Selectivity of commonly used pharmacological inhibitors for cystathionine synthase (CBS) and cystathionine lyase (CSE)

Background and Purpose Hydrogen sulfide (H2S) is a signalling molecule that belongs to the gasotransmitter family. Two major sources for endogenous enzymatic production of H2S are cystathionine synthase (CBS) and cystathionine lyase (CSE). In the present study, we examined the selectivity of commonly used pharmacological inhibitors of H2S biosynthesis towards CSE and CBS. Experimental Approach To address this question, human CSE or CBS enzymes were expressed and purified from Escherichia coli as fusion proteins with GSH-S-transferase. After purification, the activity of the recombinant enzymes was tested using the methylene blue method. Key Results -cyanoalanine (BCA) was more potent in inhibiting CSE than propargylglycine (PAG) (IC50 14 +/- 0.2 M vs. 40 +/- 8 M respectively). Similar to PAG, L-aminoethoxyvinylglycine (AVG) only inhibited CSE, but did so at much lower concentrations. On the other hand, aminooxyacetic acid (AOAA), a frequently used CBS inhibitor, was more potent in inhibiting CSE compared with BCA and PAG (IC50 1.1 +/- 0.1M); the IC50 for AOAA for inhibiting CBS was 8.5 +/- 0.7 M. In line with our biochemical observations, relaxation to L-cysteine was blocked by AOAA in aortic rings that lacked CBS expression. Trifluoroalanine and hydroxylamine, two compounds that have also been used to block H2S biosynthesis, blocked the activity of CBS and CSE. Trifluoroalanine had a fourfold lower IC50 for CBS versus CSE, while hydroxylamine was 60-fold more selective against CSE. Conclusions and Implications In conclusion, although PAG, AVG and BCA exhibit selectivity in inhibiting CSE versus CBS, no selective pharmacological CBS inhibitor is currently available.