Читать книгу Gas Biology Research in Clinical Practice - Группа авторов - Страница 12

Because of the nature of the gas to bind to the metal-centered prosthetic groups of macromolecules, it is not unreasonable to hypothesize that biological gases bind to enzymes in metabolic systems that constitute a major class of such proteins. Based on this assumption, we have recently applied metabolomic analyses assisted by capillary electrophoresis combined with mass spectrometry (CE-MS) in order to mine footprints of the gases of interest on alterations in small molecular metabolites in the metabolic systems [13, 14]. In recent studies, we examined the effects of CO on the metabolic systems using several different experimental models where the gas was upregulated significantly. Results suggested that CO has the ability to inhibit the transsulfuration pathway that is rate-limited by cystathionine β-synthase (CBS), a heme-containing enzyme [14].

Based on these data, we examined the roles of the enzyme for a CO-specific receptor candidate in vivo. Several lines of biochemical evidence support the concept that CBS acts as a CO sensor. First, studies using recombinant CBS have shown that CO inhibits CBS with a K_i value of approximately 5 μmol/l [15], being comparable to the concentration occurring in the liver. Second, murine hepatocytes express both CO-producing HO and H₂S-producing CBS with additional HO-1 induced in both hepatocytes and Kupffer cells under stress conditions. The close proximity of the enzyme distributions taken together with measured CO concentrations in the models, and the kinetics of CBS activity led us to hypothesize that CBS is acting as a CO sensor in vivo. In vivo pulse-chase analyses suggest that CBS is the enzyme that actually modulates the metabolic flux of this pathway, as judged so far from the results collected by ¹⁵N-methionine flux analyses using CE-MS [14]. To note is that CBS is the enzyme that generates H₂S. CO-overproducing livers showed a decrease in labile H₂S amount, whereas the livers of heterozygous CBS knockout mice did not show any notable decrease in H₂S in response to stress-inducible levels of CO, suggesting that the gas inhibits the activity of CBS in vivo. Such a stress-inducible suppression of H₂S in the liver stimulates -dependent choleresis that helps the solubility of organic anions in bile [14]. Mechanisms by which H₂S modulates biliary excretion might involve glibenclamide-sensitive Na⁺-K⁺-2Cl^- channels in the biliary system, although whether the gas might directly bind to the channel remains unknown.

We believe that the gas involving CO has multiple modes of biological actions through varied specific receptors. CBS appears to be one of such multiple receptors that regulate organ functions. In order to mine novel receptors that have not yet been identified, further investigation with different technological approaches should obviously be necessary.