While an analogue of DHOCTO had been detected in an earlier study on deoxycholate degradation by another Pseudomonas sp. (Leppik, 1983), a structure similar to THOCDO has, to our knowledge, not been described in any study on bacterial degradation of bile salts. Within the shoulder tailing off from the DHOCTO peak (Fig. 4), LC-MS analysis revealed an ion [M+H]+ with m/z 403.24, which could be the monoene derivative of DHOCTO. With the transposon mutant strain R1, we

also observed the transient accumulation of the monoene derivative of DHOPDC in the culture supernatants (Birkenmaier et al., 2007). The compound P3, which formed the second largest peak in HPLC analysis, www.selleckchem.com/products/wnt-c59-c59.html coeluted with and had a UV spectrum very similar to that of the previously identified Δ1,4-3-ketocholate (X) from culture supernatants of the transposon mutant strain R1 (Birkenmaier et al., 2007). To characterize the mutant strain Chol1-KO[skt] further, it was tested for growth with intermediates of cholate degradation. Strain Chol1-KO[skt] could grow with DHADD (VIII) and THSATD (IX). Importantly, strain Chol1-KO[skt] could also grow with DHOPDC (XIII) that was provided with filter-sterilized supernatants of a culture of the acad mutant strain R1 that had been grown with succinate in the presence

of cholate as described previously (Birkenmaier et al., 2007). The growth of strain Chol1-KO[skt] with DHOPDC clearly showed that the skt-gene must be responsible for a reaction step preceding the formation of DHOPDC. The accumulation of DHOCTO and THOCDO supports this conclusion because progestogen antagonist both compounds could have arisen from hydrolyzed CoA-esters III and IV that are presumptive intermediates of β-oxidation of the acyl side chain of cholate (Fig. 1). Thus, the accumulation of DHOCTO and THOCDO indicates that at least the first

two steps of β-oxidation starting from Δ1,4-3-ketocholyl-CoA (II) could be catalyzed in the skt mutant. This narrowed the probable function of the skt-encoded protein down to being either a 3-hydroxy-acyl-CoA dehydrogenase or a β-ketothiolase. A closer analysis of the predicted protein reveals that Skt and its orthologs in other cholate-degrading bacteria (Fig. 2) have similarities to the β-ketothiolase domain Anidulafungin (LY303366) of eukaryotic sterol carrier protein SCP-x (Stolowich et al., 2002). SCP-x, which is also referred to as a nonspecific lipid transfer protein, is a fusion protein with a smaller C-terminal and a larger N-terminal domain. While the C-terminal domain (also called the SCP-2 domain) is responsible for intracellular targeting and the uptake of sterols, the N-terminal domain has 3-ketoacyl-CoA-thiolase activity for branched-chain-acyl-CoA esters. Interestingly, SCP-x is also responsible for the final step of cholate biosynthesis in mammals (Kannenberg et al., 1999; Russell, 2003).