Transforming growth factor beta 1 (TGF-β1) is a member of the transforming growth factor beta superfamily of cytokines.  TGF-ß1 precursor contains 390 amino acids with an N-terminal signal peptide of 29 amino acids required for secretion from a cell, a 249 amino acids pro-region ( latency associated peptide or LAP), and a 112 amino acids C-terminal region that becomes the active TGF-β1 upon activation.

Both LAP and TGF-ß1 exist as homodimers in circulation, but the disulfide linked homodimers of LAP and TGF-β1 remain non-covalently associated, forming the small latent TGF-β1 complex (SLC, 100 kD).  The large latent TGF-β1 Complex (LLC, 235 – 260 kD) contains a third component, the latent TGF-β binding protein (LTBP), which is linked to LAP by a single disulfide bond.  The LTBP does not confer latency, but for efficient secretion of the complex to extracellular sites.  Free active TGF-β1 can be released (activated) by many factors including enzymes and low or high pH. The following picture illustrates the relationship among different forms of TGF-ß1.

TGF-β1 is nearly 100% conserved across mammalian species. It has diverse biological functions in multiple cellular processes such as regulating proliferation and differentiation of various cell types. TGF-ß1 is also an important immunoregulatory cytokine, which is involved in the maintenance of self-tolerance, Th17 differentiation, and T cell homeostasis etc.

It is expected that normal serum, plasma, or other biological fluid contains low concentration of free active TGF-β1 and high concentration of Latent TGF-β1. It is the free active form TGF-ß1 that binds TGF-ß receptor and exerts biological functions. However, it has been difficult to quantify the free active TGF-ß1 because of insufficient sensitivities of most assay products currently available on the market. It is necessary to measure both the free active form and total TGF-ß1 in biological samples to understand the TGF-ß1 functions.