Isotope Labeled Reactivity-Based Metabolomics Reveal Cysteine as a Potent In Vivo Scavenger of Lactoylglutathione

Metabolite adducts are increasingly being recognized as important intermediates in cellular metabolism and as circulating messengers. Methylglyoxal (MG) is a highly reactive metabolite involved in diabetes and aging, via formation of a wide variety of protein adducts. Though new types of protein adducts with biological function continuously appear, little is known regarding the potentially more diverse MG-metabolite adductome. To address this, we here devise a “symmetric” isotopic labeling and reactivity-based metabolomics approach. We find more than 100 direct MG- as well as lactoylglutathione (LGSH)-derived adducts and among them characterize 10 as D- and L-lactoylated amino acids. We find cysteine to generate mainly L-Lac-Cys from MG, whereas the diasteromer , D-lactoyl-cysteine, mainly originate via a rapid reaction with LGSH with rates on par with click-reactions. The reaction salvages glutathione and also scavenges/regulate prote in cysteine lactoylation, in aggregate assigning cysteine with both glyoxalase 1 and 2 like function.  Several of the adducts are circulating human metabolites, as well as D-Lac-Cys, L-Lac-Cys and D-Lac-Phe are increased in the urine of diabetic mice and may serve as a novel type of biomarker for metabolic diseases.