Submitted to: Phytochemical Society of North America Meeting and Newsletter
Publication Type: Abstract Only
Publication Acceptance Date: 7/9/2007
Publication Date: 7/21/2007
Citation: Liu, J., Stipanovic, R.D., Puckhaber, L.S., Bell, A.A. 2007. Stereospecific oxydative coupling of hemigosypol to form (+)-gossypol in moco cotton is controlled by a dirigent protein [abstract]. Phytochemical Society of North America Meeting and Newsletter. p. 57.
Technical Abstract: Gossypol is biosynthesized by the free radical coupling of two molecules of hemigossypol to yield two optically active enantiomers, (+)-gossypol and (-)-gossypol. Most commercial cottonseeds have a (+) to (-)-gossypol ratio of about 3:2. However, this ratio can be as high as 98:2 in moco cotton. Cell free extracts of moco cotton tissue preferentially converted hemigossypol into (+)-gossypol (74% (+)-gossypol vs. 26% (-)-gossypol). Addition of hydrogen peroxide in the assay produced four fold more gossypol and also increased (+)-gossypol percentage to 79 percent. Adding exogenous oxidases like peroxidase or laccase increased gossypol yield but did not decrease the (+)-gossypol percentage. Exogenous oxidases themselves produce a racemic mixture of gossypol from hemigossypol. These results are consistent with the involvement of dirigent protein in the stereospecific coupling of hemigossypol for the formation of (+)-gossypol and are inconsistent with involvement of a stereospecific oxidase. The peroxidase activity of the crude extract is mainly responsible for the generation of hemigossypol free radicals in cotton. Partial purification of the crude moco cotton extract yielded a protein preparation that does not possess any catalytic ability to generate the free radicals and is unable to convert hemigossypol to gossypol but nevertheless guided the stereospecific coupling of hemigossypol free radicals generated by a cotton peroxidase(s) for (+)-gossypol formation in moco cotton. This partially purified (+)-gossypol-forming dirigent protein had two native molecular weights of 77.2 and 122.5 kDa probably corresponding to a monomer and dimmer, respectively.