sex hormone binding globulin
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| Crystal structure of SHBG binding dihydrotestosterone. From PDB 1D2S.[1] | |
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Sex hormone-binding globulin, Androgen-binding protein
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| Identifiers | |
| Symbol | SHBG |
| Alt. Symbols | ABP |
| Entrez | 6462 |
| HUGO | 10839 |
| OMIM | 182205 |
| RefSeq | NM_001040 |
| UniProt | P04278 |
| Other data | |
| Locus | Chr. 17 p13-p12 |
Sex hormone-binding globulin (SHBG) is a glycoprotein that binds to sex hormones, specifically testosterone and estradiol. Other steroid hormones such as progesterone, cortisol, and other corticosteroids are bound by transcortin.
Transport of sex hormones
These sex hormones circulate in the bloodstream, bound mostly to SHBG and to some degree bound to serum albumin. Only a small fraction is unbound, or "free," and thus biologically active and able to enter a cell and activate its receptor. The SHBG inhibits the function of these hormones. Thus bioavailability of sex hormones is influenced by the level of SHBG.
SHBG production
SHBG is produced by the liver cells and is released into the bloodstream. Other sites that produce SHBG are the brain, uterus, and placenta and vagina. In addition SHBG is produced by the testes; testes-produced SHBG is also called androgen-binding protein. The gene for SHBG is located on chromosome 17.
Control
SHBG levels appear to be controlled by a delicate balance of enhancing and inhibiting factors. Its level is decreased by high levels of insulin and insulin-like growth factor-I (IGF-I). Also, high androgen levels decrease SHBG, while high estrogen and thyroxine levels increase it.
Conditions with high or low levels
Conditions with low SHBG include polycystic ovary syndrome, diabetes, and hypothyroidism. Conditions with high SHBG include pregnancy, hyperthyroidism, and anorexia nervosa. There has recently been research to link high SHBG levels with breast and testicular cancer as well.
Measurement of sex hormones
When determining levels of circulating estradiol or testosterone, either a total measurement could be done that includes the "free" and the bound fractions, or only the "free" hormone could be measured. A free androgen index expresses the ratio of testosterone to the sex hormone binding globulin and can be used to summarise the activity of free testosterone.
See also
References
- ^ Grishkovskaya I, Avvakumov GV, Sklenar G, Dales D, Hammond GL, Muller YA (2000). "Crystal structure of human sex hormone-binding globulin: steroid transport by a laminin G-like domain". EMBO J 19 (4): 504–12. DOI:10.1093/emboj/19.4.504. PMID 10675319.
| Proteins: carrier proteins | |
|---|---|
| Hormone | Follistatin -
Growth hormone binding protein - Insulin-like growth factor binding protein - Neurophysins (Neurophysin I, II) Sex hormone binding globulin/Androgen binding protein - Transcortin - Thyroxine-binding globulin - Transthyretin |
| Metal/element | calcium (Calcium-binding protein, Calmodulin-binding proteins) - copper (Ceruloplasmin) - iron (Iron-binding proteins, Transferrin receptor) |
| Vitamin | Retinol binding protein (4) - Transcobalamins |
| Other | Acyl carrier protein - Adaptor protein - Cholesterylester transfer protein - F-box protein - GTP-binding protein - Latent TGF-beta binding protein - Light-harvesting complex - Membrane transport protein |
| Globulins: beta globulins |
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| Angiostatin - Haemopexin - Beta-2 microglobulin - Factor H - Plasminogen - Properdin - Sex hormone binding globulin - Transferrin |
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