C6 oxidation

Fig. (10). C6-oxidation of 6-deoxocastasterone, the proposed enzymatic reaction of CYP85.

Nevertheless, there are many questions still open because of problems to detect enzyme activities corresponding to each step of the pathway. The model of biosynthesis pathway was put together by studying the metabolism of exogenously applied intermediates in cell cultures of various origins and combining these results with data of native brassinosteroid patterns. It is more or less accepted that there are three pathways in parallel, the early and the late C6 oxidation pathway, as well as the 24/ -epimers follow ing the same route. Some observations in the analysis of native brassinosteroid patterns suggest a possible connection between the pathways. It was shown that seeds of Arabidopsis contain castasterone and 24-epi-brassinolide [34]. Also members of both 24-epimers, brassinolide and 24-epi-brassinolide were detected in tomato seeds [Winter, unpublished]. 

Side-chain oxidized derivatives of ascorbic acid are also implicated in the catabolism of ascorbic acid in plants. Loewus et al. (62) have established the intermediacy of ascorbic acid in the biosynthesis of tartaric acid in the grape. Labeling studies have established a metabolic pathway that must involve C5 and C6 oxidation of ascorbic acid. 

Fig. 3. Biosynthesis of brassinolide from campesterol. Brassinolide is synthesized from castasterone which is derived from campestanol by either late C6-oxidation pathway or early C6-oxidation pathway. Campesterol is converted to campestanol by hydrogenation. Italic letters refer to the lesions in the biosynthesis and sensitivity mutants of Arabidopsis det2, dwf4, cpd and bril) and pea (Ik and Ika). The asterisked pathways and compounds are hypothetical.

Typhasterol is then hydroxylated at C2 to give castasterone having a pair of vicinal hydroxyls which is very frequently and abundantly found in plants. The pathway from teasterone to castasterone was also confirmed in the seedlings of C. roseus, tobacco and rice, indicating that the early C6 oxidation pathway is operative in intact plants [49]. 

Two years later, a more elaborate reaction mechanism was proposed by Li and co-workers.This mechanism involved reduction and oxidation of copper on the enzyme itself and binding of oxygen with formation of several radicals, but ending in a Cl, C4 or C6 oxidized glucose molecule. Supporting evidence for this mechanism was obtained later. In the highly similar... 

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