Diamine Oxidase

Diamine oxidase (EC1.4.3.6) is an enzyme that catalyzes the oxidation of histamine, putrescine, ethylene diamine and other diamines to aldehydes. Diamine oxidase is a highly active intracellular enzyme in the submucosal villi of human and mammalian small intestine, which plays an important role in histamine and polyamine metabolism. Its activity is closely related to the synthesis of nucleic acid and protein in mucosal cells, which can reflect the integrity and damage degree of intestinal mechanical barrier. The enzyme can decompose not only histamine, but also amine produced by amino acid decarboxylation in intestinal mucosa, which plays a detoxification role. The enzyme activity can be inhibited by carbonyl reagents such as cyanide, hydroxylamine, semicarbazide and so on.

Distribution

Diamine oxidase widely exists in animal tissues (intestinal mucosa, lung, liver, kidney, etc.), plant tissues and microorganisms. Different from monoamine oxidase, diamine oxidase in animal tissues is water-soluble and exists in the supernatant of homogenate. There is coenzyme pyridoxal phosphate in the quite refined renal diamine oxidase. Diamine oxidase has been crystallized in ruminant plasma, which can well oxidize benzylamine and spermine, containing copper and pyridoxal phosphate. Diamine oxidase in plants also oxidizes monoamines well, containing copper and showing pink. Microbial diamine oxidase also has high specificity to the matrix and can be induced to form.

Structure

Human diamine oxidase is a homodimer of two 85 kDa subunits (A and B). There is one homodimer present in the asymmetric unit. There is only very weak electron density for one additional residue at the N terminus of chain B. The missing residues at the N-terminus contain the 19-residue signal peptide that was omitted from the construct. No residues are missing from the C terminus of the mature protein. Human diamine oxidase hares the archetypal CAO (copper-containing amine oxidases) fold comprised of the three domains, D2, D3 and D4 from each subunit that make up the mushroom cap. The D1 domain is a unique feature of the E. coli enzyme among those that have been structurally characterized. The dimer interface is formed by residues from the D4 units that form an annulus at the center and extensive areas of contact made by arms that reach across from one subunit to the other. The D2 and D3 domains lie at the periphery of the cap. Connecting D2 to D3 is a short linker of 8 residues. Both D2 and D3 are similar in length and are composed of a 4 stranded β sheet and 3 α-helices. Bridging from one side of the structure to the other, and linking D3 to D4, is a meandering 51 residue linker that threads beneath the center of the two D4 domains before joining to the larger of the two central β-sheets that start domain D4. The largest domain, D4, contains the active site and the protruding arms linking the subunits. Arm 1, termed the ‘upper arm’, is an extended β-hairpin. The ‘lower arm’ consists of a four stranded β-sheet comprised of residues from two sections of the polypeptide chain. Each of the D4 subunits is dominated by a central β-sandwich comprising twisted 8 and 6 stranded β-sheets. The active site of each subunit is located between and towards the edge of these β-sheets with both contributing elements of the active site.

Figure 1. Structure of human diamine oxidase. (Mcgrath A.P. et al. 2009)

Function

The expression level of diamine oxidase was the highest in digestive tract and placenta. In humans, extravillous trophoblastic cells in the placenta express diamine oxidase and secrete it into the blood of pregnant women. The decrease of diamine oxidase in maternal blood in early pregnancy may be an indicator of trophoblastic pregnancy diseases such as early-onset preeclampsia. Under normal circumstances, diamine oxidase does not exist in human blood circulation, or only in very few cases, but the increase of this substance in pregnant women indicates that it is a protective mechanism against bad histamine. Diamine oxidase is also secreted by eosinophils. If there is a lack of diamine oxidase in the human body, it will be released.

Application

In normal conditions, the activity of diamine oxidase in plasma was very low, but increased significantly after intravenous injection of heparin. There are heparin cell surface receptors on the basolateral side of the cells, which have a high affinity for heparin, and heparin can release diamine oxidase from endothelial cells of small intestinal microvessels. Diamine oxidase in the blood is rapidly ingested and cleared by the liver through the portal vein and excreted through urine and bile in an inactive form. When liver function is damaged, the uptake and clearance of diamine oxidase is reduced, which affects the accuracy of diamine oxidase value, and can lead to the increase of serum diamine oxidase. The liver function of the subjects was measured, and the activity of diamine oxidase increased progressively from the duodenal ileum. The value of diamine oxidase after heparin showed a correlation in small intestinal injury, acute gastroenteritis, radiation enteritis, intestinal mucosal atrophy and small intestinal ischemia.

Intestinal mucosal diseases often lead to a decrease in diamine oxidase content. The main mechanism causing the decrease of diamine oxidase is small intestinal mucosal lesion, which reduces the number of small intestinal cells, hinders the synthesis of diamine oxidase, promotes the degradation of diamine oxidase, and reduces the transport of diamine oxidase to endothelial cells and storage in endothelial cells. Therefore, the activity of diamine oxidase in tissue or serum can be used as a useful marker for the determination of small intestinal diseases.

Reference

1. Mcgrath A.P. et al. Structure and Inhibition of Human Diamine Oxidase. BIOCHEMISTRY, 2009, 48(41):9810-9822.