2-nitro-3-(p-hydroxyphenyl)propionate and aci-1-nitro-2-(p-hydroxyphenyl)ethane, two intermediates in the biosynthesis of the cyanogenic glucoside dhurrin in Sorghum bicolor (L.) Moench.

Abstract

The biosynthetic pathway for the cyanogenic glucoside dhurrin derived from tyrosine has been studied in vitro by using ['80]

The bifurcation of the cyanogenic glucoside and glucosinolate biosynthetic pathways

“…The current study advances our mechanistic understanding of the reactions catalyzed by CYP79A1 and CYP71E1 with respect to the structure of geometrical isomer of the p-hydroxyphenylacetaldoxime produced. In previous studies, it was demonstrated by radiolabeling experiments that the dhurrin pathway included both the E and Z geometrical oxime isomers as obligatory intermediates and that E-oxime was produced first and subsequently converted into the Z-oxime isomer (Halkier et al, 1991). However, in studies using isolated CYP79A1 reconstituted in liposomes, E-as well as Z-oxime were observed as products (Halkier et al, 1989(Halkier et al, , 1991.…”

“…In previous studies, it was demonstrated by radiolabeling experiments that the dhurrin pathway included both the E and Z geometrical oxime isomers as obligatory intermediates and that E-oxime was produced first and subsequently converted into the Z-oxime isomer (Halkier et al, 1991). However, in studies using isolated CYP79A1 reconstituted in liposomes, E-as well as Z-oxime were observed as products (Halkier et al, 1989(Halkier et al, , 1991. In the present study, improved technologies and faster experimental conditions enabled us to demonstrate that the CYP79A1 enzyme specifically produces the E structural isomer, although a minute amount of the Z structural isomer was always detected (Figures 3 and 4).…”

“…This offers a mechanism explaining why all hitherto isolated glucosinolates have retained the same fixed stereochemical configuration of their characteristic oxime function. In cyanogenic glucoside synthesis, the catalytic reactions catalyzed by CYP79A1 also includes a symmetrical functional group, the N,N-dihydroxy group in the N,N-dihydroxytyrosine intermediate (Halkier et al, 1991). The functionality of the two sequentially introduced hydroxyl groups would at first hand be expected to be identical.…”

“…The functionality of the two sequentially introduced hydroxyl groups would at first hand be expected to be identical. Nevertheless, in the conversion of N,N-dihydroxytyrosine into p-hydroxyphenylacetaldoxime, biosynthetic experiments using 18 O 2 and tyrosine and N-hydroxytyrosine as substrates demonstrated that the oxygen atom initially introduced in the conversion of tyrosine into N-hydroxytyrosine is specifically lost (Halkier et al, 1991). This shows that all intermediates involved have been retained, bound in the active site of CYP79A1, preventing free rotation around the C-N single bond in the course of the entire reaction sequence.…”

Synthesis of Benzylglucosinolate in Tropaeolum majus L. (Isothiocyanates as Potent Enzyme Inhibitors)

“…In studies of the biosynthesis of cyanogenic glycosides, in vitro experiments in which microsomal enzyme preparations are used have contributed greatly to the characterization of the intermediates involved (Mdler and Conn, 1979;Halkier and Mdler, 1989;Halkier et al, 1989Halkier et al, , 1991Conn, 1991). In view of this, it would obviously be desirable to obtain a similar microsomal enzyme system catalyzing the biosynthesis of glucosinolates.…”