Zinc in Cell Culture

Importance and uses of zinc in serum-free eukaryotic, including hybridoma and Chinese Hamster Ovary (CHO) cell cultures

Zinc, a Serum-Free Medium Supplement, Useful In Biomanufacturing, Tissue Engineering and Specialty Media

Zinc is an essential element in cell culture. However, classical media do not typically contain it in their basal formulations. This is because sera used at 5 to 10% provides sufficient zinc for cell growth and survival. Efforts to develop serum-free media necessitate the addition of zinc to the cell culture system. However, this requirement is not necessarily obvious. Certain media such as Iscove's Modified Dulbecco's Medium (IMDM) which is often used as a base for development of serum-free hybridoma media do not contain zinc in their basal formulations. However, these media are often supplemented with insulin. Insulin is a physiological transport molecule for zinc and insulins used to supplement media at mg/L levels contain sufficient zinc to, at least partially, support cells in culture.

The range of zinc supplementation in a select group of classical media varies from 0.7 pM in Williams E media designed for the culture of liver cells to 3 nM in Ham's Nutrient Media, F-12 and its derivatives: F-12 Coon's (F12C) Modification; MCDB media 151 and 302 and Serum-Free/Protein Free Hybridoma Medium. The concentration of zinc in DMEM/Ham's Nutrient Mixture F-12 (50:50), a medium often used as a base for the development of proprietary media for CHO cells, is 1.5 nM. The zinc concentration in F-12 Kaighn's (F12K) Modification; and MCDB media 105, 110, and 153 is 0.5 nM. MCDB 201and Ham's F-10 both contain 100 pM of zinc and MCDB media 131 contains 1 pM of Zinc.

Primary Functions of Zinc in Cell Culture Systems

Zinc is essential for life. It is has been found in over 100 enzymes that are involved in a wide range of life sustaining processes.

• Zinc stabilizes cell membranes by binding to sulfhydryl groups on cell surface proteins.
• Zinc is found in Zn, Cu superoxide dismutase (SOD). Both zinc and copper also have a role in regulating the transcription of the SOD gene. Therefore, zinc helps protect cells from superoxide radical damage.
• Several enzymes have zinc as a coordinating cation: alcohol dehydrogenase, glutamic acid dehydrogenase; carbonic anhydrase, and carboxypeptidase. Several NAD-dependent dehydrogenases are zinc proteins.
• Zinc is part of a family of transcription regulatory proteins know as zinc finger proteins. It plays a role in regulation of RNA synthesis.
• Nucleic acid metabolizing enzymes such as RNA and DNA polymerases, deoxythymidine kinase, and reverse transcriptase require zinc

Chemical Attributes of Zinc that make it a Useful Serum-Free Medium Supplement

Zinc is a transition metal, that unlike iron and copper does not contribute to oxidative stress in vitro. It exists in physiological solutions as an electrophilic divalent cation that binds weakly to low-molecular-weight ligands such as amino acids, peptides, organic acids and phosphates. It binds efficiently to protein disulfide bridges. The intracellular storage and carrier molecules for zinc are members of the metallothionein family of proteins, and in vitro, it is transported on albumin. These proteins are rich in disulfide bridges.

The binding of zinc to albumin is not extremely tight. When cells are present, zinc becomes associated with the sulfhydryl bridges of proteins of the cell membrane. Secondary and tertiary changes in membrane protein conformations favor this. Zinc has been reported to help stabilize cell membranes.

Other molecules present in vitro may affect the binding of zinc to albumin or the cell. EDTA strongly binds zinc, and when present may reduce its availability to cells. EDTA may be inadvertently introduced into the culture medium. This can occur if cells that have been processed with EDTA are not properly washed before being introduced into the culture medium. Reducing agents, such as glutathione, 2-mercaptoethanol, or DTT, when present in the medium, may disrupt protein disulfide bridges causing release of zinc.

Zinc may be lost from the medium due to precipitation. This may only occur when the medium is oxidatively stressed. Oxidative stress creates several anions, such as oxides, peroxides and sulfides that form zinc precipitates.