Natural low molecular mass organic compound with oxidase activity

Organocatalysts, low-molecular-mass organic compounds composed of nonmetallic elements, are often used in organic synthesis, but there have been no reports of organocatalysts of biological origin that function in vivo.  Here we report that actinorhodin (ACT), which is produced by an actinomycetes, Streptomyces coelicolor A3(2), is an organocatalyst.  During the screening of a new oxidase from streptomycetes, we found that supernatant of S. coelicolor A3(2) showed catalytic activity to oxidize l-ascorbic acid (L-ASC).  The oxidase could not be purified, however, a correlation between the depth of color and the oxidase activity was obtained.  Thus, we focus on ACT known as the most famous pigment produced by this strain.  Next, we discovered that the addition of purified ACT to the reaction mixture containing L-ASC as a substrate led to a decrease in the O₂ concentration.  Like L-ASC, some thiol compounds [e.g. L-cysteine] also acted as substrates.  By means of oxygen electrode and LC-MS/MS analyses, we carried out quantitative analysis of ACT in the reaction mixture before and after the oxidation reaction.  We also investigated some characteristics of ACT from enzymological standpoints.  As a result, a small amount of ACT oxidized an excess amount of substrate, and ACT was not consumed during the reaction.  These findings demonstrated that ACT is a “natural” organocatalyst and catalyzed the following oxidation reactions: L-ASC + O₂ -> L-dehydroascorbic acid + H₂O₂; and 2 L-cysteine + O₂ -> L-cystine + H₂O₂.