Biosynthesis of a cholesterol-derived brassinosteroid, 28-norcastasterone, in <i>Arabidopsis thaliana</i>

다국어 초록 (Multilingual Abstract)

<P>A metabolic study revealed that 28-norcastasterone in <I>Arabidopsis</I> is synthesized from cholesterol via the late C-6 oxidation pathway. On the other hand, the early C-6 oxidation pathway was found to be interrupted because cholestanol is converted to 6-oxocholestanol, but further metabolism to 28-norcathasterone was not observed. The 6-oxoBRs were found to have been produced from the respective 6-deoxoBRs administered to the enzyme solution, thus indicating that these 6-oxoBRs are supplied from the late C-6 oxidation pathway. Heterologously expressed CYP85A1 and CYP85A2 in yeast catalysed this C-6 oxidation, with CYP85A2 being much more efficient than CYP85A1. Abnormal growth of <I>det2</I> and <I>dwf4</I> was restored via the application of 28-norcastasterone and closer precursors. Furthermore, <I>det2</I> and <I>dwf4</I> could not convert cholesterol to cholestanol and cholestanol to 6-deoxo-28-norcathasterone, respectively. It is, therefore, most likely that the same enzyme system is operant in the synthesis of both 28-norcastasterone and castasterone. In the presence of <I>S</I>-adenosyl-<SMALL>L</SMALL>-methionine, the cell-free enzyme extract catalysed the C-24 methylation of 28-norcastasterone to castasterone, although the conversion rates of 28-norteasterone to teasterone and 28-nortyphasterol to typhasterol were much lower; this suggests that 28-norcastasterone is the primary precursor for the generation of C<SUB>28</SUB>-BRs from C<SUB>27</SUB>-BRs.</P>