Alpha-1 antitrypsin

Alpha-1 Antitrypsin or α1-antitrypsin (A1AT) is a protease inhibitor belonging to the is_associated_with::serpin superfamily. It is generally known as serum trypsin inhibitor. Alpha 1-antitrypsin is also referred to as alpha-1 proteinase inhibitor (A1PI) because it inhibits a wide variety of proteases. It protects tissues from is_associated_with::enzymes of inflammatory cells, especially is_associated_with::neutrophil elastase, and has a is_associated_with::reference range in blood of 1.5 - 3.5 is_associated_with::gram/liter (in US the reference range is generally expressed as mg/dL or micromoles), but the concentration can rise manyfold upon acute inflammation. In its absence, neutrophil elastase is free to break down is_associated_with::elastin, which contributes to the elasticity of the lungs, resulting in respiratory complications such as is_associated_with::emphysema, or COPD (chronic obstructive pulmonary disease) in adults and cirrhosis in adults or children.

Function
A1AT is a 52-is_associated_with::kDa is_associated_with::serpin and, in is_associated_with::medicine, it is considered the most prominent serpin; the terms α1-antitrypsin and protease inhibitor (Pi) are often used interchangeably.

Most serpins inactivate is_associated_with::enzymes by binding to them is_associated_with::covalently, requiring very high levels to perform their function. In the acute phase reaction, a further elevation is required to "limit" the damage caused by activated is_associated_with::neutrophil granulocytes and their enzyme is_associated_with::elastase, which breaks down the is_associated_with::connective tissue fiber is_associated_with::elastin.

Like all is_associated_with::serine protease inhibitors, A1AT has a characteristic is_associated_with::secondary structure of is_associated_with::beta sheets and alpha helices. is_associated_with::Mutations in these areas can lead to non-functional proteins that can is_associated_with::polymerise and accumulate in the is_associated_with::liver (infantile hepatic cirrhosis).

Role in disease
Disorders of this protein include is_associated_with::alpha 1-antitrypsin deficiency, an is_associated_with::autosomal codominant is_associated_with::hereditary disorder in which a deficiency of alpha 1-antitrypsin leads to a chronic uninhibited tissue breakdown. This causes the degradation especially of lung tissue, and eventually leads to characteristic manifestations of pulmonary emphysema. Evidence has shown that cigarette smoke can lead to oxidation of is_associated_with::methionine 358 of α1-antitrypsin (382 in the pre-processed form containing the 24 amino acid signal peptide), a residue essential for binding elastase; this is thought to be one of the primary mechanisms by which cigarette smoking (or second-hand smoke) can lead to emphysema. Because A1AT is expressed in the liver, certain mutations in the is_associated_with::gene encoding the protein can cause misfolding and impaired secretion, which can lead to is_associated_with::liver cirrhosis.

An extremely rare form of Pi, termed PiPittsburgh, functions as an is_associated_with::antithrombin (a related serpin), due to a mutation (Met358Arg). One person with this mutation has been reported to have died of a lethal is_associated_with::bleeding diathesis.

Liver biopsy will show abundant PAS-positive globules within periportal hepatocytes.

Nomenclature
The protein was originally named "antitrypsin" because of its ability to is_associated_with::covalently bind and irreversibly inactivate the enzyme is_associated_with::trypsin in vitro. Trypsin, a type of is_associated_with::peptidase, is a digestive enzyme active in the is_associated_with::duodenum and elsewhere.

The term alpha-1 refers to the protein's behavior on is_associated_with::protein electrophoresis. On electrophoresis, the protein component of the blood is separated by is_associated_with::electric current. There are several clusters, the first being albumin, the second being the alpha, the third beta and the fourth gamma (is_associated_with::immunoglobulins). The non-albumin proteins are referred to as is_associated_with::globulins.

The alpha region can be further divided into two sub-regions, termed "1" and "2". Alpha 1-antitrypsin is the main is_associated_with::protein of the is_associated_with::alpha-globulin 1 region.

Another name used is alpha-1 proteinase inhibitor (α1-PI).

Genetics
The is_associated_with::gene is located on the long arm of the fourteenth is_associated_with::chromosome (14q32.1).

Over 100 different variants of α1-antitrypsin have been described in various populations. North-Western is_associated_with::Europeans are most at risk for carrying one of the most common mutant forms of A1AT, the Z mutation (Glu342Lys on M1A, rs28929474).

Biochemical Properties
A1AT is a single-chain glycoprotein consisting of 394 amino acids in the mature form and exhibits a number of is_associated_with::glycoforms. The three N-linked glycosylations sites are mainly equipped with so-called diantennary N-is_associated_with::glycans. However, one particular site shows a considerable amount of heterogeneity since tri- and even tetraantennary N-is_associated_with::glycans can be attached to the is_associated_with::Asparagine 107 (ExPASy amino acid nomenclature). These is_associated_with::glycans carry different amounts of negatively charged sialic acids, this causes the heterogeneity observed on normal A1AT when analysed by is_associated_with::isoelectric focussing. In addition, the fucosylated triantennary N-glycans were shown to have the is_associated_with::fucose as part of a so-called is_associated_with::Sialyl Lewis x is_associated_with::epitope, which could confer this is_associated_with::protein particular protein-cell recognition properties. The single is_associated_with::cysteine residue of A1AT in position 256 (is_associated_with::ExPASy nomenclature) is found to be covalently linked to a free single is_associated_with::cysteine by a is_associated_with::disulfide bridge.

Analysis
The level of A1AT in serum is most often determined by adding an antibody that binds to A1AT, then using is_associated_with::turbidimetry to measure how much A1AT is present. Other detection methods include use of enzyme-linked-immuno-sorbent-assays and radial immunodiffusion.

Different analytical methods are used to determine A1AT is_associated_with::phenotype. As is_associated_with::protein is_associated_with::electrophoresis is imprecise, A1AT phenotype is analysed by is_associated_with::isoelectric focusing (IEF) in the pH range 4.5-5.5, where the protein migrates in a gel according to its isoelectric point or charge in a is_associated_with::pH gradient.

Normal A1AT is termed M, as it is migrates toward the center of such an IEF gel. Other variants are less functional, and are termed A-L and N-Z, dependent on whether they run proximal or distal to the M band. The presence of deviant bands on IEF can signify the presence of is_associated_with::alpha 1-antitrypsin deficiency. Since the number of identified mutations has exceeded the number of letters in the alphabet, subscripts have been added to most recent discoveries in this area, as in the Pittsburgh mutation described above.

As every person has two copies of the A1AT is_associated_with::gene, a is_associated_with::heterozygote with two different copies of the gene may have two different bands showing on electrofocusing, although heterozygote with one null mutant that abolishes expression of the gene will only show one band.

In is_associated_with::blood test results, the IEF results are notated as in PiMM, where Pi stands for protease inhibitor and "MM" is the banding pattern of that patient.

Alpha 1-antitrypsin levels in the blood depend on the is_associated_with::genotype. Some mutant forms fail to fold properly and are, thus, targeted for destruction in the is_associated_with::proteasome, whereas others have a tendency to is_associated_with::polymerise, being retained in the is_associated_with::endoplasmic reticulum. The serum levels of some of the common genotypes are:


 * PiMM: 100% (normal)
 * PiMS: 80% of normal serum level of A1AT
 * PiSS: 60% of normal serum level of A1AT
 * PiMZ: 60% of normal serum level of A1AT
 * PiSZ: 40% of normal serum level of A1AT
 * PiZZ: 10-15% (severe is_associated_with::alpha 1-antitrypsin deficiency)

Other rarer forms have been described; in all there are over 80 variants.
 * PiZ is caused by a is_associated_with::glutamate to is_associated_with::lysine mutation at position 342 (366 in pre-processed form)
 * PiS is caused by a is_associated_with::glutamate to is_associated_with::valine mutation at position 264 (288 in pre-processed form)

Therapeutic use
Recombinant alpha 1-antitrypsin is not yet commercially available, but is under investigation as a therapy for alpha 1-antitrypsin deficiency.

Therapeutic concentrates are prepared from the is_associated_with::blood plasma of blood donors. The US FDA has approved the use of three alpha 1-antitrypsin products derived from a human plasma: Prolastin, Zemaira, and Aralast. These products for intravenous augmentation A1AT therapy can cost up to $100,000 per year per patient. They are administered intravenously at a dose of 60 mg/kg once a week.

A recent study analyzed and compared the three FDA-approved products in terms of their primary structure and is_associated_with::glycosylation. All three products showed minor differences compared to the normal human plasma A1AT, and are introduced during the specific purifications procedures. However, these detected differences are not believed to have any negative implications to the patients.

Aerosolized-augmented A1AT therapy is under study. This involves inhaling purified human A1AT into the lungs and trapping the A1AT into the lower respiratory tract. This method proves more successful than intravenous-augmented A1AT therapy because intravenous use of A1AT results in only 10%-15% of the A1AT reaching the lower respiratory tract, whereas 25%-45% of A1AT can reach the lower respiratory tract through inhalation. However, inhaled A1AT may not reach the elastin fibers in the lung where elastase injury actually occurs. Further study is currently underway.

History
The possibility of allelic variants of A1AT leading to disease was first investigated by Axelsson and Laurell in 1965.