GNMT

The enzyme was first described by Blumenstein and Williams (1960) in guinea pig liver.[8] However, this enzyme was not purified until 1972 in the rabbit liver by Kerr.[9] In 1984, Cook and Wagner demonstrated that a liver cytosolic folate binding protein is identical to GNMT.[10] The human GMNT gene was cloned in 2000 by Chen and coworkers.[6]

GNMT is an abundant enzyme in liver cytosol and consists of 0.9% to 3% of the soluble protein present in liver.[11] In addition to liver, GNMT activity has been found in a number of other tissues including pancreas and kidney.[9] GNMT is most abundant in the peri-portal region of the liver and exocrine tissue of the pancreas.[11] The GNMT proteins located in tissues that are actively in secretion, such as the proximal kidney tubules, the submaxillary glands and the intestinal mucosa.[11] GNMT is also expressed in various neurons presented in the cerebral cortex, hippocampus, substantia nigra and cerebellum.[12] The presence of GNMT in these cells suggests that this enzyme may play a role in secretion.

The properties of GNMT protein from rabbits, rats and humans, either purified from liver/pancreas, or expressed in Escherichia coli, have been well characterized. All GNMTs have very similar molecular and kinetic properties.[11][13][14][15][16] Comparison of the cDNA and protein sequences of human, rabbit, pig and rat GNMTs shows similarities of over 84% at the nucleotide level and about 90% at the amino acid level. All GNMTs are 130 kDa tetramers consisting of four identical subunits, each having a Mr of 32 kDa.[15] The structure of recombinant rat, mouse and human GNMTs have been solved.[17][18] The four nearly spherical subunits are arranged to form a flat and square tetramer with a large hole in the center. The active sites are located in the near center of each subunit.

Glycine N-methyltransferase catalyzes the synthesis of N-methylglycine (sarcosine) from glycine using S-adenosylmethionine(SAM) (AdoMet) as the methyl donor. GNMT acts as an enzyme to regulate the ratio of S-adenosylmethionine(SAM) to S-adenosylhomocysteine (SAH) (AdoHcy)[19] and participates in the detoxification pathway in liver cells.[7] GNMT competes with tRNA methyltransferases for SAM and the product, S-adenosylhomocysteine (SAH), is a potent inhibitor of tRNA methyltransferases and a relatively weak inhibitor of GNMT.[9] GNMT regulates the relative levels of SAM and SAH. Since SAM is the methyl donor for almost all cellular methylation reactions.[19] GNMT is therefore likely to regulate cellular methylation capacity.[19][20] An endogenous ligand of GNMT, 5-methyltetrahydropteroylpentaglutamate (5-CH3-H4PteGIu5) is a powerful inhibitor of this enzyme.[21] Thus, GNMT has been proposed to link the de novo synthesis of methyl groups to the ratio of SAM to SAH, which in turn serves as a bridge between methionine and one-carbon metabolism.[19][21]

In addition to the methyltransferase activity, the 4S polycyclic aromatic hydrocarbon (PAH)-binding protein and GNMT are one and the same protein.[22] The catalytic site resembles a molecular basket, unlike most other SAM-dependent methyltransferases,[17] which therefore suggests that GNMT may be capable of capturing unidentified chemicals as a part of a detoxification process. Therefore, GNMT has been proposed to be a protein with diverse functionality.[23]

GNMT has been shown to detoxify some environmental carcinogens such as polyaromatic hydrocarbons and aflatoxin.[24]

There is mounting evidence that supports the involvement of GNMT deficiency in liver carcinogenesis.[25]

The glycoside natural product 1,2,3,4,6-penta-O-galloyl-β-d-glucopyranoside (PGG) isolated from Paeonia lactiflora, an Asian flower plant, induces GNMT mRNA and protein expression in Huh7 human hepatoma cells.[26]