Human serum albumin

Human serum albumin is the most abundant protein in human blood plasma. It is produced in the liver. Albumin constitutes about half of the blood serum protein. It is soluble and monomeric.

The gene for albumin is located on chromosome 4 and mutations in this gene can result in various anomalous proteins. The human albumin gene is 16,961 nucleotides long from the putative 'cap' site to the first poly(A) addition site. It is split into 15 exons that are symmetrically placed within the 3 domains thought to have arisen by triplication of a single primordial domain.

Albumin is synthesized in the liver as preproalbumin, which has an N-terminal peptide that is removed before the nascent protein is released from the rough endoplasmic reticulum. The product, proalbumin, is in turn cleaved in the Golgi vesicles to produce the secreted albumin.

The reference range for albumin concentrations in blood is 3.4 to 5.4 g/dL. It has a serum half-life of approximately 20 days. It has a molecular mass of 67 kDa.

Function

 * Maintains osmotic pressure
 * Transports thyroid hormones
 * Transports other hormones, in particular, ones that are fat-soluble
 * Transports fatty acids ("free" fatty acids) to the liver
 * Transports unconjugated bilirubin
 * Transports many drugs; serum albumin levels can affect the half-life of drugs
 * Competitively binds calcium ions (Ca2+)
 * Buffers pH
 * Serum albumin, as a negative acute-phase protein, is down-regulated in inflammatory states. As such, it is not a valid marker of nutritional status; rather, it is a marker in inflammatory states
 * Prevents photodegradation of folic acid

Measurement
Plasma albumin is a component of liver function tests (LFTs), but may be ordered separately. Albumin can be measured in serum (yellow-top tube), plain tube with no additives (red-top tube), or heparin plasma (green-top tube). The reference interval is 36 - 52 g/L. (upper limit increased from 47 g/L on the 15th June 2007). One of the methods used is bromocresol green on a Roche Modular or Olympus AU2700 analyser.

Hypoalbuminemia
Low blood albumin levels (hypoalbuminemia) can be caused by:


 * Liver disease; cirrhosis of the liver is most common
 * Excess excretion by the kidneys (as in nephrotic syndrome)
 * Excess loss in bowel (protein-losing enteropathy, e.g., Ménétrier's disease)
 * Burns (plasma loss in the absence of skin barrier)
 * Redistribution (hemodilution [as in pregnancy], increased vascular permeability or decreased lymphatic clearance)
 * Acute disease states (referred to as a negative acute-phase protein)
 * Mutation causing analbuminemia (very rare)

Hyperalbuminemia
Typically, this condition is a sign of severe or chronic dehydration. Chronic dehydration needs to be treated with zinc as well as with water. Zinc reduces cell swelling caused by increased intake of water (hypotonicity) and also increases retention of salt. In the dehydrated state, the body has too high an osmolarity and, it appears, discards zinc to prevent this. Zinc also regulates transport of the cellular osmolyte taurine, and albumin is known to increase cellular taurine absorption. Zinc has been shown to increase retinol (vitamin A) production from beta-carotene, and in lab experiments retinol reduced human albumin production. It is possible that a retinol (vitamin A) deficiency alone could cause albumin levels to become raised. Patients recovering from chronic dehydration may develop dry eyes as the body uses up its vitamin A store. It is interesting to note that retinol causes cells to swell with water (this is likely one reason that too much vitamin A is toxic). Hyperalbuminemia is also associated with high protein diets.

Therapeutic Uses
Human albumin solution or HAS is available for medical use, usually at concentrations of 5-25%.

Human albumin is often used to replace lost fluid and help restore blood volume in trauma, burns and surgery patients. A systematic review; Human albumin solution for resuscitation and volume expansion in critically ill patients found 37 trials with 8716 trial participants, that compared albumin with no albumin or with a crystalloid solution, in critically ill patients. The review of trials found no evidence that albumin, compared with cheaper alternatives such as saline, reduces the risk of dying.

It has not been shown to give better results than other fluids when used simply to replace volume, but is frequently used in conditions where loss of albumin is a major problem, such as liver disease with ascites.

Glycosylation
It has been known for a long time that human blood proteins like hemoglobin and serum albumin  may undergo a slow non-enzymatic glycation, mainly by formation of a Schiff base between ε-amino groups of lysine (and sometimes arginine) residues and glucose molecules in blood (Maillard reaction). This reaction can be inhibited in the presence of antioxidant agents. Although this reaction may happen normally, elevated glycoalbumin is observed in diabetes mellitus.

Glycation has the potential to alter the biological structure and function of the serum albumin protein.

Moreover, the glycation can result in the formation of Advanced Glycosylation End-Products (AGE), which result in abnormal biological effects. Accumulation of AGEs leads to tissue damage via alteration of the structures and functions of tissue proteins, stimulation of cellular responses, through receptors specific for AGE-proteins, and generation of reactive oxygen intermediates. AGEs also react with DNA, thus causing mutations and DNA transposition. Thermal processing of proteins and carbohydrates brings major changes in allergenicity. AGEs are antigenic and represent many of the important neoantigens found in cooked or stored foods. They also interfere with the normal product of nitric oxide in cells.

Although there are several lysine and arginine residues in the serum albumin structure, very few of them can take part in the glycation reaction. It is not clear exactly why only these residues are glycated in serum albumin, but it is suggested that non-covalent binding of glucose to serum albumin prior to the covalent bond formation might be the reason.

Loss via kidneys
In the healthy kidney, albumin's size and negative electric charge exclude it from excretion in the glomerulus. This is not always the case, as in some diseases including diabetic nephropathy, a major complication of uncontrolled diabetes in which proteins can cross the glomerulus. The lost albumin can be detected by a simple urine test. Depending on the amount of albumin lost, a patient may have normal renal function, microalbuminuria, or albuminuria.

Amino acid sequence
The approximate sequence of human serum albumin is:

MKWVTFISLL FLFSSAYSRG VFRRDAHKSE VAHRFKDLGE ENFKALVLIA FAQYLQQCPF EDHVKLVNEV TEFAKTCVAD ESAENCDKSL HTLFGDKLCT VATLRETYGE MADCCAKQEP ERNECFLQHK DDNPNLPRLV RPEVDVMCTA FHDNEETFLK KYLYEIARRH PYFYAPELLF FAKRYKAAFT ECCQAADKAA CLLPKLDELR DEGKASSAKQ RLKCASLQKF GERAFKAWAV ARLSQRFPKA EFAEVSKLVT DLTKVHTECC HGDLLECADD RADLAKYICE NQDSISSKLK ECCEKPLLEK SHCIAEVEND EMPADLPSLA ADFVESKDVC KNYAEAKDVF LGMFLYEYAR RHPDYSVVLL LRLAKTYETT LEKCCAAADP HECYAKVFDE FKPLVEEPQN LIKQNCELFE QLGEYKFQNA LLVRYTKKVP QVSTPTLVEV SRNLGKVGSK CCKHPEAKRM PCAEDYLSVV LNQLCVLHEK TPVSDRVTKC CTESLVNRRP CFSALEVDET YVPKEFNAET FTFHADICTL SEKERQIKKQ TALVELVKHK PKATKEQLKA VMDDFAAFVE KCCKADDKET CFAEEGKKLV AASQAALGL

The italicized first 24 amino acids are signal and propeptide portions not observed in the transcribed, translated, and transported protein but present in the gene. There are 609 amino acids in this sequence with only 585 amino acids in the final product observed in the blood.

Interactions
Human serum albumin has been shown to interact with FCGRT.