Glycidamide

History

Two of the first researchers who acknowledged the existence of glycidamide were Murray and Cloke in 1934. They performed experiments on glycidamide formation (see Synthesis for more details).

Since glycidamide is a metabolite of acrylamide, not much studies has been done on glycidamide on its own. Most of the studies focus on the effects of acrylamide, whereas less studies focus specifically on the effects of glycidamide. There are many studies that combine acrylamide and glycidamide, but the focus is still mainly on acrylamide.

Structure and reactivity

Glycidamide is a reactive epoxide metabolite from acrylamide. Glycidamides are fine crystals with lumps with a light orange color. It has an asymmetrical structure with electrophilic properties and can react with nucleophiles. This results in covalent binding of the electrophile. There is no data on the odor of glycidamide.

Glycidamide gives a positive response in the Ames/Salmonella mutagenicity assay, which indicates that it can cause mutations in the DNA.

Synthesis

Glycidamide is naturally formed by oxidation of acrylamide by cytochrome P450 2E1 (CYP2E1). This reaction follows a Michaelis-Menten process. Due to this reaction, glycidamide becomes critical to the genotoxicity of acrylamide. Saturated fatty acids protect the acrylamide from forming glycidamide. When during food processing, oil is used that contains unsaturated fatty acids, the amount of glycidamide formed is much higher.

The first experiments on glycidamide formation were done by Murray and Cloke (1934). They tried to form glycidamides from α,β-ethylenic nitriles. In order to do so, they used the modified Radziszewski reaction. The Radziszewski reaction refers to a method for the preparation of amides, described by Radziszewski in 1885. The amides are prepared “by the action of 3% hydrogen peroxide on nitriles in the presence of alkali and at a temperature of 40 degrees Celsius”. The reaction was modified by adding methanol, ethanol and acetone. Some nitriles did indeed give glycidamides.

Reactions

Glycidamide reacts with DNA to form DNA adducts and is more reactive to DNA than acrylamide. Several glycidamide-DNA adducts have been characterized (Beland, 2015). The main DNA adducts are N7-(2-carbamoyl-2-hydroxyethyl)-guanine (or N7-GA-Gua) and N3-(2-carbamoyl-2-hydroxyethyl)adenine (or N3-GA-Ade). Glycidamide also reacts with haemoglobine (Hb) to form a cysteine adduct, S-(20hydroxy-2carboxyethyl)cysteine. With this reaction, N-terminal valine adducts are also formed.

Available forms (isoforms)

There are two isomers of this connection: (R)-Glycidamide and the mirror image (S)-Glycidamide. The racemate (RS)-Glycidamide is a 1:1 mixture of both isomers.

Epoxide

The epoxide is a strong alkylating agent, which can open a ring to form a reactive ion. This reactive ion can bind to DNA and alkylate it. Alkylation of DNA forms DNA adducts and can cause mutagenicity. There are often tumors observed at the site of exposure.

Neurotoxicity

Inhibition of the sodium/potassium ATPase protein present in the plasma membrane of the nerve cell is caused by glycidamide. Intracellular sodium increases and intracellular potassium decreases due to this inhibition. This causes depolarization of the nerve membrane. The depolarization triggers a reverse sodium/calcium exchange, which will cause calcium-mediated axon degeneration.

Metabolism

The liver is a very active organ in the metabolism of xenobiotics. Substances in the liver modify the compounds to make them more soluble in water, in order to excrete them through bile and urine. This modification, however, can result in a greater toxicity of the compound. Whether this is the case for glycidamide remains unclear.

Glycidamide can be detoxified through different pathways to glycidamide-glutathione conjugates. There is an enzymatic pathway via glutathione-S-transferase and a non-enzymatic pathway. These glycidamide-glutathione conjugates are further metabolized to mercapturic acids by different peptidases and transferases, such as gamma-glutamyl-transpeptidase, dipeptidase and N-acetyltransferase. The mercapturic acids that can be formed are N-acetyl-S-(2-carbamoylethyl)-cysteine (AAMA), N-acetyl-S-(1-carbamoyl-2-hydroxyethyl)-cysteine (GAMA2), and N-acetyl-S-(2-carbamoyl-2-hydroxyethyl)-cysteine (GAMA3) (Huang et al., 2011). These mercapturic acids are excreted through urine.

Glycidamide can also be hydrolyzed to glyceramide both spontaneously or enzymatically by microsomal epoxide hydrolase. This too can be excreted through urine.

Toxicity

The DEREK NEXUS assessment shows that it is plausible (meaning that there is weight of evidence in favor of this proposition) that glycidamide is carcinogenic, mutagenic, neurotoxic, developmental toxic and oestrogenic. It also shows that it is plausible that glycidamide causes chromosome damage and irritation of the eye and skin. The results of this assessments are confirmed by the Hazard Identification of lookchem. It states that glycidamide may cause cancer and heritable genetic damage. It also causes skin and eye irritation.

Animal studies

Mice and rats are used for most of the studies of glycidamide. These are used, because the formation of glycidamide adducts is directly proportional in man and rat.

Studies with Big Blue mice and rats have shown mutations and DNA adducts that are consistent with those arising from glycidamide. Besaratinia and Pfeifer (2004) for instance, found cytotoxic effects in Big Blue mouse embryonic fibroblasts after being exposed to increasing concentrations of glycidamide. Another study found tumors in the mice bodies after treatment with glycidamide.

A study by National Toxicology Program (2014) provided evidence of carcinogenic activity of glycidamide in several species of rats and mice. For two years, rats and mice were exposed to varying doses of glycidamide in drinking water. In the rats and mice were several carcinogenic effects found, such as carcinomas, fibroadenomas and malignant mesotheliomas.