Elucidating the biodegradation pathway for the pharmaceutical metformin in wastewater: Implications in human and wastewater microbiomes

Abstract

Metformin (1,1-dimethylbiguanide) is a wonder drug and a pervasive pollutant which is taken by type-II diabetes patients and more recently for improving obesity and cancer treatment outcomes. The drug’s direct mode of action is currently unknown but is thought to be dependent on microbial reactions to the drug. Large therapeutic doses (1-2 grams daily) and the global use of metformin result in over 100 million kilograms of the drug entering aquatic ecosystems each year. The biodegradation of metformin was known in wastewater treatment plants to be transformed by microbes to guanylurea but, prior to this work, no enzymes involved in the biodegradation had been identified. As a result of this work, the metformin biodegradation pathway has been completely elucidated starting with metformin hydrolysis to form dimethylamine and guanylurea by metformin hydrolase (MfmAB). The guanylurea hydrolase (GuuH) was discovered, which degrades guanylurea into guanidine, ammonia and carbon dioxide. Lastly, the enzyme CgdAB was discovered that acts as a carboxyguanidine deiminase bacteria use to assimilate nitrogen from guanidine. A secondary pathway of metformin biodegradation was also identified by discovering a biguanidase (BguH) that degrades biguanide and 1-methylbiguanide to form guanylurea, suggesting that metformin can also be demethylated by microbes. These enzymes were characterized by kinetics, X-ray crystallography or computational modelling, and bioinformatics. While the metabolism of metformin is now known in wastewater ecosystems, metabolism in the human gut has not been established but may contribute to the potency or therapeutic effect of the drug. Testing enzymes that are found in human gut microbes, homologous to the metformin hydrolase, did not show activity on metformin. However, a subset of these gut enzymes, from Gammaproteobacteria that hydrolyze agmatine, was potently inhibited by metformin and the natural product galegine, from which the drug was derived from. Agmatine is a known effector of human host metabolism and has been reported to augment metformin’s therapeutic effects for type-II diabetes. This gut-derived inhibition mechanism gives new insights on metformin’s action in the gut and may lead to significant discoveries in improving metformin therapy.