Monoamine Oxidase

Monoamine oxidase (MAO) (EC 1.4.3.4) are a class of flavoenzymes capable of catalyzing a variety of amines, such as the neurotransmitters dopamine, norepinephrine, serotonin (5-HT), and tyramine, phenylethylamine (PEA) and some exogenous amines include the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Currently, two isozymes, MAO-A and MAO-B, are known. These enzymes are easily identifiable due to the selective difference between the substrate and the inhibitor. MAO-A can be inhibited by low concentrations of chlorocillin and preferentially catalyzes 5-HT and norepinephrine. MAO-B selectively catalyzes phenethylamine (PEA) and benzylamine and is inhibited by (R)-selegiline. Tyramine, dopamine, and tryptamine all serve as substrates for both enzymes.

Distribution

MAO is widely distributed in tissues and organs in the body, especially in the liver, kidney, stomach and small intestine. It is mainly located on the outer surface of the mitochondrial membrane and is tightly bound to the membrane. The flavin adenine dinucleotide is used as a coenzyme. Another type exists in connective tissue, without FAD, pyridoxal phosphate as coenzyme. The activity of MAO in brain tissue increases with age and glial cells increases. MAO can decompose catecholamine hormones and indirectly reflect cardiac sympathetic nerve function. It has been confirmed that the relative molecular masses of MAOs from different sources differ greatly, with about 100,000 for the smaller and more than 1,000,000 for the greater.

Catalytic Mechanism

Studies have shown that the amino acid residues His382 and Thr158 in MAO-B are essential for catalysis, while Phe208 in MAO-A and lle199 in MAO-B determine the substrate specificity. The evidence shows that there is also a cysteine residue in the active site of MAO, which may also be related to catalysis. At present, there are three approaches of the catalytic mechanism: single electron transfer (SET), hydrogen atom transfer (HAT), and nucleophilic or bipolar mechanisms.

Detection Methods

Aldehyde-benzoquinone colorimetric method: This method produces a brownish red under alkaline conditions by MAO oxidation of benzylamine and the formation of aldehydazine with 2,4-dinitrophenylhydrazine. The colorimetric determination is performed at 470 nm to calculate the concentration of MAO.

MCDP colorimetric method: This method produces hydrogen peroxide by oxidizing benzylamine with MAO. Hydrogen peroxide reacts with MCDP in the presence of peroxidase to form colored methylene blue. Colorimetric determination is performed at 660 nm to calculate the concentration of MAO. This method requires the addition of a stop solution before the determination, not suitable for the detection of large quantities of specimens.

Continuous monitoring method: This method is to catalyze benzylamine to produce ammonia by MAO, ammonia generates glutamic acid in the presence of α-ketoglutarate, NADPH, and GLDH, and NADPH reduces to NADP+, which causes a decrease in absorbance at 340 nm. Monitoring the decreasing rate can determine the MAO activity in the sample. This method is quick, simple, and suitable for automated analysis. It can reduce human error, has good accuracy and precision, and is suitable for most clinical laboratory applications.

Clinical Applications

The level of serum monoamine oxidase activity can reflect the degree of liver fibrosis and is an important indicator for the diagnosis of cirrhosis. The positive rate of MAO activity increased in liver cirrhosis patients was above 80%, and the highest value was up to 3.5 times the control value. The MAO activity in patients with various types of hepatitis in the acute phase did not increase most, but in the case of fulminant severe hepatitis, due to hepatocyte necrosis, mitochondria release a large amount of MAO, resulting in increased MAO activity, with a positive rate of 73.3%. The amplitude was positively related to the degree of severity of illness. MAO activity was significantly different between patients with hepatitis and cirrhosis, but there was no significant difference in MAO activity among various types of hepatitis. Diabetes mellitus and congestive heart failure can be secondary to hepatic fibrosis due to liver stagnation, as well as increased human MAO activity. Thyroid hyperfunction may be due to decomposition and synthesis of fibrous tissue, acromegaly due to excessive fiber synthesis and other reasons, resulting in different levels of MAO activity increased. Some patients with connective tissue disease without fiber proliferation do not have elevated MAO activity. The patients with primary liver cancer, secondary liver cancer, teratoma, biliary cirrhosis, schistosomiasis cirrhosis, and chronic cholestatic serohepatitis had no change in the activity of MAO.