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

Abstract

BACKGROUND AND PURPOSE

Hydrogen sulfide (H ₂S) is a signalling molecule that belongs to the gasotransmitter family. Two major sources for endogenous enzymatic production of H ₂S are cystathionine β synthase (CBS) and cystathionine γ lyase (CSE). In the present study, we examined the selectivity of commonly used pharmacological inhibitors of H ₂S 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

β-cyanoalanine (BCA) was more potent in inhibiting CSE than propargylglycine (PAG) (IC ₅₀ 14 ± 0.2 μM vs. 40 ± 8 μM 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 (IC ₅₀ 1.1 ± 0.1 μM); the IC ₅₀ for AOAA for inhibiting CBS was 8.5 ± 0.7 μM. 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 H ₂S biosynthesis, blocked the activity of CBS and CSE. Trifluoroalanine had a fourfold lower IC ₅₀ 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.