3-Dehydro-teasterone

The next step in the proposed pathway is the epimerization of teasterone to typhasterol producing 3-dehydro-teasterone as intermediate (Fig. 8)). No mutant is thus far described being responsible for this conversion. Nevertheless, a corresponding enzyme activity has been detected in different plants. 

The first data confirming this oxidoreductive epimerization were obtained by measuring the enzyme activity in protein extracts from a tomato cell suspension [24], It could be demonstrated, using a very sensitive fluorimetric detection method, that two different enzymes were involved in this subpathway. No enzyme activity could be detected with 24-ep/-teasterone as substrate and NAD+ or NADP+ as electronacceptors. But using the proposed intermediate 3-dehydro-24-epi-teasterone as substrate, enzymatic conversion to 24-epi-teasterone was measured in a microsomal fraction of tomato cell cultures (Fig. (9)). 3-dehydro-24-epi-teasterone-reductase showed a specific activity of 361 fkat/mg protein with NADPH as the only accepted electrondonor. 

Interestingly, the conversion of 3-dehydro-24-e/ i-teasterone to 24-epi-teasterone follows the opposite direction of the proposed biosynthesis pathway. Feeding experiments support this observation Exogenously applied 3-dehydro-24-e/ /-teasterone was converted to 24-epj-teasterone as the major metabolite (95%), and to only traces (5%) of 24-epi-typhasterol [23]. Thus, it is very likely that there is a different pathway in tomato cell cultures. In addition, preliminary studies with protein extracts of Arabidopsis cell cultures also reveal differences in enzyme distribution and specificity depending on the origin of the culture [Winter, unpublished]. 

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