Cystathionine-β-synthase (CBS), the first (and rate-limiting) enzyme in the transsulfuration pathway, is an important mammalian enzyme in health and disease. Its biochemical functions under physiological conditions include the metabolism of homocysteine (a cytotoxic molecule and cardiovascular risk factor) and the generation of hydrogen sulfide (H₂S), a gaseous biological mediator with multiple regulatory roles in the vascular, nervous, and immune system. CBS is up-regulated in several diseases, including Down syndrome and many forms of cancer; in these conditions, the preclinical data indicate that inhibition or inactivation of CBS exerts beneficial effects. This article overviews the current information on the expression, tissue distribution, physiological roles, and biochemistry of CBS, followed by a comprehensive overview of direct and indirect approaches to inhibit the enzyme. Among the small-molecule CBS inhibitors, the review highlights the specificity and selectivity problems related to many of the commonly used “CBS inhibitors” (e.g., aminooxyacetic acid) and provides a comprehensive review of their pharmacological actions under physiological conditions and in various disease models.

Please cite this paper as: Effect of honey and its major royal jelly protein 1 on cytokine and MMP‐9 mRNA transcripts in human keratinocytes. Experimental Dermatology 2010; 19: e73–e79.Abstract:  Honey has been used since ancient times as a remedy in wound healing. However, even though the results from randomized clinical trials document that honey accelerates wound healing, no study dealing with its influence on human skin cells (epidermal keratinocytes and dermal fibroblast) has been performed. We demonstrate that keratinocytes, which are known to be involved in wound healing, are responsible for elevated production of mediators including cytokines (TNF‐α, IL‐1β and TGF‐β) and matrix metalloproteinase‐9 (MMP‐9) after incubation with honey. Real‐time PCR was performed for the quantification of mRNA level of selected cytokines and MMP‐9. Furthermore, we show that the increased level of MMP‐9 in the epidermis following incubation with honey leads to degradation of type IV collagen in the basement membrane. These data indisputably demonstrate that honey activates keratinocytes and support the findings that honey may accelerate wound healing process.

Significance: Cystathionine β-synthase (CBS), the pivotal enzyme of the transsulfuration pathway, regulates flux through the pathway to yield compounds, such as cysteine, glutathione, taurine, and H ₂ S, that control cellular redox status and signaling. Our crystal structure of an engineered human CBS construct bound to S -adenosylmethionine (AdoMet) reveals the unique binding site of the allosteric activator and the architecture of the human CBS enzyme in its activated conformation. Together with the basal conformation that we reported earlier, these structures unravel the molecular mechanism of human CBS activation by AdoMet. Current knowledge will allow for modeling of numerous pathogenic mutations causing inherited homocystinuria and for design of compounds modulating CBS activity.

Significance: Cystathionine β-synthase (CBS), the pivotal enzyme of the transsulfuration pathway, regulates the flux through the pathway to yield compounds such as cysteine, glutathione, taurine, and H ₂ S that control the cellular redox status and signaling. Our crystal structures of the full-length wild-type and D444N mutant human CBS enzymes show a unique arrangement of the regulatory CBS motifs, thus making it possible to infer how the enzyme is stimulated by its allosteric activator S-adenosyl-L-methionine and how native tetramers are formed. The structure will allow modeling of numerous mutations causing inherited homocystinuria and the design of compounds modulating CBS activity.

CBS (cystathionine β-synthase) is a multidomain tetrameric enzyme essential in the regulation of homocysteine metabolism, whose activity is enhanced by the allosteric regulator SAM (S-adenosylmethionine). Missense mutations in CBS are the major cause of inherited HCU (homocystinuria). In the present study we apply a novel approach based on a combination of calorimetric methods, functional assays and kinetic modelling to provide structural and energetic insight into the effects of SAM on the stability and activity of WT (wild-type) CBS and seven HCU-causing mutants. We found two sets of SAM-binding sites in the C-terminal regulatory domain with different structural and energetic features: a high affinity set of two sites, probably involved in kinetic stabilization of the regulatory domain, and a low affinity set of four sites, which are involved in the enzyme activation. We show that the regulatory domain displays a low kinetic stability in WT CBS, which is further decreased in many HCU-causing mutants. We propose that the SAM-induced stabilization may play a key role in modulating steady-state levels of WT and mutant CBS in vivo. Our strategy may be valuable for understanding ligand effects on proteins with a complex architecture and their role in human genetic diseases and for the development of novel pharmacological strategies.