胆汁酸生合成における炭素-炭素結合切断反応に関する研究

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Cholic acid biosynthesis from cholesterol involves C-24/C-25 bond cleavage.A precursor of this carbon-carbon bond cleavage reaction is 3α, 7α, 12α-trihydroxycoprostan-26-oic acid(THCA) and this acid is thought to be metabolized into cholic acid by a mechanism similar to that of the β-oxidation of fatty acids. Among the intermediates, experimental evidence for the intermediary role of THCA-CoA, 24-ene-THCA-CoA and 24-hydroxy-THCA-CoA has been accumulated. In contrast, little has been known on the formation and metabolism of 24-oxo-THCA-CoA. Our efforts on this line are described here.The potential substrates including the three THCA-CoA derivatives mentioned above were chemically synthesized, and incubated with rat liver light mitochondria fraction or Hep G2. 24-Oxo-THCA-CoA, if produced during incubation, should be transformed into 27-nor-3α, 7α, 12α-trihydroxy-24-coprostan-24-on ("ethyl ketone") by the subsequent alkaline treatment inducing facile decarboxylation of the intermediate β-keto-carboxylic acid. GC-MS analysis of the trimethylsilyl ether derivatives coming from THCA-CoA, 24-ene-THCA-CoA and 24- hydroxy-THCA CoA clearly detected the "ethyl ketone". These results indicated the enzymic transformation of these three CoA-esters into 24-oxo-THCA-CoA.Similarly 24-oxo-THCA-CoA was incubated and the subsequent HPLC analysis of the p-bromophenacyl ester derivative indicated the enzymic formation of cholic acid(or cholyl-CoA from 24-oxo-THCA-CoATaken together, it is concluded that 24-oxo-THCA-CoA is the final intermediate of cholic acid biosynthesis.