Stereo- and regioselective hydroxylation of α-ionone by Streptomyces strains

A total of 215 Streptomyces strains were screened for their capacity to regio- and stereoselectively hydroxylate β- and/or α-ionone to the respective 3-hydroxy derivatives. With β-ionone as the substrate, 15 strains showed little conversion to 4-hydroxy- and none showed conversion to the 3- hydroxy product as desired. Among these 15 Streptomyces strains, S. fradiae Tu 27, S. arenae Tu 495, S. griseus ATCC 13273, S. violaceoniger Tu 38, and S. antibioticus Tu 4 and Tu 46 converted α-ionone to 3-hydroxy-α-ionone with significantly higher hydroxylation activity compared to that of β- ionone. Hydroxylation of racemic α-ionone [(6R)-(-)/(6S)-(+)] resulted in the exclusive formation of only the two enantiomers (3R,6R)- and (3S,6S)- hydroxy-α-ionone. Thus, the enzymatic hydroxylation of α-ionone by the Streptomyces strains tested proceeds with both high regio- and stereoselectivity.