Synthesis and Biological Activity of Homohypotaurine Obtained by the Enzyme-Based Conversion of Homocysteine Sulfinic Acid Using Recombinant Escherichia Coli Glutamate Decarboxylase

<i>l</i>-Homocysteine, formed from S-adenosyl methionine following demethylation and adenosine release, accumulates when the methionine recycling pathway and other pathways become impaired, thus leading to hyperhomocysteinemia, a biomarker in cardiovascular diseases, neurological/psychiatric disorders, and cancer. The partial oxidation of the <i>l</i>-homocysteine thiol group and its decarboxylation on C-alpha lead to the formation of <i>l</i>-homocysteinesulfinic acid (<i>l</i>-HCSA) and homohypotaurine (HHT), respectively. Both compounds are not readily available from commercial suppliers, which hinders the investigation of their biological activities. Herein, the chemical synthesis of <i>l</i>-HCSA, from <i>l</i>-homocystine, was the starting point for establishing the bio-based synthesis of HHT using recombinant <i>Escherichia coli</i> glutamate decarboxylase (<i>Ec</i>GadB), an enzyme already successfully employed for the bio-based synthesis of GABA and its phosphinic analog. Prior to HHT synthesis, <i>k</i>_cat (33.92 ± 1.07) and <i>K</i>_M (38.24 ± 3.45 mM) kinetic constants were determined for <i>l</i>-HCSA on <i>Ec</i>GadB. The results of our study show that the <i>Ec</i>GadB-mediated synthesis of HHT can be achieved with good yields (i.e., 40% following enzymatic synthesis and column chromatography). Purified HHT was tested in vitro on primary human umbilical vein endothelial cells and rat cardiomyoblasts and compared to the fully oxidized analog, homotaurine (OT, also known as tramiprosate), in widespread pharmaceutical use. The results show that both cell lines display statistically significant recovery from the cytotoxic effects induced by H₂O₂ in the presence of HHT.