Masses calculated from the elemental formulas. pKa values are published measurements; β-alanine is not a standard residue, so the site calculators cannot take these sequences directly.
A Moscow discovery
Carnosine was discovered in 1900 by the Russian biochemist Vladimir Gulevich (Gulewitsch) and his colleague S. Amiradžibi, who isolated it from meat extract. The name comes from the Latin carnis, "of flesh". It was one of the first biologically active peptides to be identified, and at the time one of the most abundant small nitrogen compounds known in muscle. Anserine, its methylated relative, was found in goose muscle in 1929 and named after the goose genus Anser.
Structure
Carnosine is β-alanyl-L-histidine. Its first residue is β-alanine, in which the amino group sits on the β-carbon rather than the α-carbon. β-Alanine is not one of the 20 proteinogenic amino acids, and carnosine is made by an enzyme, carnosine synthase, rather than by the ribosome. It therefore cannot be entered into the MW calculator, whose alphabet covers only the standard residues. The values in the fact sheet were calculated from the elemental formula.
The related dipeptides differ only in the histidine ring or in the first residue:
| Dipeptide | Composition | Formula | Monoisotopic (Da) | Where found |
|---|---|---|---|---|
| Carnosine | β-Ala–His | C₉H₁₄N₄O₃ | 226.1066 | Skeletal muscle and brain of most vertebrates |
| Anserine | β-Ala–3-methyl-His | C₁₀H₁₆N₄O₃ | 240.1222 | Muscle of birds, fish and many mammals |
| Balenine (ophidine) | β-Ala–1-methyl-His | C₁₀H₁₆N₄O₃ | 240.1222 | Muscle of whales and snakes |
| Homocarnosine | GABA–His | C₁₀H₁₆N₄O₃ | 240.1222 | Brain |
The last three are exact isomers with identical formulas and masses. Mass spectrometry alone cannot tell them apart; chromatographic separation or characteristic fragment ions are needed. This is the same limitation that makes Leu and Ile indistinguishable, discussed in the guide to peptide mass spectrometry.
A buffer tuned to muscle
Carnosine has three ionisable groups. Measured pKa values are about 2.6 for the carboxyl group, 6.8 for the imidazole ring of histidine, and 9.5 for the β-amino group. The imidazole value is the important one. In free histidine it is near 6.0, but in carnosine the neighbouring groups raise it to about 6.8, which is close to the pH of resting muscle (about 7.0–7.1).
A buffer works best at pH values close to its pKa, so carnosine is well placed to absorb protons in exactly the range that muscle cells pass through during intense exercise, when lactate and protons accumulate and intracellular pH can fall below 6.5. Estimates of carnosine's contribution to the buffering capacity of human skeletal muscle vary widely between studies and methods, from under 10% to around 20%, with larger shares in fast-twitch fibres, which are used in sprinting and rely heavily on anaerobic glycolysis.
Using the Henderson–Hasselbalch reasoning described in the isoelectric point guide, carnosine's net charge is close to zero around pH 8, its isoelectric point being roughly halfway between the imidazole and amino pKa values, at about 8.2. At physiological pH the molecule is a zwitterion with a partially protonated imidazole.
Distribution and metabolism
Carnosine is found at high concentrations in skeletal muscle, at lower concentrations in the heart and brain, and in the olfactory bulb. Levels differ markedly between species. Animals that sprint or dive, such as racehorses, greyhounds and whales, have particularly high muscle concentrations of carnosine or its methylated analogues. In humans, concentrations are higher in fast-twitch than in slow-twitch fibres, and on average higher in men than in women.
Muscle cells make carnosine from β-alanine and histidine, and the availability of β-alanine usually limits synthesis. β-Alanine comes partly from the diet, mainly from carnosine and anserine in meat and fish, and partly from breakdown of uracil in the liver. Studies have reported lower muscle carnosine in people on long-term vegetarian diets, consistent with this.
Humans differ from most other mammals in having a very active carnosine-degrading enzyme, serum carnosinase (CN1), in the blood. Dietary carnosine is therefore rapidly split into β-alanine and histidine after absorption, and intact carnosine barely reaches muscle. Muscle carnosine is rebuilt inside the cells.
Other proposed functions
Beyond buffering, a range of other roles has been proposed, with different strengths of evidence:
- Metal ion chelation. Carnosine binds copper and zinc through its imidazole, amino and carboxyl groups. The zinc complex polaprezinc is used as a gastric mucosal protectant in Japan and some other countries.
- Scavenging reactive carbonyls. Carnosine reacts with reactive aldehydes formed during lipid oxidation and sugar metabolism, such as 4-hydroxynonenal, and forms adducts that can be detected in muscle and urine. This "carbonyl quenching" is a plausible protective mechanism supported by laboratory studies.
- Antioxidant activity. Carnosine scavenges some reactive oxygen species in vitro. How much this contributes in living tissue, alongside glutathione and enzymatic systems described in the glutathione profile, is less clear.
Many further claims, including anti-ageing effects, appear in popular sources. Most rest on laboratory or animal studies and have not been established in humans.
β-Alanine and muscle carnosine
Because β-alanine is the limiting precursor, it is also the obvious lever for changing muscle carnosine. Controlled studies since the mid-2000s have shown that sustained dietary supplementation with β-alanine raises muscle carnosine content substantially over several weeks, and systematic reviews report small performance benefits in high-intensity exercise lasting roughly one to four minutes, the range in which muscle acidosis limits performance. A harmless tingling sensation of the skin, paraesthesia, is a commonly reported side effect.
This is one of the better-documented examples in sports science of a dietary intervention with a clear biochemical mechanism. It is described here for its biochemistry; decisions about personal use are a matter for a qualified professional.
Carnosine and anserine in food
Meat and fish are the main dietary sources. Beef and pork are rich in carnosine; chicken breast, turkey, tuna and salmon contain large amounts of anserine. Plant foods contain essentially none. The dipeptides contribute to the characteristic taste of meat extracts and broths, and their measurement has been used to verify the species origin of meat, since the ratio of carnosine to anserine differs between species.
Analysis
Carnosine and its analogues are small, very polar and positively charged at acidic pH, so they are poorly retained on standard reversed-phase columns. They are usually analysed by HILIC or ion-exchange chromatography, or by reversed-phase HPLC after derivatisation, often coupled with mass spectrometry. The difficulties of retaining small polar peptides are covered in the guide to peptide HPLC. Muscle content is traditionally reported per kilogram of dry muscle, which makes values from different studies comparable regardless of water content.
Frequently asked questions
Is carnosine the same as carnitine?
No. The names are similar because both were first found in meat, but carnitine is not a peptide: it is a small quaternary ammonium compound involved in transporting fatty acids into mitochondria.
Why is β-alanine used instead of ordinary alanine?
The enzyme that makes carnosine specifically uses β-alanine. The β-linkage also makes the peptide resistant to most ordinary peptidases, much as the γ-linkage protects glutathione.
Why does histidine have a higher pKa in carnosine than on its own?
Neighbouring charged groups influence each other's ionisation. In carnosine the carboxylate is negatively charged at physiological pH and stabilises the protonated, positively charged imidazole, which raises its pKa.
Can I calculate carnosine's mass with the MW calculator?
Not directly, because β-alanine is not a standard amino acid. Entering AH (L-alanyl-histidine) gives the same formula and mass, since α- and β-alanine are isomers, but the pKa values and therefore the calculated charge will differ.
References
- Boldyrev AA, Aldini G, Derave W (2013) Physiology and pathophysiology of carnosine. Physiological Reviews 93:1803–1845.
- Harris RC, Tallon MJ, Dunnett M, et al. (2006) The absorption of orally supplied β-alanine and its effect on muscle carnosine synthesis in human vastus lateralis. Amino Acids 30:279–289.
- Saunders B, Elliott-Sale K, Artioli GG, et al. (2017) β-Alanine supplementation to improve exercise capacity and performance: a systematic review and meta-analysis. British Journal of Sports Medicine 51:658–669.
- Gulewitsch W, Amiradžibi S (1900) Über das Carnosin, eine neue organische Base des Fleischextraktes. Berichte der deutschen chemischen Gesellschaft 33:1902–1903.