Brassinosteroid metabolites

H and chemical shifts of brassinosteroid metabolites are shown in Tables 4 and 5, respectively. 

Brassinosteroids (BRs) are hydroxylated derivatives of cholestane and their structure variations comprise substitutions pattern on ring A, B and the C-17 side chain (Fig. 6.10). The BRs are classified as C27, C28/ or C29 BRs, depending on the substitutions and the length of the side chain. More than 70 BRs as well as more than 42 BR metabolites have been isolated and identified (Bajguz and Tretyn, 2003). 

Microbial Transformation of Brassinosteroids. Until now, information concerning the microbial transformations of brassinosteroids has not been published. Such biotransformations could open novel pathways to additionally functionalized members for structure-activity investigations as well as provide information about possible metabolic processes of such compounds. Furthermore, the intermediate metabolites may have practical application. 

The rice lamina inclination assay is very sensitive to brassinosteroids (11). In the second set of studies, we examined the metabolism of radioactive castasterone in the rice lamina assay (Yokota, T. et. al., unpublished data). The fate of tritiated castasterone was monitored for 72 hr. During incubation, again brassinolide was not detected. However, polar metabolites accumulated and the amount increased during 72 hr. (Figure 7). The polar metabolites seemed not to be changed after hydrolysis using either enzyme, hydrochloric acid or sodium hydroxide. 

The results of the rice lamina inclination test (RLIT) indicated an extraordinary high activity of 25-hydroxy-24-epibrassinolide (90). This compound is about ten times more active than 24-epibrassinolide (12), indicating that the hydroxylation at C-25 is an activating step in the brassinosteroid metabolism. Therefore, 25-hydroxy-24-epibrassinolide (90) is, next to brassinolide (1), one of the most active brassinosteroids known until now. In comparison with 25-hydroxy-24-epibrassinolide (90), the 26-hydroxylated metabolite (91) was clearly less active. As in other groups of steroidal hormones, for instance vitamine D metabolites, hydroxylation at C-25 seems to be essential for high activity. ... 

Two compounds common in plant metabolism are believed to be precursors of isoprenoid cytokinins in plants adenosine-5 -monophosphate (AMP) and A -isopentenylpyrophos-phate (iPP). As a final product of the mevalonate pathway, the latter substance serves also as a precursor for a wide spectrum of metabolites including some other plant hormones, as abscisic acid, gibberellins and brassinosteroids. The hypothetical scheme of reactions resulting in the formation of iPA, Z and DHZ is given in Fig. 2. The enzyme of entry into isoprenoid cytokinin formation is A -isopentenylpyrophosphate 5 -AMP-A -iso-pentenyltransferase (EC 2.5.1.8, trivially named cytokinin synthetase ). This enzyme activity was first detected in a cell-free preparation from the slime mould Dictyostelium discoideum [7,8]. Later the enzyme from higher plants (cytokinin-independent tobacco callus [9,10] and immature Zea mays kernels [11]) was described and the data were recently summarised in [12], The enzyme is very specific as far as the substrate is concerned [13,14] only the nucleotide AMP can be converted and only iPP (with a double bond in A position) may function as a side chain donor. 

Brassinosteroids represent a class of phytohormones and are widespread in the plant kingdom. However, there are only a few reports on these metabolites in the two large Solanales families. Though their physiological effects on plants are not yet fully understood, it is obvious that they have specific effects on plant growth and development. Furthermore, they may protect plants from drought, extreme temperatures, heavy metals, salinity, and herbicidal injury. Bajguz and Tretyn... 

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