The enzymes of the transsulfuration pathway, cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE), are important for the endogenous production of hydrogen sulfide (H(2)S), a gaseous signaling molecule. The relative contributions of CBS and CSE to H(2)S generation in different tissues are not known. In this study, we report quantification of CBS and CSE in murine liver and kidney and their contribution to H(2)S generation in these tissues and in brain at saturating substrate concentrations. We show that CBS protein levels are significantly lower than those of CSE; 60-fold and 20-fold in liver and kidney, respectively. Each enzyme is more abundant in liver compared with kidney, twofold and sixfold for CBS and CSE, respectively. At high substrate concentrations (20 mM each cysteine and homocysteine), the capacity for liver H(2)S production is approximately equal for CBS and CSE, whereas in kidney and brain, CBS constitutes the major source of H(2)S, accounting for ∼80% and ∼95%, respectively, of the total output. At physiologically relevant concentrations of substrate, and adjusting for the differences in CBS versus CSE levels, we estimate that CBS accounts for only 3% of H(2)S production by the transsulfuration pathway enzymes in liver.

Background: Long range allosteric communication between the heme and PLP cofactors occurs in human cystathionine ␤-synthase. Results: Seven mutations in the PLP pocket were studied, including one described in homocystinuric patients. Conclusion: The mutations perturb the heme electronic environment 20 Å away and increase propensity for forming an inactive species. Significance: Bidirectional communication between heme and PLP occurs via an ␣-helix; its disruption is associated with disease.

Hydrogen sulfide (H2S) is a gaseous signaling molecule that participates in various physiological processes such as regulation of blood pressure. Cystathionine beta‐synthase (CBS) and gamma‐cystathioninase (CSE) are the two enzymes that are responsible for the formation of H2S in the transsulfuration pathway. Quantitative Western blot analysis was used to determine the relative abundance of each enzyme in murine liver and kidney. In this study, we report quantification of CBS and CSE in murine liver and kidney and their contribution to H2S generation in these tissues and in brain. Our results show that the level of CBS protein is significantly lower than CSE; 60‐fold and 20‐fold in liver and kidney respectively. Levels of both enzymes are higher in liver compared to kidney, 2‐fold and 6‐fold for CBS and CSE respectively. At high substrate concentrations (20 mM each cysteine and homocysteine), CBS and CSE contribute equally to H2S production in liver extract while CBS constitutes the major source of H2S in kidney and brain with CSE contributing approximately 20% and 6% respectively. Determination of tissue levels of these enzymes is important for therapeutic reasons that aim to specifically modulate H2S synthesis in tissue‐specific manner.