Analysis of Cosmetic Efficacy Peptide Ingredients (Part 5): Carnosine

In the field of efficacy skincare, oxidation and glycation are two fundamental drivers of skin aging. Oxidative damage disrupts cellular structures and collagen fibers, while glycation cross-links collagen, causing it to lose elasticity and leading to dull, sagging skin. Together, they accelerate the appearance of fine lines, sagging, dullness, and roughness. Carnosine is a classic active peptide capable of simultaneously addressing both antioxidant and anti-glycation pathways. As an endogenously synthesized dipeptide, it offers high biocompatibility, multi-target action, and mildness, making it a key ingredient in anti-aging lines from international brands like L'Oréal, Olay, and CHANDO, and a versatile peptide ingredient in anti-aging formulations.

1. Basic Information and Molecular Properties

Name: L-Carnosine

Alias: L-Carnosine, Levocarnosine

CAS No.: 305-84-0

Molecular Formula: C9H14N4O3

Molecular Weight: 226.24

Core Structure: A natural dipeptide formed by the amide bond linkage of β-alanine and L-histidine.

Appearance and Properties: White crystalline solid with excellent water solubility. It is non-irritating to the skin and non-cytotoxic.

Carnosine was first discovered in 1900 by Russian chemist Gulevich in beef muscle extract. It is an endogenous active substance widely present in vertebrates, distributed in human skeletal muscle, brain, heart, and gastrointestinal tissues, with the highest concentration (up to 20 mM) found in skeletal muscle.

Carnosine's multi-faceted bioactivity stems entirely from its precise molecular design, featuring two core active sites: 1) The imidazole ring from the L-histidine residue, with a pKa of 6.72, closely matches the physiological pH of the human body. This ring is the core functional group responsible for scavenging free radicals, capturing reactive carbonyl species, chelating metal ions, and buffering pH, forming the structural basis for its diverse activities. 2) The molecule contains three ionizable groups (carboxyl, β-alanine amino, and imidazole nitrogen), endowing it with excellent water solubility and transdermal penetration ability. The stable amide bond structure allows it to maintain integrity in the skin environment. As a naturally occurring substance, it also exhibits excellent biocompatibility.

2. Synthetic Pathways

2.1 In Vivo Synthesis and Metabolism

In humans, carnosine is synthesized by the enzyme carnosine synthetase (CARNS1), a process requiring magnesium ions and ATP, with the gene encoding this enzyme being ATPGD1 (also known as CARNS1). Since L-histidine is abundant in the body but β-alanine is limited, the synthesis rate of carnosine is primarily restricted by β-alanine availability.

Carnosine degradation is primarily mediated by two enzymes: 1) Serum carnosinase (CN1), found in serum and brain, is highly specific and active, rapidly degrading circulating carnosine within 2-3 hours after a meal. 2) Tissue carnosinase (CN2), present in the liver, kidneys, and spleen, is a non-specific dipeptidase. This rapid degradation is the core reason for carnosine's short circulatory half-life (approximately 5 minutes) in humans and a key stability challenge for its application.

2.2 Industrial Synthesis and Production Methods

Current industrial production of carnosine primarily employs chemical synthesis, while biosynthesis methods are under continuous optimization.

Chemical Synthesis: The mainstream industrial method uses phthalyl-L-carnosine as a key intermediate, producing high-purity L-carnosine via a hydrazinolysis reaction. The core process involves: 1) Hydrazinolysis Reaction: Phthalyl-L-carnosine, water, and hydrazine hydrate are combined in a reactor, heated under reflux, followed by pH adjustment with acetic acid and filtration to remove byproducts, yielding the hydrazinolysis solution. 2) Purification: Dichloromethane and benzaldehyde are added to the solution, stirred at room temperature, and allowed to separate. The aqueous phase is washed with dichloromethane, then treated with activated carbon and ammonia for decolorization. After filtration and vacuum concentration, methanol is added for crystallization. The product is filtered, dried, yielding high-purity L-carnosine free of hydrazine residues. This mature process minimizes byproducts, offers high purification efficiency, and achieves a final purity exceeding 98%, making it the dominant production method for cosmetic-grade carnosine.

Biosynthesis (Emerging/Optimization Direction): This method utilizes genetic engineering to optimize key enzymes like GADL1 for β-alanine synthesis, enhancing its production efficiency. Carnosine is then synthesized in a biological system catalyzed by carnosine synthetase. This approach is environmentally friendly with few byproducts. Early limitations included low enzymatic efficiency and yield, hindering industrial viability. Recent directed evolution of key enzymes has significantly improved yields, positioning this as an important future direction for carnosine production.

3. Mechanisms of Action in Skincare

Carnosine's core value in skincare lies in its synergistic "antioxidant + anti-glycation" dual-pathway anti-aging action, directly targeting the root causes of skin aging, while also offering ancillary benefits like anti-inflammatory repair, photoprotection, and pH buffering.

3.1 Antioxidant Action: Blocking Oxidative Damage

UV radiation, environmental pollution, and metabolic stress lead to excessive accumulation of reactive oxygen species (ROS) in the skin, triggering lipid peroxidation, collagen degradation, and cellular DNA damage—core drivers of skin aging. Carnosine establishes a comprehensive antioxidant defense system through five pathways:

• Direct Free Radical Scavenging: The imidazole ring acts as a hydrogen acceptor, directly quenching various ROS like hydroxyl radicals, superoxide anions, and peroxyl radicals, reducing direct cellular damage.
• Termination of Lipid Peroxidation Chain Reactions: It binds to toxic aldehydes like malondialdehyde (MDA) and 4-hydroxy-2-nonenal (HNE) produced during lipid peroxidation, preventing the spread of peroxidation and protecting cell membrane integrity.
• Transition Metal Ion Chelation: It chelates transition metal ions like iron and copper, inhibiting the Fenton reaction they catalyze, thereby reducing free radical generation at the source and preventing metal ion-induced collagen degradation.
• Neutralization of Inflammatory Oxidants: It rapidly reacts with hypochlorous acid to form less active derivatives, reducing oxidative damage during inflammation and alleviating discomfort like redness and stinging.
• Activation of Endogenous Antioxidant Systems: It downregulates the expression of the pro-oxidant NADPH oxidase and enhances the activity of endogenous antioxidants like superoxide dismutase (SOD) and glutathione peroxidase, strengthening the skin's own antioxidant capacity.

3.2 Anti-Glycation Action: Protecting Collagen Structure

Excess free sugars in the body can undergo non-enzymatic glycation with skin collagen and elastin, forming advanced glycation end products (AGEs). This leads to collagen fiber cross-linking, fragmentation, and loss of elasticity, resulting in skin laxity, fine lines, and dullness/roughness. Carnosine is a classic anti-glycation ingredient in skincare, acting through two core mechanisms: 1) Sacrificial Binding Action, Blocking Glycation at the Source: The amino group and imidazole ring of carnosine act as preferential sites, binding with free sugars and reactive carbonyl species before they can react with structural proteins like collagen and elastin, thus protecting them from damage. 2) Halting Glycation Progression and Reversing Early Damage: Carnosine can bind to mildly glycated proteins, preventing their further cross-linking into AGEs. It also inhibits glycation-mediated collagen degradation signaling, reducing AGE accumulation in the skin, thereby improving glycation-induced laxity and dullness.

3.3 Ancillary Anti-Inflammatory and Repair Effects, Strengthening the Skin Barrier

Beyond its core antioxidant and anti-glycation functions, carnosine can form complexes with zinc ions, inhibiting the NF-κB inflammatory signaling pathway and reducing the release of pro-inflammatory factors like TNF-α, IL-6, and IL-8, thereby calming skin inflammation. It also promotes fibroblast proliferation and accelerates wound healing, making it effective for soothing and repairing skin post-procedures or with compromised barriers.

4. Core Skincare Efficacy and Applications

The efficacy of carnosine in skincare, elucidated through its multi-target mechanisms, is supported by extensive in vitro and human clinical studies. Its core benefits can be summarized in four key areas:

4.1 Dual-Pathway Synergistic Anti-Aging

Carnosine inhibits ROS-mediated degradation of the extracellular matrix via its antioxidant pathway, while preventing abnormal collagen cross-linking via its anti-glycation action. Additionally, it delays the senescence of skin fibroblasts and promotes collagen and elastin synthesis. Human clinical trials demonstrate that anti-aging formulations containing carnosine, used consistently for 8 weeks, significantly improve the appearance of dry lines, fine lines, and nasolabial fold depth, while enhancing skin firmness and elasticity parameters. It is a broad-spectrum anti-aging active suitable for all skin types.

4.2 Anti-Glycation and Skin Brightening

For skin dullness, uneven tone, and post-inflammatory hyperpigmentation caused by high-sugar diets and irregular lifestyles, carnosine effectively inhibits AGE formation, reducing the yellowish tone associated with glycation. Synergized with its free radical scavenging ability, it further improves dullness caused by oxidative stress. Clinical observation data indicate that consistent use for 28 days can significantly enhance skin brightness, radiance, and restore a healthy glow.

4.3 Soothing, Repair, and Barrier Support

Carnosine's anti-inflammatory and antioxidant properties help alleviate inflammatory reactions and oxidative damage induced by UV radiation, chemical exfoliation (e.g., AHAs), or aesthetic procedures, reducing discomfort like redness, stinging, and itching. Simultaneously, it helps consolidate the skin barrier structure and enhance tolerance to external aggressors, making it particularly suitable for anti-aging needs of sensitive or barrier-compromised skin.

4.4 Ancillary Photoprotection

Carnosine effectively scavenges excess free radicals induced by UV radiation, mitigating photoaging-associated cellular DNA damage and collagen degradation. It also helps reduce UV-induced erythema and inflammation. Therefore, carnosine can serve as a supporting active ingredient in daytime sunscreen products, synergistically enhancing the skin's photoprotection.

5. Formulation Application and Compatibility Recommendations

5.1 Dosage and Formulation Compatibility

Cosmetic-grade high-purity carnosine (HPLC purity ≥98%) is typically effective in formulations at concentrations ranging from 0.1% to 2.0%. Studies show that a 0.5% concentration delivers significant antioxidant and anti-glycation benefits. It is compatible with most skincare formulations. Key application scenarios and recommended ranges are:

• Anti-Aging/Anti-Glycation Serums: 0.5% - 2.0%
• Firming/Anti-Wrinkle Creams/Lotions: 0.2% - 1.0%
• Brightening/Mask Treatments: 0.3% - 1.0%
• Daytime Protection/Sunscreen Products: 0.1% - 0.5%
• Post-Procedure Repair Products: 0.2% - 0.8%

5.2 Synergistic Combination Strategies

Carnosine's multi-target action can synergize with various active ingredients for enhanced efficacy. Proven classic combinations include:

• Carnosine + Ergothioneine/Idebenone: For synergistic antioxidant action. Carnosine provides broad-spectrum free radical scavenging, while ergothioneine focuses on mitochondrial protection. Their combination creates a more comprehensive antioxidant defense network, strengthening anti-aging and brightening effects.
• Carnosine + Niacinamide: Carnosine inhibits glycation triggers at the source, while niacinamide effectively blocks melanin transfer to keratinocytes and supports barrier repair. This combination is suitable for addressing dull, sallow tones and fragile skin barriers.
• Carnosine + Collagen-Signal Peptides (e.g., Palmitoyl Pentapeptide-4): Carnosine protects existing collagen from oxidative and glycative damage, while collagen-signal peptides promote new collagen synthesis. This "protect + synthesize" synergistic approach is highly effective against skin laxity and fine lines.
• Carnosine + GHK-Cu (Copper Peptide): Carnosine scavenges free radicals and reduces inflammation, while GHK-Cu focuses on barrier repair and tissue regeneration. This combination is gentle and effective for anti-aging and repair in sensitive skin or post-procedure care.

5.3 Incompatibilities and Precautions

To preserve carnosine's activity and long-term stability in formulations, attention to compatibility and storage conditions is crucial. It should be avoided in the same phase as strong oxidizers like high-concentration hydrogen peroxide to prevent oxidative degradation. The formulation pH should not be consistently below 3. If combining with strong acids like high-concentration AHAs or pure L-ascorbic acid, strategies like separate application times or encapsulation (e.g., microencapsulation) are recommended. Additionally, the use of strong chelating agents (e.g., disodium EDTA) should be controlled to minimize potential interference with carnosine's activity. To address potential challenges in skin penetration and stability, delivery technologies like liposomal encapsulation or nano-encapsulation are recommended to enhance stability and bioavailability.

6. Summary

Carnosine is a rare endogenous peptide in skincare that possesses both potent antioxidant and anti-glycation properties. With its excellent biocompatibility, multi-target anti-aging mechanisms, and mild nature suitable for all skin types, it has become a classic, versatile ingredient in anti-aging formulations. Moving beyond single-target anti-aging logic, it addresses the two fundamental drivers of skin aging—oxidation and glycation. It can serve as a core ingredient in anti-glycation/anti-aging formulations or as a supporting component to synergize with various actives, making it an indispensable core peptide ingredient in contemporary high-performance anti-aging skincare.