Alpha-ketoglutarate (AKG), also known as 2-ketoglutaric acid, 2-oxoglutamate, 2-oxoglutaric acid, oxoglutaric acid, and 2-oxopentanedioic acid, is an organic compound with diverse roles in metabolism and potential clinical applications. It is an organic compound containing a ketone group (C=O) adjacent to a carboxyl group (COOH). These compounds are important intermediates in the catabolism of lipids and proteins, playing a crucial role in energy production and metabolism. AKG serves as a key intermediate in the citric acid cycle (also known as the Krebs cycle), a central pathway for energy production in cells.
Understanding Alpha-Keto Acids
In organic chemistry, keto acids or ketoacids, also called oxo carboxylic acids, are organic compounds that contain a carboxylic acid group (−COOH) and a ketone group (>C=O). In several cases, the keto group is hydrated. Alpha-keto acids, alpha-ketoacids, or 2-oxoacids have the keto group adjacent to the carboxylic acid. They often arise by oxidative deamination of amino acids, and reciprocally, they are precursors to the same. alpha-ketoglutaric acid, a 5-carbon ketoacid derived from glutamic acid. Beta-keto acids, beta-ketoacids, or 3-oxoacids, such as acetoacetic acid, have the ketone group at the second carbon from the carboxylic acid. They generally form by the Claisen condensation. Gamma-keto acids, Gamma-ketoacids, or 4-oxoacids have the ketone group at the third carbon from the carboxylic acid. Keto acids appear in a wide variety of anabolic pathways in metabolism. When ingested sugars and carbohydrate levels are low, stored fats and proteins are the primary source of energy production. Glucogenic amino acids from proteins and/or Glycerol from Triglycerides are converted to glucose.
Key Facts About α-Keto Acids
- α-Keto acids are key intermediates in the citric acid cycle, also known as the Krebs cycle, which is a central pathway for energy production in cells.
- The oxidative decarboxylation of α-keto acids, such as pyruvate and α-ketoglutarate, is a critical step in the catabolism of carbohydrates, lipids, and proteins.
- Transamination reactions involving α-keto acids and amino acids are essential for the interconversion of these biomolecules and the regulation of nitrogen balance in the body.
- Deficiencies or imbalances in α-keto acid metabolism can lead to various metabolic disorders, such as maple syrup urine disease and homocystinuria.
- α-Keto acids can also be used as substrates for the synthesis of other important biomolecules, such as neurotransmitters and hormones.
Role in Metabolism
In cellular metabolism, the generation and decomposition of AKG are involved in a variety of metabolic pathways. In the TCA cycle, AKG is decarboxylated to succinyl-CoA and CO2 by AKG dehydrogenase (encoded by ogdh-1), a key control point of the TCA cycle. Otherwise, AKG can be generated from isocitrate by oxidative decarboxylation catalysed by isocitrate dehydrogenase (IDH). Also, AKG can be produced anaplerotically from glutamate by oxidative deamination using glutamate dehydrogenase, and as a product of pyridoxal phosphate-dependent trans-amination reactions in which glutamate is a common amino donor. AKG supplementation in human adult stage is sufficient whereas it is found to be insufficient in the senescent stage
The Citric Acid Cycle and α-Keto Acids
The citric acid cycle, also known as the Krebs cycle, is a central metabolic pathway that is closely linked to the metabolism of α-keto acids. Many α-keto acids, such as pyruvate, acetyl-CoA, and α-ketoglutarate, are either directly produced as intermediates in the citric acid cycle or enter the cycle after undergoing various catabolic processes. The oxidation and decarboxylation of these α-keto acids within the citric acid cycle is a crucial step in the complete oxidation of carbohydrates, lipids, and proteins, ultimately leading to the generation of ATP, the primary energy currency of the cell. The citric acid cycle, therefore, plays a pivotal role in the efficient metabolism of α-keto acids and the overall energy production in the body.
Catabolism of Lipids and Proteins
α-Keto acids are central intermediates in the catabolic pathways of both lipids and proteins. In the catabolism of lipids, the oxidative decarboxylation of α-keto acids, such as pyruvate and acetyl-CoA, is a crucial step in the breakdown of fatty acids and the subsequent entry of their carbon skeletons into the citric acid cycle for energy production. Similarly, in the catabolism of proteins, the transamination of amino acids with α-keto acids, like α-ketoglutarate, leads to the formation of new amino acids that can then be further degraded, with the α-keto acid intermediates entering the citric acid cycle.
Read also: The role of alpha-keto acids in metabolism.
Consequences of Imbalances
Disruptions in the metabolism of α-keto acids can have significant consequences for an individual's health and well-being. Deficiencies or imbalances in the metabolism of specific α-keto acids can lead to the accumulation of these compounds or the depletion of downstream metabolites, resulting in various metabolic disorders. For example, a deficiency in the enzyme responsible for the oxidative decarboxylation of α-ketoglutarate can lead to the condition known as maple syrup urine disease, characterized by the buildup of branched-chain α-keto acids and the associated neurological and developmental problems. Similarly, an imbalance in the metabolism of homocysteine, an α-keto acid intermediate, can contribute to the development of homocystinuria, which is linked to an increased risk of cardiovascular and neurological complications.
Potential Health Benefits and Uses
Alpha-ketoglutarate is a natural compound that works in many pathways in the body, including making muscle and healing wounds. It's also used as medicine. Alpha-ketoglutarate seems to help the body rebuild muscle and reduce muscle loss after surgery or trauma. It might also have anti-aging effects.
Muscle Metabolism and Protein Synthesis
AKG is a nitrogen scavenger and a source of glutamate and glutamine that stimulates protein synthesis and inhibits protein degradation in muscles. AKG as a precursor of glutamate and glutamine is a central metabolic fuel for cells of the gastrointestinal tract as well. AKG can decrease protein catabolism and increase protein synthesis to enhance bone tissue formation in the skeletal muscles and can be used in clinical applications.
Wound Healing and Collagen Production
It has been demonstrated that AKG is involved in collagen metabolism through a variety of mechanisms. First, AKG is a cofactor of prolyl-4-hydroxylase (P4H). P4H is located within the endoplasmic reticulum (ER), and catalyze the formation of 4-hydroxyproline, which is crucial for the formation of the collagen triple helix.
Incomplete hydroxylation of proline residues within the repeated amino acid motif: any amino acid-proline-glycine (X-Pro-Gly), results in incomplete formation of the collagen triple helix. Incorrectly folded triple helices are not secreted into cytoplasm, and are subsequently degraded in the ER. Second, AKG contributes to facilitate collagen synthesis by increasing the pool of proline residues via glutamate and about 25% of the dietary AKG is converted to proline in the enterocytes.
Read also: Explore the details of BCKDH Deficiency
Proline is a primary substrate for collagen synthesis, and plays a central role in collagen metabolism. As seen in Fig. 1, proline is formed through the conversion of pyrroline 5-carboxylate (P5C), an intermediate in the inter-conversion of proline, ornithine and glutamate. Recently, it was reported that in addition to being a source of proline residues through the P5C-pathway, P5C activates collagen production through the activation of prolidase, a key enzyme in proline recyling . In this regard, AKG, which is a precursor of P5C, also has a close relationship to proline metabolism in the cell and organism. In a study performed in growing pigs, it was displayed that enteral AKG administration increased the level of proline in the portal and arterial blood by 45% and 20%, respectively, when compared to animals that were not given AKG.
Bone Tissue Formation
Another mechanisms of AKG influence on bone tissue results from its impact on the endocrine system of the organism. Glutamine and glutamate is transformed in ornithine and then to arginine. Both ornithine and arginine stimulate the secretion of growth hormone (GH) and insulin-like growth factor I (IGF-I) . The osteotropic effect of functional axis GH-IGF-I is widely known and well described.
AKG may also affect bone structure by the interaction of glutamate-glutamate receptors (GluR). The presence of GluR has been confirmed on osteoblasts and osteoclasts, whereas Genever et al reported its significance in bone tissue metabolism. Additionally, there is a preliminary evidence to show that dietary AKG counteracts the bone losses in rats with experimental osteopenia induced by ovariectomy and fundectomy.
Immune Function
AKG is also called the immune nutrient factor and it play an important role in the general immune metabolism. It is already known that AKG is an important source of glutamine and glutamate, is defined as glutamine homologue and derivative. Glutamine is an important fuel for lymphocytes and macrophages. Macrophages and neutrophils are involved in the early, non-specific host-defence responses and play an important role in the pathophysiology and/or protection against sepsis. Previous reports showed that during inflammatory states such as sepsis and injury, the consumption of glutamine by circulating and immune cells increases. Studies have revealed that supplemental glutamine augments the in vitro bactericidal activity of neutrophils in burned or postoperative patients. The study by Gianotti et al. (1995) showed that oral glutamine supplementation decreases bacterial translocation in experimental gut-origin sepsis.
Anti-Aging Effects
A recent study (Chin et al., 2014) shows that AKG can extend the lifespan of adult Caenorhabditis elegans by inhibiting ATP synthase and TOR. They discovered that the tricarboxylic acid cycle intermediate AKG delays ageing and extends the lifespan of C. elegans by ∼ 50% with a concentration-dependent manner of 8 mM AKG producing the maximal lifespan extension in wild-type N2 worms. Chin et al (Chin et al., 2014) also demonstrated that AKG not only extends lifespan, but also delays age-related phenotypes, such as the decline in rapid, coordinated body movement. In this study, it reported that AKG has greater potential values in aging.
Read also: The Role of BCKDC
Mitochondrial ATP synthase is a significant ubiquitous enzyme in energy metabolism of virtually all living cells. It is a membrane-bound rotary motor enzyme that is a key energy carrier for cellular energy metabolism. Chin et al (Chin et al., 2014) provided evidence that the lifespan increase by AKG requires ATP synthase subunit β and is dependent on target of rapamycin (TOR) downstream. They used a small-molecule target identification strategy termed drug affinity responsive target stability (DARTS), found the ATP synthase subunit β is a novel binding protein of AKG. They discovered AKG inhibits ATP synthase, leads to reduced ATP content, decreased oxygen consumption, and increased autophagy in both C. elegans and mammalian cells, similar to ATP synthase 2 (ATP-2) knockdown.
Target of rapamycin (TOR), belongs to a conserved group of serine/threonine kinases from the phosphatidylinositol kinase-related kinase (PIKK) family, regulates growth and metabolism in all eukaryotic cells. Previous researches have demonstrated that inhibition of TOR activity can delay the aging process, as evidenced by increased life span in yeast, worms, flies, and mice with mutations in TOR pathway components. AKG does not interact with TOR directly and mainly decreases TOR pathway activity through the inhibition of ATP synthase (Fig. 3). AKG longevity partially depends on AMPK and FoxO. The AMP-activated protein kinase (AMPK) is an evolutionarily conserved cellular energy sensor with key roles in aging and lifespan.
Fork head box ‘Other’ (FoxO) proteins, a subgroup of the Fork head transcription factor family, have an pivotal role in mediating the impacts of insulin and growth factors on diverse physiological functions, including cell proliferation, apoptosis and metabolism. Consistent with the implicate of TOR in AKG longevity, the FoxO, a transcription factor PHA-4, which is required to extend lifespan in response to reduced TOR signaling, is likewise essential for AKG-induced longevity. In addition, autophagy, which is activated both by TOR inhibition and by dietary restriction, is significantly increased in worms treated with AKG.
Kidney Disease
People use alpha-ketoglutarate for long-term kidney disease. Taking alpha-ketoglutarate by mouth seems to improve results of certain lab tests used to monitor the effectiveness of hemodialysis in patients receiving this treatment.
Keto-analogues administration plays an important role in clinical chronic kidney disease (CKD) adjunctive therapy, however previous studies on their reno-protective effect mainly focused on kidney pathological changes induced by nephrectomy. KA presented a protective effect on IR induced renal injury and fibrosis by attenuating inflammatory infiltration and apoptosis via inhibition of nuclear factor-kappa B (NF-κB) and mitogen-activated protein kinase (MAPK) pathways. KA supplement protected mice from IR induced renal injury and metabolic disorders.
Research on Renal Protection
In animal study, IR surgery was performed to mimic progressive chronic kidney injury, while KA was administrated orally. For clinical research, a retrospective cohort study was conducted to delineate the usage and effects of KA on attenuating CKD exacerbation. End-point CKD event was defined as 50% reduction of initial estimated glomerular filtration rate (eGFR).
Animal Study Findings
- PAS staining showed a mass of tubular expansions, casts and loss of tubular brush border in IR + Nacl group. With the treatment of KA, tubular expansions were ameliorated and casts were reduced.
- LTL staining showed IR caused more than 2.0-folds decrease of healthy tubules, KA preserved nearly 1.8 folds increased remnant healthy tubules compared to IR + Nacl group.
- The detection of Kim-1 showed a same phenomenon as LTL. An extensive injure renal tubules were founded in IR + Nacl group. In IR + KA group, the number of kim-1 positive tubules decreased.
- Serum BUN and plasma TC and TG were detected by using kits. KA reduced IR induced enhancement of BUN and TG. Otherwise, KA showed a slight decrease in IR induced TC disorder, the concentration of TC in IR + Nacl was 4.74 mmol/L while in IR + KA was 4.19 mmol/L.
- Pathological staining Sirius red (red), myofibroblast marker α-SMA (red) and collagen I (red) staining presented increased interstitial myofibroblast and collagen deposition in IR + Nacl group. KA improved both myofibroblasts and collagen. Renal interstitial fibrosis was improved in IR + KA group.
- The detections of fibrosis related factors: PDGFR-β, fibronectin, collagen I and TGF-β showed KA had an improved effect on IR induced renal fibrosis in protein and/or RNA levels.
- CD45 (green), CD3 (red), Ly6G (red) and DAPI (blue) staining presented a more than 8.0-folds of CD45 positive T cell, 60-folds of CD3 positive T cells and 40 folds of Ly6G positive neutrophils in IR + Nacl group after IR surgery. KA improved mature T cell and neutrophil granulocytes infiltration.
- Western blot analysis and histograms of apoptosis markers: Bcl-2, BAX and caspase3. After IR surgery, the apoptosis related factors BAX and Caspase 3 increased. KA reduced the expressions of BAX and Caspase 3, without improved the anti-apoptotic factor Bcl-2. However, the ratio of Bcl-2/BAX increased in IR + KA group, meaning a anti-apoptotic effect of KA on IR.
- Western blot analysis and histograms of NF-κB pathway: p-p65 raised dramatically in IR + Nacl group compared to the Sham group. Its targeted molecule NFATc-1 increased after IR surgery too. KA inhibited the activation of NF-κB pathway, as well as NFATc-1.
- MAPK pathway (p-p38 and p-ERK). KA presented a similar effect on MAPK pathway as NF-κB pathway. A nearly 2.0-folds increase of p-p38 was detected in IR + Nacl group compared to the Sham group. Half was decreased after the usage of KA. The expression of p-ERK was also reduced in protein level in KA group after IR.
Clinical Research Findings
The probability to reach the end-point was lower in KA group than no-KA group at CKD stages 4 and 5 when adjusted in a log rank test (p = 0.0241). For patients at stage 3 to 5, KA also showed a protective effect on renal function before 30 months, after 30 months, patients in KA group presented a deteriorate renal situation.
Other Potential Uses
There is interest in using alpha-ketoglutarate for a number of other purposes, but there isn't enough reliable information to say whether it might be helpful. These include:
- Aging skin
- Athletic performance
- Liver disease
- Complications after surgery
Absorption and Metabolism
In the cellular metabolism, it is impossible to utilize AKG from the TCA cycle in the synthesis of amino acids, for this to occur, one must provide AKG as a pure dietary supplement. It was demonstrated that AKG was significantly better absorbed from the upper small intestine than from the distal sections. Low pH, Fe2+ and/or SO2−4 ions can enhance AKG absorption. AKG has a short lifetime, is probably dependent on quick metabolism in the enteorcyetes and liver. Over 60% of enteral AKG passes through the intestine in different forms and is not oxidized to the degree of 100% as glutamine and glutamate. In the enterocytes, AKG is converted into proline, leucine and other amino acids.
Moreover, enteric feeding of AKG supplements can significantly increase circulating plasma levels of such hormones as insulin, growth hormone and insulin like growth factor-1 (IGF-1) and all derivatives of AKG (e.g. glutamine or glutamate) are immediately converted to CO2 during their passage across the gut epithelium. In the cellular metabolism, AKG provides an important source of glutamine and glutamate that stimulates protein synthesis, inhibits protein degradation in muscle, and constitutes an important metabolic fuel for cells of the gastrointestinal tract.
Glutamine is an energy source for all types of cells in the organism constituting more than 60% of the total amino acid pool, so AKG as a precursor of glutamine, is a main source of energy for intestinal cells and a preferred substrate for both enterocytes and other rapidly dividing cells. In addition, glutamate, released from nerve fibers in bone tissue, is synthesized by the reductive amination of AKG in peri-vein hepatocytes and can give rise to an increase in proline synthesis, which plays a central role in the synthesis of collagen. In the liver, glutamine serves as a precursor for ureagenesis, gluconeogenesis and acute phase protein synthesis, plays an important role in the inter-organ flow of nitrogen and carbon.
Glutamine has traditionally been considered to be a non-essential amino acid in health, but in catabolic states and stress, it is an essential fuel source for cells of the gastrointestinal tract, rapidly dividing leucocytes and macrophages in the immune system and can be rapidly depleted despite the significant release from muscle tissue. Otherwise, it was also shown that AKG can improve absorption of Fe2+. Thus, AKG and its derivatives can play a role as a Fe2+ absorption enhancer both in rapidly growing animals and humans with Fe2+ insufficiency. Furthermore, AKG, ascorbate and Fe2+ steer hydroxylation of peptide-bound proline to hydroxyproline via prolyl hydrolase, increasing the conversion of pro-collagen to collagen and bone matrix formation.
Side Effects and Precautions
When taken by mouth: Alpha-ketoglutarate is possibly safe when used for up to 3 years.When applied to the skin: Alpha-ketoglutarate is possibly safe when used for up to 8 weeks.
Special Precautions and Warnings
Pregnancy and breast-feeding: There isn't enough reliable information to know if alpha-ketoglutarate is safe to use when pregnant or breast-feeding. Stay on the safe side and avoid use.
Dosage
There isn't enough reliable information to know what an appropriate dose of alpha-ketoglutarate might be. Keep in mind that natural products are not always necessarily safe and dosages can be important. Be sure to follow relevant directions on product labels and consult a healthcare professional before using.