Insight

Vitamins are a class of trace organic compounds essential for humans and animals to maintain normal physiological functions. Despite being required in minute quantities, deficiency of essential vitamins will trigger corresponding deficiency disorders. At the cellular level, vitamins act as coenzymes, biological antioxidants and even hormones. Therefore, vitamins are indispensable components in chemically defined media for bioproduction, and their stability profoundly impacts cell culture and production performance.

Inside cells, riboflavin is converted into flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). Riboflavin itself, along with FMN and FAD, serves as cofactors for intracellular redox reactions.

Riboflavin exhibits high stability against temperature, oxygen and acidic conditions, yet it is prone to degradation under alkaline environments. In oxygen-containing systems, riboflavin is markedly photosensitive. Upon exposure to ultraviolet or visible light, it is excited into a transient singlet excited state and subsequently degrades into a series of compounds dominated by formylmethylflavin and lumichrome. Generally, other medium components impose negligible effects on riboflavin. However, its photodegradation and oxidation processes compromise the stability of methionine, cysteine, tryptophan and tyrosine.

To prevent photodegradation of riboflavin, culture media shall be stored away from light. Complexing agents and encapsulants can also be supplemented to inhibit degradation. For instance, cyclodextrins stabilize riboflavin via molecular encapsulation.

As a coenzyme, folic acid participates in numerous metabolic reactions. Acting as a carrier of one-carbon units such as methyl and formyl groups, it plays vital roles in amino acid metabolism, purine and pyrimidine biosynthesis, and the formation of S-adenosylmethionine (SAM).

Under physiological conditions, folic acid is inactivated by heat, acid, light and reducing agents. Notably, folic acid in aqueous oxygen-containing solutions is highly susceptible to light-induced degradation, whereas it remains photostable under anoxic conditions.

Since photodegradation is the primary degradation pathway of folic acid, light exclusion is the most straightforward mitigation measure. Considering the promotional effect of oxygen on degradation, antioxidants including phenolic compounds (butylated hydroxyanisole, nordihydroguaiaretic acid, ethyl hydroacetate) and ascorbates can be added to stabilize folic acid solutions.

Cyanocobalamin functions as a key coenzyme in propionate metabolism, amino acid metabolism and one-carbon metabolism. Similar to most vitamins, it interconverts among multiple active forms within cells. Among cobalamin derivatives, cyanocobalamin possesses the highest chemical stability.

Cobalamins with carbon-linked β-ligands (e.g., methyl and cyano groups) undergo rapid photolysis, which cleaves the β-ligand and reduces trivalent cobalt to divalent cobalt to form B₁₂r. In aerobic environments, the cobalt center is re-oxidized, and the β-ligand is substituted by a hydroxyl group to generate hydroxocobalamin.

Cyanocobalamin is chemically incompatible with ascorbic acid. Ascorbic acid mediates single-electron reduction of cyanocobalamin to produce B₁₂r, which is further oxidized into hydroxocobalamin. Studies demonstrate that the degradation rate of hydroxocobalamin induced by ascorbic acid is approximately 10 times that of cyanocobalamin. Other reducing agents such as sodium bisulfite, reducing sugars and ferrous salts exert similar destructive effects on cyanocobalamin. Nicotinamide and riboflavin slightly accelerate the photolysis of other cobalamins into hydroxocobalamin, and riboflavin further promotes the photodegradation of hydroxocobalamin.

Current stabilization strategies for cyanocobalamin mainly focus on suppressing the formation of hydroxocobalamin triggered by oxidants and light exposure. Ferric chloride, ferric oxide saccharate, phosphate buffers and potassium ferrocyanide are well-recognized additives for stabilizing cyanocobalamin.

Thiamine acts as a coenzyme in various decarboxylation reactions, including the conversion of pyruvate to acetyl-CoA and α-ketoglutarate to succinyl-CoA during energy metabolism, as well as pentose formation in glucose metabolism.

Thiamine is sensitive to high temperature, oxidation, reduction and light, and its degradation mechanism and rate are highly pH-dependent. Its stability is also affected by other medium components: sulfite accelerates its decomposition into pyrimidine methanesulfonic acid and thiazole; riboflavin promotes its oxidation into thiamine disulfide; thiol oxidants facilitate thiamine degradation, while thiol reductants exert an inhibitory effect.

Small molecular additives are commonly used to enhance thiamine stability. Free thiols are the most widely applied, which stabilize thiamine by scavenging disulfides and eliminating dissolved oxygen as antioxidants. In addition, organic compounds such as monosodium glutamate, glycine and serine chelate trace metals that trigger degradation, thereby protecting thiamine.

Vitamin B₆ comprises six interconvertible compounds: pyridoxine, pyridoxal, pyridoxamine and their respective 5′-phosphate derivatives. At least one form of vitamin B₆ is typically incorporated into culture media. It participates in fatty acid metabolism, folate metabolism, coenzyme Q biosynthesis, gluconeogenesis, heme biosynthesis, as well as amino acid racemization, transamination and elimination reactions.

Pyridoxine hydrochloride is the predominant form of vitamin B₆ used in culture media, owing to its superior stability, low chemical reactivity and high solubility compared with other B₆ variants. All forms of vitamin B₆ are photosensitive, with 4-pyridinecarboxylic acid identified as the major degradation product. Pyridoxal readily reacts with primary amines to form Schiff bases, which in turn catalyze amine degradation while pyridoxal itself is converted into pyridoxal or pyridoxamine. Heat, trivalent iron ions, copper ions, aluminum ions and phosphate ions also facilitate the reversible formation of Schiff bases.

The primary stabilization approach is to adopt stable pyridoxine instead of reactive pyridoxal. Pyridoxine also exhibits antioxidant properties and helps stabilize other critical medium components. Since metal ions exacerbate the reactivity of vitamin B₆, appropriate metal chelation serves as an effective measure to inhibit the degradation of vitamin B₆ and other medium ingredients.

Biotin (Vitamin B₇) is essential for bicarbonate-dependent carboxylation reactions, and thus plays a core role in metabolic pathways including the tricarboxylic acid cycle, fatty acid synthesis and catabolism of branched-chain amino acids. Pantothenic acid is a constituent of 4′-phosphopantetheine and coenzyme A. These compounds act as acyl carriers and carbonyl activators in numerous metabolic processes by forming thioester bonds between terminal thiol groups and carboxyl groups of metabolic substrates to activate carboxyl groups for subsequent reactions.

Nicotinamide and nicotinic acid belong to the vitamin B complex. They are converted into nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) in vivo. The NAD/NADH redox couple is a core component of numerous reductases and dehydrogenases, supporting a wide range of enzymatic reactions including energy metabolism pathways (glycolysis, electron transport chain and tricarboxylic acid cycle) and anabolic reactions for fatty acid and steroid synthesis.

Biotin and nicotinamide possess excellent chemical stability and rarely degrade under routine medium storage conditions. Pantothenic acid is chemically labile, so it is conventionally supplemented as calcium pantothenate in culture media. No obvious adverse interactions exist between these three compounds and other medium components.

Collectively, the key factors triggering vitamin degradation in culture media include pH variation, oxidation, light exposure, high temperature and chemical reactions with specific medium ingredients. B-group vitamins are relatively stable in solid powder form. When dissolved in solution, storage under neutral pH, dark environment and 2–8 °C can substantially mitigate degradation. These standard conditions suppress acid/alkali-catalyzed reactions, photooxidation, UV-induced decomposition and thermal degradation, and have been widely adopted in laboratory operations.

Nevertheless, large-scale liquid medium preparation in commercial production cannot always guarantee complete light exclusion and refrigerated storage. Accordingly, targeted strategies are required to improve medium stability: selecting chemically robust vitamin variants (e.g., replacing pyridoxal with pyridoxine and using calcium pantothenate); and restraining adverse reactions between interactive components. Notably, efficient metal chelation remarkably reduces the degradation of multiple B-group vitamins catalyzed by transition metals.

For oxidation-prone vitamins such as folic acid and pyridoxine, novel antioxidant molecules like thiazolidines effectively block oxidative degradation pathways. Molecular encapsulation is a reliable strategy for highly unstable vitamins, which shields vitamins from reactive medium environments over a certain period. Cyclodextrins, which encapsulate vitamins (e.g., riboflavin) within their hydrophobic cavities, and nanoemulsions have been proven effective for this purpose.

Although targeted stabilization methods have been developed for individual vitamins, maintaining the overall stability of vitamins in chemically defined media remains challenging. Since various B-group vitamins coexist in the same medium, stabilization measures for one vitamin may negatively affect another. Therefore, medium development requires optimizing a comprehensive set of conditions to minimize overall vitamin degradation. Current research on vitamin stability lays a foundation for improving batch-to-batch consistency of chemically defined media and enhancing bioproduction performance in the future.