Involvement of Cytochrome P450 in Glucosinolate Biosynthesis in White Mustard (A Biochemical Anomaly)

One of the first steps in glucosinolate biosynthesis is the conversion of amino acids to their aldoximes. The biochemistry of this process is controversial, and several very different enzyme systems have been described. The major glucosinolate in white mustard (Sinapis alba) is sinalbin, which is derived from tyrosine via its aldoxime, and this conversion is catalyzed by a cytochrome P450 (Cyt P450) monooxygenase. Phenylethyl- and alkenylglucosinolates are also present in white mustard leaves, as are the enzymes catalyzing the relevant aldoxime formation from homophenylalanine and methionine homologs, respectively. These enzymes are similar to those found in Brassica sp. and are distinct from the tyrosine-dependent enzyme in that they contain no heme and are unaffected by Cyt P450 inhibitors. They are instead inhibited by the flavoprotein inhibitor diphenylene iodonium and by Cu2+. In both white mustard and oilseed rape (Brassica napus) methyl jasmonate specifically stimulates indolylglucosinolate biosynthesis and yet has no effect on sinalbin accumulation in either cotyledons or leaves of white mustard. White mustard appears to be unique among crucifers in having a Cyt P450 aldoxime-forming enzyme for biosynthesis of one glucosinolate, although it also contains all of the non-Cyt P450 enzyme systems found in other members of the family. Sinalbin biosynthesis in white mustard is therefore an inappropriate model system for the synthesis of other glucosinolates in crucifers, including canola and oilseed rape.