Introduction to methods in invertebrate muscle biology

Biogenesis of Hydrogen Sulfide and Thioethers by Cystathionine Beta-Synthase

AIMS

The transsulfuration pathway enzymes cystathionine beta-synthase (CBS) and cystathionine gamma-lyase are thought to be the major source of hydrogen sulfide (H₂S). In this study, we assessed the role of CBS in H₂S biogenesis.

RESULTS

We show that despite discouraging enzyme kinetics of alternative H₂S-producing reactions utilizing cysteine compared with the canonical condensation of serine and homocysteine, our simulations of substrate competitions at biologically relevant conditions suggest that cysteine is able to partially compete with serine on CBS, thus leading to generation of appreciable amounts of H₂S. The leading H₂S-producing reaction is condensation of cysteine with homocysteine, while cysteine desulfuration plays a dominant role when cysteine is more abundant than serine and homocysteine is limited. We found that the serine-to-cysteine ratio is the main determinant of CBS H₂S productivity. Abundance of cysteine over serine, for example, in plasma, allowed for up to 43% of CBS activity being responsible for H₂S production, while excess of serine typical for intracellular levels effectively limited such activity to less than 1.5%. CBS also produced lanthionine from serine and cysteine and a third of lanthionine coming from condensation of two cysteines contributed to the H₂S pool.

INNOVATION

Our study characterizes the H₂S-producing potential of CBS under biologically relevant conditions and highlights the serine-to-cysteine ratio as the main determinant of H₂S production by CBS in vivo.

CONCLUSION

Our data clarify the function of CBS in H₂S biogenesis and the role of thioethers as surrogate H₂S markers. Antioxid. Redox Signal. 28, 311-323.

Hydrogen sulfide: redox metabolism and signaling

The recognition of hydrogen sulfide (H(2)S) as an endogenously produced gas with signaling potential has stimulated research on a multitude of physiological effects mediated in the cardiovascular, immune, gastrointestinal, genitourinary, endocrine, and central nervous systems. The heightened activity in the area of H(2)S biology led to convening of the first international conference on H(2)S in Shanghai in the summer of 2009 and to two Forum issues published in 2010 by Antioxidants & Redox Signaling on the physiological effects of H(2)S. Yet, fundamental questions regarding the biogenesis and regulation of H(2)S, the bioenergetics of its catabolism, its tissue concentrations, and elucidation of its molecular targets remain. Some of these issues are the subject of the current Forum on H(2)S.

The role of cystathionine beta-synthase in homocysteine metabolism

Cystathionine beta-synthase (CBS) is the first enzyme in the transsulfuration pathway, catalyzing the conversion of serine and homocysteine to cystathionine and water. The enzyme contains three functional domains. The middle domain contains the catalytic core, which is responsible for the pyridoxal phosphate-catalyzed reaction. The C-terminal domain contains a negative regulatory region that is responsible for allosteric activation of the enzyme by S-adenosylmethionine. The N-terminal domain contains heme, and this domain regulates the enzyme in response to redox conditions. Besides its canonical reaction, CBS can catalyze alternative reactions that produce hydrogen sulfide, a novel neuromodulator in the brain. Mutations in human CBS result in homocystinuria, an autosomal recessive disorder characterized by defects in a variety of different organ systems. The most common CBS allele is 833T>C (I278T), which is associated with pyridoxine-responsive homocystinuria. A complementation system in S. cerevisiae has been developed for analysis of human CBS mutations. Using this system, it has been discovered that deletion of the C-terminal domain of CBS can suppress the functional defects of many patient-derived mutations. This finding suggests it may be possible to develop drugs that interact with the C-terminal domain of CBS to treat elevated homocysteine in humans.

Sulfur-containing cyclic ketimines and imino acids. A novel family of endogenous products in the search for a role

Aminoethylcysteine, lanthionine, cystathionine and cystine are mono-deaminated either by L-amino-acid oxidase or by a transaminase exhibiting the properties described for glutamine transaminase. The deaminated products cyclize producing the respective ketimines. Authentic samples of each ketimine were prepared by reacting the appropriate aminothiol compound with bromopyruvate, except cystine ketimine which required the interaction of thiopyruvate with cystine sulfoxide. Reduction of the first three mentioned ketimines with NaBH4 yields the respective derivatives with the saturated rings of thiomorpholine and hexahydrothiazepine. The same reduction is carried out enzymically by a reductase extracted from mammalian tissues. Properties of the members of this family of compounds are described. Gas chromatography followed by mass spectrometry permits the identification of most of these products. HPLC is very useful for the determination of the ketimines by taking advantage of specific absorbance at 380 nm obtained by prior derivatization with phenylisothiocyanate. Adaptation of these and other analytical procedures to biological samples disclosed the presence of most of these compounds in bovine brain and in human urine. By using [35S]lanthionine ketimine as a representative member of the ketimine group, the specific, high-affinity, saturable and reversible binding to bovine brain membranes has been demonstrated. The binding is removed by aminoethylcysteine ketimine and by cystathionine ketimine indicating the occurrence in bovine brain of a common binding site for ketimines. The reduced ketimines are totally ineffective in competing with [35S]lanthionine ketimine. Alltogether these findings are highly indicative for the existence in mammals of a novel class of endogenous sulfur-containing cyclic products provided with a possible neurochemical function to be investigated further.

Cystathionase: high-performance liquid chromatography. Molecular cloning in lambda gt11. Nonradioactive immunodetection of fusion protein

A method of purification of rat liver cystathionase by high-performance liquid chromatography (HPLC) utilizing non-ideal gel filtration method is proposed. Resolution factors-flow rate, pH values, ionic strength of the mobile phase-were optimized. Antibodies to the enzyme were purified using an immunosorbent synthesized on the basis of epoxylated Toyopearl-65. Radioimmunoassay and immunoblotting demonstrated antibody monospecificity towards cystathionase. These monospecific antibodies were utilized for detecting enzyme amounts (up to 30 pg) using the avidin-biotin system. Rat cDNA expression library in phage lambda gt11 was screened. The cystathionase cDNA clone was isolated, and the structure of the insert was determined.

Methionine biosynthesis in Enterobacteriaceae: biochemical, regulatory, and evolutionary aspects

The genes coding for the enzymes involved in methionine biosynthesis and regulation are scattered on the Escherichia coli chromosome. All of them have been cloned and most have been sequenced. From the information gathered, one can establish the existence (upstream of the structural genes coding for the biosynthetic genes and the regulatory gene) of "methionine boxes" consisting of two or more repeats of an octanucleotide sequence pattern. The comparison of these sequences allows the extraction of a consensus operator sequence. Mutations in these sequences lead to the constitutivity of the vicinal structural gene. The operator sequence is the target of a DNA-binding protein--the methionine aporepressor--which has been obtained in the pure state, for which S-adenosylmethionine acts as the corepressor. Mutations in the corresponding gene lead to the constitutive expression of all the methionine structural genes. The physicochemical properties of the methionine aporepressor are being investigated.