Studies on biosynthesis of okaramines, insecticidal indole alkaloids.

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An isolate Penicillium simplicissimum ATCC 90288 (formerly AK-40) produces okaramines A and B when grown on okara. They are indole alkaloids with unique structures which exhibit insecticidal activity against third instar larvae of silkworm uopn oral administration. Our continuous efforts to find new okaramine congeners to elucidate the structre-activity relationship and also to enable the biosynthesis of okaramines have led to the isolation of okaramines D,E,and F.Their chemical structures were determined to be new congeners of okaramine.Okaramines seem to be derived from two moles of tryptophan and two mole of isoprene. In order to clarify this hypothesis, we tried feeding experiment in solid culture. Unfortunately we found that no DL-tryptophan-d_8 was incorporated into okaramines. [2-^<13>C] -Acetic acid was not incorporated too.One of the reasons why the compounds, which are considered to be precursors of okaramines, were not incorporated into okaramines is the condition of cultureing the fungus. We searched for another cultural condition, liquid culture. A strain ATCC 90288 showed a good growth in a static liquid culture. Czapek, Sabourad, malt extract, maltyeast extract, and potato-glucose media were used, showing that malt extract and potatoglucose media were good for the growth. The amount of okaramines were measeured as insecticidal activity, indicating that the productivity was poor. As a next trial we supplemented the okara extract to the media. It is of interest that the strain showed better growth when the extract of okara was added to the medium. Okaramines were produced even in a Czapek or Sabouraud media, semi-synthetic media, when the extract of okara being added. Feeding experiment will be carried out using this liquid culture system.