Cystathionine beta-Synthase as a Carbon Monoxide-Sensitive Regulator of Bile Excretion

Carbon monoxide (CO) is a stress-inducible gas generated by heme oxygenase (HO) eliciting adaptive responses against toxicants; however, mechanisms for its reception remain unknown. Serendipitous observation in metabolome analysis in CO-overproducing livers suggested roles of cystathionine beta-synthase (CBS) that rate-limits transsulfuration pathway and H2S generation, for the gas-responsive receptor. Studies using recombinant CBS indicated that CO binds to the prosthetic heme, stabilizing 6-coordinated CO-Fe(II)-histidine complex to block the activity, whereas nitric oxide (NO) forms 5-coordinated structure without inhibiting it. The CO-overproducing livers down-regulated H2S to stimulate HCO3--dependent choleresis: these responses were attenuated by blocking HO or by donating H2S. Livers of heterozygous CBS knockout mice neither down-regulated H2S nor exhibited the choleresis while overproducing CO. In the mouse model of estradiol-induced cholestasis, CO overproduction by inducing HO-1 significantly improved the bile output through stimulating HCO3- excretion; such a choleretic response did not occur in the knockout mice. Conclusion: Results collected from metabolome analyses suggested that CBS serves as a CO-sensitive modulator of H2S to support biliary excretion, shedding light on a putative role of the enzyme for stress-elicited adaptive response against bile-dependent detoxification processes. (HEPATOLOGY 2009;49:141-150.)