5'-nucleotidase

5′-nucleotidase (EC 3.1.3.5) is an enzyme which catalyzes the phosphorylytic cleavage of 5′-nucleotides.^[2] Although originally found in snake venom,^[3] the activity of 5′-nucleotidase has been described for bacteria and plant cells, and is widely distributed in vertebrate tissue.^[4] In mammalian cells the enzyme is predominantly located in the plasma membrane and its primary role is in the conversion of extracellular nucleotides (e.g. 5′-AMP), which are generally impermeable, to the corresponding nucleoside (e.g. adenosine) which can readily enter most cells.^[5] Consequently, the enzyme plays a key role in the metabolism of nucleotides.

The enzyme has a wide substrate specificity for nucleotides and has been shown to hydrolyze 5′-nucleotides rapidly, ribose-5-phosphate slowly, and other phosphate esters extremely slowly (if at all).^[6]

The enzyme catalyses the following reaction:

a 5′-nucleotide + H₂O ⇌ a nucleoside + phosphate

The 5′-nucleotidase-catalyzed reaction of an AMP nucleotide to adenosine nucleoside is shown below:

• Accepted Name: 5′-nucleotidase
• Systematic Name: 5′-ribonucleotide phosphohydrolase
• Synonyms: uridine 5′-nucleotidase, 5′-adenylic phosphatase, adenosine 5′-phosphatase, AMP phosphatase, adenosine monophosphatase, 5′-mononucleotidase, AMPase, UMPase, snake venom 5′-nucleotidase, thymidine monophosphate nucleotidase, 5′-AMPase, 5′-AMP nucleotidase, AMP phosphohydrolase, IMP 5′-nucleotidase.^[7][8][9]

Studies of the soluble form of human ecto-5′-nucleotidase, without a GPI anchor, have shown that the C-terminal domain holds the substrate-binding pocket, and that the aromatic purine motif of the substrate is stacked between two phenylalanine residues.^[10] Furthermore, a catalytic mechanism has been proposed involving an in-line nucleophilic attack by a hydroxyl moiety that is coordinated by zinc on the substrate phosphorus, with the nucleoside acting as a leaving group.^[10]

Studies of mammalian 5′-nucleotidases have shown that there exist at least four different forms of the 5′-nucleotidase enzyme: one membrane-bound form and three soluble forms.^[4] The membrane-bound form is anchored to the plasma membrane via GPI at its C-terminus.^[4] One of the soluble forms appears to be derived from the GPI-anchored ex-5′-nucleotidase and has an extracellular location.^[4] The two cytosolic forms of the enzyme have similar characteristics, but can be differentiated on the basis of their preferential affinities for nucleotide substrates.^[4] The GPI-anchored form exists as a dimer, with the two subunits linked via a disulfide bridge. The soluble forms can exist as dimers or tetramers. Generally at least 50% of the enzyme is found in the surface-bound form.^[4]

The concentration of 5′-nucleotidase protein in the blood is often used as a liver function test in individuals that show signs of liver problems. The combined assays of serum 5′-nucleotisase and alkaline phosphatase (AP) activities are extremely helpful in differential diagnosis since serum 5′-nucleotidase activity is increased in obstructive hepatobiliary disorders, but not in osseous disorders, whereas serum AP activity is generally increased in both categories of diseases.^[11] In other words, the test is used to determine if elevated protein levels are due to skeletal damage or liver damage.^[12] Normal levels of 5′-nucleotidase are 2-17 units per liter.^[13] Elevated levels may indicate cholestasis, destruction of liver cells, hepatitis (liver inflammation), liver ischemia, a liver tumor, or use of liver-damaging drugs. Pregnancy and certain medications (acetaminophen, halothane, isoniazid, methyldopa, nitrofurantoin) may interfere with the test.^[12][13]

The test may also be referred to as 5′NT Levels Blood Test, CDF73 Levels Blood Test, and Ecto-5′-Nucleotidase Levels Blood Test.^[12]

Ecto-5′-nucleotidase is considered a maturation marker for T cells and B cells. This is due to the fact that the enzyme activity is approximately 10-times higher for peripheral T cells than thymocytes, 5-6 times higher in adult peripheral B cells than fetal B cells, and largely absent in non-T cell and non-B cell lymphocytes.^[5][14] In immunodeficiency diseases with arrested lymphocyte maturation, ex-5′-nucleatidase activity is generally low.^[5] Such diseases include severe combined immuno-deficiency, Wiskott–Aldrich syndrome, congenital X-linked agammaglobulinemia, selective IgA deficiency and acquired immune deficiency syndrome (AIDS).^{[5][15][16][17]}

Numerous studies have shown that erythrocyte pyrimidine 5′-nucleotidase activity is significantly lowered in patients with lead poisoning, and that pyrimidine 5′-nucleotidase activity can be used as an index of lead poisoning.^[18] It is believed that lead induced deficiency of the enzyme in maturing erythroid cells is responsible for basophilic stippling and hemolysis in a manner analogous to the pathogenesis of the hereditary enzyme deficiency syndrome. The mechanism of inhibition of 5′-nucleotidase in lead poisoning may contribute to the hemolytic syndromes that occur in patients with acute lead poisoning.^[5] Since erythrocyte pyrimidine 5′NT activity is inhibited in vitro by various metals (e.g., copper, zinc, cadmium, lead, mercury, and tin), it is likely that inhibition of pyrimidine 5′-nucleotidase may contribute to the hemolytic syndromes that occur in patients with acute poisoning by these metals.^[5][19][20]

Cytosolic 5-nucleotidase II superactivity has been associated with autism spectrum disorder via a disorder of carnitine biosynthesis.^[21]

• 5′-nucleotidase at the U.S. National Library of Medicine Medical Subject Headings (MeSH)