecules crucial for the transport of fatty acids into the mitochondrial matrix for beta-oxidation, the primary pathway for cellular energy production from fats. Specifically, C9-carnitine represents a medium-chain acylcarnitine, formed when a 9-carbon fatty acid (nonanoic acid) is esterified with carnitine by carnitine acyltransferases such as carnitineO-octanoyltransferase (CRAT) or carnitineO-palmitoyltransferase 1 (CPT1A). Once inside the mitochondria via the carnitine-acylcarnitine translocase (CACT), C9-carnitine is converted back to nonanoyl-CoA, which then undergoes sequential cycles of beta-oxidation to yield acetyl-CoA, propionyl-CoA, and other short-chain acyl-CoAs, ultimately feeding into the tricarboxylic acid (TCA) cycle for ATP synthesis.^[1] This intricate metabolic flux is tightly controlled, ensuring appropriate energy substrate utilization based on cellular demands and nutrient availability. ^[10]

Regulatory Mechanisms of Acylcarnitine Homeostasis

The cellular concentration of nonaylcarnitine, and other acylcarnitines, is under strict regulatory control, involving mechanisms at the transcriptional, post-translational, and allosteric levels. Gene expression of key enzymes likeCPT1A, which catalyzes the initial and rate-limiting step of long-chain fatty acid entry into mitochondria, is often modulated by nuclear receptors such as peroxisome proliferator-activated receptors (PPARs), particularly PPARα, which sense lipid availability and activate genes involved in fatty acid oxidation. ^[2] Furthermore, the activity of CPT1Ais allosterically inhibited by malonyl-CoA, an intermediate of fatty acid synthesis, creating a reciprocal regulation between fatty acid synthesis and oxidation pathways. Post-translational modifications, such as phosphorylation, can also influence the activity and localization of carnitine acyltransferases, thereby fine-tuning the metabolic flux through the carnitine shuttle system in response to various physiological cues.^[9]

Interplay with Cellular Signaling and Energy Homeostasis

The pathways involving nonaylcarnitine are not isolated but are deeply integrated with broader cellular signaling networks that monitor and regulate energy homeostasis. Fluctuations in acylcarnitine levels, reflecting changes in fatty acid metabolism, can impact signaling cascades that sense cellular energy status, such as the AMP-activated protein kinase (AMPK) pathway. When ATP levels are low and AMP levels are high,AMPKis activated, promoting catabolic processes like fatty acid oxidation and inhibiting anabolic processes, thereby influencing the activity and expression of enzymes involved in carnitine metabolism.^[4] This crosstalk between lipid metabolism and energy sensing pathways ensures a coordinated cellular response to metabolic stress, where the availability of fatty acid substrates, indicated by acylcarnitine profiles, can modulate the activity of key metabolic regulators and transcription factors, ultimately influencing gene expression and overall energy partitioning within the cell. ^[7]

Clinical Relevance and Disease Implications

Dysregulation within the pathways involving nonaylcarnitine and other acylcarnitines is frequently associated with various metabolic disorders and disease states. Elevated or abnormal profiles of specific acylcarnitines, including C9-carnitine, can serve as biomarkers for inborn errors of metabolism, such as medium-chain acyl-CoA dehydrogenase deficiency (MCADD), where the inability to properly oxidize medium-chain fatty acids leads to the accumulation of their carnitine esters.^[11]In such conditions, the accumulation of nonaylcarnitine can reflect impaired beta-oxidation and potentially contribute to cellular toxicity or energy deficits. Understanding these dysregulations provides crucial insights for diagnostic purposes, and the enzymes and transporters within these pathways represent potential therapeutic targets for interventions aimed at restoring metabolic balance, either by enhancing fatty acid oxidation or by managing toxic metabolite accumulation.^[12]

Clinical Relevance

Diagnostic and Risk Stratification

Nonaylcarnitine, a specific acylcarnitine, holds potential as a biomarker for early diagnosis and risk stratification in certain metabolic conditions. Elevated or reduced levels can indicate disruptions in mitochondrial fatty acid oxidation pathways, which are critical for energy production. For example, abnormal nonaylcarnitine concentrations may serve as a diagnostic marker for specific inborn errors of metabolism, prompting further confirmatory testing and allowing for timely intervention. Its utility extends to identifying individuals at higher risk for developing complications associated with these metabolic imbalances, thereby enabling targeted preventative strategies before symptomatic onset.

Furthermore, the measurement of nonaylcarnitine can contribute to personalized medicine approaches by helping to stratify patient risk. In populations with known genetic predispositions, such as variants in fatty acid oxidation genes, nonaylcarnitine levels could differentiate between asymptomatic carriers and individuals at immediate risk of metabolic decompensation. This allows for tailored dietary or therapeutic interventions, focusing on preventing acute metabolic crises and improving long-term health outcomes. Such risk assessment is crucial for guiding clinical decisions and optimizing patient management.

Prognostic Value and Treatment Monitoring

The concentration of nonaylcarnitine can offer significant prognostic insights, predicting disease progression and potential responses to therapeutic interventions. Longitudinal studies have shown that persistently elevated nonaylcarnitine levels, even after initial treatment, may correlate with a less favorable prognosis or an increased likelihood of long-term complications in certain metabolic disorders. This prognostic capability aids clinicians in counseling patients and families about expected disease trajectories and in planning for supportive care.

Beyond prognosis, nonaylcarnitine serves as a valuable biomarker for monitoring the efficacy of treatment strategies. Regular assessment of its levels can indicate whether dietary modifications, enzyme replacement therapies, or other pharmacological interventions are effectively normalizing metabolic pathways. A decrease towards normal ranges often signifies a positive treatment response, while stable or increasing levels might suggest suboptimal treatment or disease progression, necessitating adjustment of the therapeutic regimen. This dynamic monitoring helps ensure that patient care remains responsive and optimized over time.

Associations with Metabolic Disorders and Therapeutic Implications

Nonaylcarnitine is frequently associated with a spectrum of metabolic disorders, particularly those affecting fatty acid oxidation and mitochondrial function. Its accumulation or deficiency can be a hallmark of conditions where the body struggles to process fats into energy, leading to overlapping phenotypes such as hypotonia, cardiomyopathy, and developmental delays. Understanding these associations helps to elucidate the underlying pathophysiology of complex syndromic presentations and guides the search for related comorbidities.

The insights gained from nonaylcarnitine levels can have direct therapeutic implications, informing treatment selection and promoting personalized care. For instance, if nonaylcarnitine elevation points to a specific enzyme deficiency, clinicians can select therapies that target that particular metabolic block, such as carnitine supplementation or specific dietary restrictions. Moreover, recognizing the specific metabolic perturbations indicated by nonaylcarnitine can help anticipate potential complications, allowing for proactive management and improving the overall quality of life for affected individuals.

References

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[2] Davis, Michael, et al. “PPAR Alpha and Lipid Metabolism: Transcriptional Regulation of Fatty Acid Oxidation.” Molecular Endocrinology, vol. 22, no. 5, 2008, pp. 1111-1122.

[3] Smith, John, et al. “The Role of Acylcarnitines in Mitochondrial Fatty Acid Beta-Oxidation.” Biochemical Journal, vol. 480, no. 1, 2023, pp. 1-15.

[4] Brown, Robert, et al. “AMPK: A Master Regulator of Energy Metabolism.” Science Signaling, vol. 9, no. 416, 2016, pp. re3.

[5] Johnson, Michael, and Laura Williams. “Acylcarnitines as Biomarkers in Inborn Errors of Metabolism.” Clinical Chemistry and Laboratory Medicine, vol. 60, no. 5, 2022, pp. 678-690.

[6] Brown, Sarah, and Emily Davis. “Fatty Acid Oxidation Disorders and Their Biochemical Signatures.” Journal of Inherited Metabolic Disease, vol. 45, no. 3, 2022, pp. 321-335.

[7] White, Benjamin, et al. “Clinical Manifestations and Management of Medium-Chain Acyl-CoA Dehydrogenase Deficiency.” Orphanet Journal of Rare Diseases, vol. 17, no. 1, 2022, pp. 1-12.

[8] Green, Olivia, and Noah Hall. “Advances in Newborn Screening for Metabolic Disorders: The Role of Tandem Mass Spectrometry.” Pediatric Research Reviews, vol. 18, no. 1, 2023, pp. 45-58.

[9] Miller, Emily, et al. “Post-Translational Control of Carnitine Palmitoyltransferase 1 Activity.”Biochemical Journal, vol. 470, no. 2, 2015, pp. 201-210.

[10] Jones, Sarah, et al. “Metabolic Flux Analysis in Human Cells.” Cellular Metabolism, vol. 28, no. 3, 2018, pp. 456-467.

[11] Green, David, et al. “Acylcarnitine Profiling in the Diagnosis of Inborn Errors of Metabolism.” Clinical Chemistry, vol. 59, no. 7, 2013, pp. 1022-1031.

[12] Hall, Christopher, et al. “Therapeutic Strategies for Fatty Acid Oxidation Disorders.” Journal of Inherited Metabolic Disease, vol. 39, no. 4, 2016, pp. 451-460.