Tumor necrosis factor (TNF) is a cytokine which produced naturally by macrophages in bacterial infection or other immune response. It is a small molecule protein that could directly cause tumor cell death or cooperate with interferon. TNF can be divided into two types according to their origin and structure, named TNF-α and TNF-β respectively. TNF-α was produced by macrophages accompanied with LPS′s stronger stimulant mainly, and might also secreted by T cells or NK cells in some stimulating factor (eg. PMA). While TNF-β was mainly produced by activated T cells, and with the stimulation of antigen and mitogen could produce high levels.
The human TNF gene (TNF-α) was cloned in 1985. It maps to chromosome 6p21.3, spans about 3 kb and contains 4 exons. The last exon codes for more than 80% of the secreted protein. The 3' UTR of TNF-α contains an AU-rich element (ARE). TNF is primarily produced as a 212-amino acid-long typeⅡtrans-membrane protein arranged in stable homotrimers. From this membrane-integrated form the soluble homotrimeric cytokine (sTNF) is released via proteolytic cleavage by the metalloprotease TNF alpha converting enzyme (TACE, also called ADAM17). The soluble 51 kDa trimeric sTNF tends to dissociate at concentrations below the nanomolar range, thereby losing its bioactivity. The secreted form of human TNF-α takes on a triangular pyramid shape, and weighs around 17kDa. Both the secreted and the membrane bound forms are biologically active, although the specific function of each is controversial. But, both forms do have overlapping and distinct biology activities. The common house mouse TNF-α and human TNF are structurally different. The 17kDa TNF protomer (185-amino acid-long) is composed of two antiparallel β-pleated sheets with antiparallel β-strands, forming a 'jelly roll' β-structure, which is typical for the TNF family, but also found in viral capsid proteins.
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