Formation of benzoquinol moiety in cornoside by salidroside mono-oxygenase, a cytochrome P450 enzyme, from Abeliophyllum distichum cell suspension cultures

Abstract.

A microsomal fraction prepared from Abeliophyllum distichum Nakai (Oleaceae) cell suspension cultures oxidized salidroside, a glucoside of 4-hydroxyphenylethyl alcohol, to cornoside possessing a unique benzoquinol ring. The enzyme named salidroside mono-oxygenase required NADPH as the only cofactor, and molecular oxygen. The reaction was strongly inhibited by CO as well as several cytochrome P450 inhibitors, such as cytochrome c and miconazole, indicating the involvement of a cytochrome P450 enzyme. Salidroside mono-oxygenase accepted salidroside as the only substrate, but did not oxidize 4-hydroxyphenylethyl alcohol, the salidroside aglucone, and 4-hydroxybenzoic acid. The optimum pH of the reaction was 7.5, and apparent K _m values for salidroside and NADPH were 44 µM and 33 µM, respectively. The benzoquinol ring formation mechanism is discussed in comparison to the mechanism for ipso substitution of 4-hydroxybenzoate by active oxygen species followed by elimination leading to hydroquinone.