7β-hydroxysteroid dehydratase Hsh3 eliminates the 7-hydroxy group of the bile salt ursodeoxycholate during degradation by Sphingobium sp. strain Chol11 and other Sphingomonadaceae

Bile salts are steroids with distinctive hydroxylation patterns that are produced and excreted by vertebrates. In contrast to common human bile salts, ursodeoxycholate (UDCA) has a 7-hydroxy group in {beta}-configuration and is used in large amounts for treatment of diverse gastrointestinal diseases. We isolated the 7{beta}-hydroxysteroid dehydratase Hsh3 that is involved in UDCA degradation by Sphingobium sp. strain Chol11. Hsh3 eliminates the 7{beta}-hydroxy group as water, leading to a double bond in the B-ring. This is similar to 7-hydroxysteroid dehydratases in this and other strains, but Hsh3 is evolutionary different from these. Purified Hsh3 accepted steroids with and without side chain as substrates and had minor activity with 7-hydroxy groups. The deletion mutant strain Chol11 {Delta}hsh3 had impacted growth with UDCA and accumulated a novel compound. The compound was identified as 3,5-dihydroxy-H-methyl-hexahydro-5-indene-1-one-propanoate, consisting of steroid rings C and D with a C3-side chain carrying the former 7{beta}-hydroxy group, indicating that Hsh3 activity is important especially for the later stages of bile salt-degradation. Hsh3 homologs were found in other sphingomonads that were also able to degrade UDCA as well as in environmental metagenomes. Thus, Hsh3 adds to the bacterial enzyme repertoire for degrading a variety of differently hydroxylated bile-salts. ImportanceThe bacterial degradation of different bile salts is not only important for the removal of these steroidal compounds from the environment but also harbors interesting enzymes for steroid biotechnology. The 7{beta}-hydroxy bile salt UDCA naturally occurs in high concentration in the feces of black bears and has important pharmaceutical relevance for the treatment of different liver-related diseases, for which it is administered in high and increasing amounts. Additionally, it is present in the bile salt pool of humans in trace amounts. While ursodeoxycholate degradation is environmentally important, the enzyme Hsh3 modifies the hydroxy group that confers the medically relevant properties and thus might be interesting for microbiome analyses and biotechnological applications.