Absorption, Distribution and Excretion
Although free amino acids dissolved in body fluids constitute only a tiny fraction of the total amino acids in the body, they are essential for maintaining the nutrition and metabolism of proteins. …While plasma is the easiest site for sampling, most amino acids are found in higher concentrations in the intracellular pools of tissue cells. Typically, large neutral amino acids like leucine and phenylalanine are in near-equilibrium with their plasma concentrations. However, some other amino acids, particularly glutamine, glutamate, and glycine, are 10 to 50 times more concentrated in the intracellular pools than in plasma. Dietary changes or pathological conditions can lead to significant variations in the concentrations of various free amino acids in plasma and tissue pools. /Amino Acids/ After ingestion, proteins are denatured by gastric acid and cleaved into smaller peptides by pepsin. Pepsin activity increases with the increase in gastric acid after eating. Proteins and peptides then enter the small intestine, where peptide bonds are hydrolyzed by various enzymes. These specific enzymes originate from the pancreas and include trypsin, chymotrypsin, elastase, and carboxypeptidase. The resulting mixture of free amino acids and small peptides is then transported to mucosal cells via various carrier systems. These carrier systems target specific amino acids, dipeptides, and tripeptides, with each system targeting only a limited range of peptide substrates. After the absorbed peptides are hydrolyzed intracellularly, the free amino acids are secreted into the portal vein bloodstream via other specific carrier systems within the mucosal cells, or further metabolized intracellularly. The absorbed amino acids enter the liver, where some are absorbed and utilized; the remainder enters the systemic circulation and is utilized by peripheral tissues. /Amino Acids/
Healthy adults ingest 70 to 100 grams of protein daily and excrete approximately 11 to 15 grams of nitrogen in their urine, primarily as urea, with smaller amounts excreted as ammonia, uric acid, creatinine, and some free amino acids. These are the final products of protein metabolism, with urea and ammonia derived from the partial oxidation of amino acids. Uric acid and creatinine also indirectly originate from amino acids. The removal of nitrogen from amino acids and its conversion into a form excreted by the kidneys can be viewed as a two-step process. The first step is typically accomplished through one of two enzymatic reactions: transamination or deamination. Transamination is a reversible reaction that utilizes ketoacid intermediates from glucose metabolism (such as pyruvate, oxaloacetate, and α-ketoglutarate) as acceptors of amino nitrogen. Most amino acids participate in these reactions, resulting in the transfer of amino nitrogen to three amino acids: pyruvate to alanine, oxaloacetate to aspartic acid, and α-ketoglutarate to glutamate. Unlike many amino acids, transamination of branched-chain amino acids occurs systemically, particularly in skeletal muscle. In skeletal muscle, the primary acceptors of amino nitrogen are alanine and glutamine (derived from pyruvate and glutamate, respectively), which subsequently enter the bloodstream. These amino acids act as important carriers, transporting nitrogen from the periphery (skeletal muscle) to the intestines and liver. In the small intestine, glutamine is extracted and metabolized into ammonia, alanine, and citrulline, which are then transported to the liver via the portal vein circulation. Nitrogen in amino acids can also be removed through deamination reactions to generate ammonia. Many amino acids can undergo deamination reactions, including direct deamination (histidine), dehydration deamination (serine, threonine), deamination via the purine nucleotide cycle (aspartic acid), and oxidative deamination (glutamate). ...Glutamate is also a product of specific degradation pathways of arginine and lysine. Therefore, nitrogen in any amino acid can be converted into two precursors for urea synthesis—ammonia and aspartic acid. Although the digestibility of dietary protein (i.e., the efficiency of amino acid removal from the small intestinal lumen) appears high, there is ample evidence that visceral tissues (including intestinal mucosal cells) also metabolize large amounts of nutritionally valuable essential amino acids. Therefore, less than 100% of the amino acids cleared from the intestinal lumen enter the peripheral circulation, and the amount of amino acids metabolized in the visceral circulation varies depending on the amino acid, with the intestine being particularly active in the metabolism of threonine. For more complete data on the absorption, distribution, and excretion of L-threonine (one of 12 amino acids), please visit the HSDB record page.

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Metabolism/Metabolites
Liver
Evidence suggests that excess threonine is converted into carbohydrates, liver lipids, and carbon dioxide. L-Threonine is an essential large neutral amino acid. L-Threonine does not participate in transamination reactions. Once the deamination products of amino acids enter the tricarboxylic acid cycle (TCA cycle, also known as the citric acid cycle or Krebs cycle) or glycolysis, their carbon skeletons become available for biosynthetic pathways, particularly the synthesis of glucose and fats. Whether the carbon skeleton of an amino acid ultimately produces glucose or fat depends on its entry into these pathways. If it enters as acetyl-CoA, it can only produce fat or ketone bodies. However, the carbon skeletons of other amino acids can enter these pathways in some way, making their carbon atoms available for gluconeogenesis. This is the basis for the classical nutritional classification of amino acids into ketogenic and glucogenic amino acids (i.e., those capable of producing ketone bodies [or fats] or glucose). Some amino acids degrade to produce both products and are therefore considered both ketogenic and glucogenic amino acids. The threonine dehydrogenase (TDG) pathway is an important pathway for threonine degradation, producing glycine in experimental animals, but its precise quantification in humans has not yet been established. Therefore, this study investigated the effects of dietary threonine (in the form of free amino acids (+Thr) or protein components (+P-Thr)) on the catabolism of threonine to CO₂ and glycine in six healthy adult men using a 4-hour continuous infusion of L-[1-(13)C]threonine and [(15)N]glycine. The [(13)C]glycine produced from labeled threonine was determined using gas chromatography-combustion isotope ratio mass spectrometry. Compared with the control group, the +Thr and +P-Thr diet groups had significantly higher threonine intakes (126, 126, and 50 μmol·kg⁻¹·h⁻¹, respectively, SD 8, P < 0.0001). The amount of threonine oxidized to CO₂ in the +Thr and +P-Thr groups was three times that of the control group (49, 45, and 15 μmol·kg⁻¹·h⁻¹, respectively, SD 6, P < 0.0001). The amount of threonine converted to glycine in the +Thr and +P-Thr groups also tended to be higher than in the control group (3.5, 3.4, and 1.6 μmol·kg⁻¹·h⁻¹, respectively, SD 1.3, P = 0.06). The TDG pathway accounts for only 7-11% of total threonine catabolism and is therefore a minor pathway in adults.
For more complete metabolite/metabolite data on L-threonine (8 metabolites in total), please visit the HSDB record page.
Liver

Toxicity Summary
L-Threonine is a precursor to the amino acids glycine and serine. As a lipophile, it can control fat accumulation in the liver. It may help combat mental illness and may be very effective for indigestion and intestinal dysfunction. Furthermore, threonine can prevent excessive liver fat. Nutrients are more easily absorbed when threonine is present in the body. Interactions Adding methionine-threonine to a low-casein (8.5%) diet (8.5CMT) can alleviate symptoms such as proteinuria in nephrotic rats without causing severe protein malnutrition. …This study…investigated whether adding L-arginine to an 8.5CMT diet exacerbated proteinuria and other symptoms in nephrotic rats. Glomerulonephritis was induced in male Wistar rats by a single intravenous injection of nephrotoxic serum, followed by feeding them a 20% casein diet (control group), an 8.5% casein diet, an 8.5CMT diet, or an 8.5CMT diet supplemented with L-arginine (8.5CMTA) for 16 days. Compared to the 8.5 CMT diet, the 8.5 CMT diet exacerbated proteinuria and glomerulonephritis. Administration of LN(G)-nitroarginine methyl ester (a nitric oxide synthase inhibitor) to nephritis rats fed the 8.5 CMT diet via drinking water for 14 days eliminated the adverse effects of L-arginine on proteinuria and glomerular histopathological damage. These results indicate that L-arginine supplementation exacerbates glomerulonephritis by promoting nitric oxide production. Rats fed a low-protein diet and orally administered 2-(2-nitro-4-trifluoromethylbenzoyl)cyclohexane-1,3-dione (NTBC) at a dose of 30 μmol/kg/day (i.e., 10 mg/kg/day), or rats fed a low-protein diet containing 5 ppm NTBC, developed corneal lesions within 3–8 days post-exposure, with an incidence of approximately 80%. This treatment also significantly inhibited 4-hydroxyphenylpyruvate dioxygenase (HPPD) activity in the liver and kidneys, induced tyrosine aminotransferase activity in the liver but not the kidneys, and resulted in marked tyrosinemia in plasma and aqueous humor. The degree of tyrosinemia and changes in tyrosine catabolic enzyme activity were similar to those reported in rats fed a normal protein diet and orally administered NTBC at a dose of 30 μmol/kg/day. However, corneal lesions appeared earlier in rats fed a low-protein diet. Adding 1% (w/w) threonine to a low-protein diet alleviated ocular adverse effects of NTBC. The protective effect provided by dietary threonine addition was not due to improved inhibition of hepatic HPPD activity or a decrease in the degree of tyrosinemia measured 8 days after treatment. In rats fed a low-protein diet supplemented with 5% (w/w) L-tyrosine, corneal damage developed rapidly, accompanied by marked tyrosinemia, increased hepatic tyrosine aminotransferase activity, and approximately 50% reduction in hepatic HPPD activity. Adding 1% (w/w) threonine to a low-protein diet can delay, but not completely prevent, corneal damage induced by a high-tyrosine diet. The mechanism by which dietary threonine supplementation alleviates NTBC-induced corneal damage remains unclear. However, our results suggest that protein deficiency limits the rats' responsiveness to HPPD-induced tyrosine overload.

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Non-human toxicity values
Rat intraperitoneal injection LD50: 3098 mg/kg

[1]. L-cysteine, glycine and dl-threonine in the treatment of hypostatic leg ulceration: a placebo-controlled study. Pharmatherapeutica, 01 Jan 1985, 4(4):227-230.

Therapeutic Uses
L-Threonine has been used clinically to increase glycine concentration in the cerebrospinal fluid of patients with spasticity. No adverse clinical responses were observed in these patients when administered 4.5 to 6.0 grams daily for 14 days. /Experimental Treatment/ To determine whether the naturally occurring amino acid threonine (a potential precursor for glycine biosynthesis in the spinal cord) affects spasticity in multiple sclerosis, we conducted a randomized crossover trial involving 26 independently walking patients. A total daily dose of 7.5 grams of threonine reduced signs of spasticity on clinical examination, but no symptom improvement was observed in either the examining physician or the patient. Unlike the sedative and motor dysfunction side effects of antispasmodic drugs commonly used to treat multiple sclerosis, no side effects or toxicities were observed with threonine… /Experimental Therapy/…According to a double-blind crossover trial protocol, 18 patients with familial spastic paraplegia (FSP) were given 4.5 grams and 6.0 grams of L-threonine daily, respectively. …Although the efficacy of L-threonine was not clinically significant, it significantly suppressed cramping symptoms. /Experimental Therapy/ A randomized, double-blind, placebo-controlled trial enrolled 22 patients with edematous leg ulcers. Patients received topical treatment with either a cream containing the amino acids L-cysteine, glycine, and DL-threonine, or a cream base only (placebo). Most patients received dressing changes three times a week for 12 weeks. …The group receiving the amino acid combination therapy showed significantly better wound healing and pain reduction than the control group. This study suggests that the combination of L-cysteine, glycine, and DL-threonine is valuable in promoting the healing of thrombotic leg ulcers.

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Drug Warning
…In this placebo-controlled crossover study, researchers investigated the effect of oral threonine supplementation (THR) on plasma amino acid concentrations in 12 patients with hyperphenylalaninemia. Prior to the first treatment period of this crossover study, patients were randomly assigned to one of two groups to receive either approximately 50 mg/kg/day of THR or an equivalent amount of maltodextrin (as a placebo). After an 8-week feeding period and an 8-week washout period, the supplementation was crossovered, and the study continued for another 8 weeks. Blood samples were collected at the beginning and end of each supplementation period. Daily supplementation with approximately 50 mg/kg of dietary THR significantly reduced plasma phenylalanine (PHE) levels (P = 0.0234). A significant positive correlation was found between plasma and urinary PHE concentrations (P < 0.001), indicating that the reduction in plasma PHE levels in patients receiving THR supplementation was not due to increased urinary PHE excretion. Conclusion: The data from this study suggest that oral THR supplementation has a significant effect on reducing plasma PHE levels, but no conclusions can be drawn regarding its mechanism of action. Although the use of threonine supplementation in patients with hyperphenylalaninemia appears attractive based on the available data, the observed mechanism of efficacy should be elucidated before introducing such patients into this treatment. A two-center, double-blind, placebo-controlled treatment trial in patients with amyotrophic lateral sclerosis (ALS) lasted six months, involving oral branched-chain amino acids (BCAAs) (12 g L-leucine, 8 g L-isoleucine, and 6.4 g L-valine daily) or L-threonine (4 g daily) in combination with pyridoxal phosphate (160 mg daily). …The amino acids were well tolerated. Our results did not show any benefit of six months of BCAA or L-threonine treatment on the progression of ALS. The higher rate of decline in lung function in patients receiving BCAAs or L-threonine may be due to chance, but adverse reactions of these amino acids cannot be ruled out. Currently, most commercially available infant formula contains approximately 20% more threonine than breast milk. Due to the higher threonine content, premature infants consuming these formulas have nearly twice the plasma threonine concentration compared to breastfed infants. Elevated plasma threonine levels can lead to elevated glycine levels in the brain, thus affecting the balance of neurotransmitters in the brain. This may impact early brain development after birth. Therefore, excessive threonine intake should be avoided during infant feeding.

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Pharmacodynamics
L-Threonine is an essential amino acid that helps maintain proper protein balance in the body. It is important for the formation of collagen, elastin, and tooth enamel, and when combined with aspartic acid and methionine, it aids in liver and fat metabolism.

C₄H₉NO₃

119.12

119.058

80-68-2

82822-12-6

6288

White to light yellow solid powder

1.3±0.1 g/cm3

345.8±32.0 °C at 760 mmHg

244 °C (dec.)(lit.)

162.9±25.1 °C

0.0±1.7 mmHg at 25°C

1.507

-1.23

3

4

2

8

93.3

2

C[C@H]([C@@H](C(=O)O)N)O

AYFVYJQAPQTCCC-GBXIJSLDSA-N

InChI=1S/C4H9NO3/c1-2(6)3(5)4(7)8/h2-3,6H,5H2,1H3,(H,7,8)/t2-,3+/m1/s1

(2S,3R)-2-amino-3-hydroxybutanoic acid

DLThreonine; DL Threonine

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

H2O : ~100 mg/mL (~839.49 mM)
DMSO :< 1 mg/mL

Note: Listed below are some common formulations that may be used to formulate products with low water solubility (e.g. < 1 mg/mL), you may test these formulations using a minute amount of products to avoid loss of samples.

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Injection Formulation 1: DMSO : Tween 80： Saline = 10 : 5 : 85 (i.e. 100 μL DMSO stock solution → 50 μL Tween 80 → 850 μL Saline)
*Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH ₂ O to obtain a clear solution.
Injection Formulation 2: DMSO : PEG300 ：Tween 80 : Saline = 10 : 40 : 5 : 45 (i.e. 100 μL DMSO → 400 μLPEG300 → 50 μL Tween 80 → 450 μL Saline)
Injection Formulation 3: DMSO : Corn oil = 10 : 90 (i.e. 100 μL DMSO → 900 μL Corn oil)
Example: Take the Injection Formulation 3 (DMSO : Corn oil = 10 : 90) as an example, if 1 mL of 2.5 mg/mL working solution is to be prepared, you can take 100 μL 25 mg/mL DMSO stock solution and add to 900 μL corn oil, mix well to obtain a clear or suspension solution (2.5 mg/mL, ready for use in animals).

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Injection Formulation 4: DMSO : 20% SBE-β-CD in saline = 10 : 90 [i.e. 100 μL DMSO → 900 μL (20% SBE-β-CD in saline)]
*Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.
Injection Formulation 5: 2-Hydroxypropyl-β-cyclodextrin : Saline = 50 : 50 (i.e. 500 μL 2-Hydroxypropyl-β-cyclodextrin → 500 μL Saline)
Injection Formulation 6: DMSO : PEG300 : castor oil : Saline = 5 : 10 : 20 : 65 (i.e. 50 μL DMSO → 100 μLPEG300 → 200 μL castor oil → 650 μL Saline)
Injection Formulation 7: Ethanol : Cremophor : Saline = 10： 10 : 80 (i.e. 100 μL Ethanol → 100 μL Cremophor → 800 μL Saline)
Injection Formulation 8: Dissolve in Cremophor/Ethanol (50 : 50), then diluted by Saline
Injection Formulation 9: EtOH : Corn oil = 10 : 90 (i.e. 100 μL EtOH → 900 μL Corn oil)
Injection Formulation 10: EtOH : PEG300：Tween 80 : Saline = 10 : 40 : 5 : 45 (i.e. 100 μL EtOH → 400 μLPEG300 → 50 μL Tween 80 → 450 μL Saline)

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Oral Formulation 1: Suspend in 0.5% CMC Na (carboxymethylcellulose sodium)
Oral Formulation 2: Suspend in 0.5% Carboxymethyl cellulose
Example: Take the Oral Formulation 1 (Suspend in 0.5% CMC Na) as an example, if 100 mL of 2.5 mg/mL working solution is to be prepared, you can first prepare 0.5% CMC Na solution by measuring 0.5 g CMC Na and dissolve it in 100 mL ddH2O to obtain a clear solution; then add 250 mg of the product to 100 mL 0.5% CMC Na solution, to make the suspension solution (2.5 mg/mL, ready for use in animals).

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Oral Formulation 3: Dissolved in PEG400
Oral Formulation 4: Suspend in 0.2% Carboxymethyl cellulose
Oral Formulation 5: Dissolve in 0.25% Tween 80 and 0.5% Carboxymethyl cellulose
Oral Formulation 6: Mixing with food powders

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Note: Please be aware that the above formulations are for reference only. InvivoChem strongly recommends customers to read literature methods/protocols carefully before determining which formulation you should use for in vivo studies, as different compounds have different solubility properties and have to be formulated differently.

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 (Please use freshly prepared in vivo formulations for optimal results.)