The biosynthesis of cyanogenic glucosides in higher plants. Identification of three hydroxylation steps in the biosynthesis of dhurrin in Sorghum bicolor (L.) Moench and the involvement of 1-ACI-nitro-2-(p-hydroxyphenyl)ethane as an intermediate.

Abstract

N-Hydroxytyrosine, (E)- and (Z)-p-hydroxyphenyl-acetaldehyde oxime, p-hydroxyphenylacetonitrile, and p-hydroxymandelonitrile are established intermediates in the biosynthesis of the tyrosine-derived cyanogenic glucoside dhurrin (Halkier, B. A., Olsen, C. E., and Møller, B. L. (1989) J. Biol. Chem. 264, 19487-19494. Simultaneous measurements of oxygen consumption and biosynthetic activity using a microsomal enzyme system isolated from etiolated sorghum seedlings demonstrate a requirement for three oxygen molecules in the conversion of tyrosine to p-hydroxymandelonitrile. Two oxygen molecules are consumed in the conversion of tyrosine to (E)-p-hydroxyphenylacetaldehyde oxime, indicating the existence of a previously undetected hydroxylation step in addition to that resulting in the formation of N-hydroxytyrosine. Radioactively labeled 1-nitro-2-(p-hydroxyphenyl)ethane was chemically synthesized and tested as a possible intermediate. Biosynthetic experiments demonstrate that the microsomal enzyme system metabolizes the nitro compound to the subsequent intermediates in dhurrin synthesis (Km = 0.05 mM; Vmax = 14 nmol/mg of protein/h). Low amounts of 1-nitro-2-(p-hydroxyphenyl)ethane are produced in the microsomal reaction mixtures when tyrosine is used as substrate. These data support the involvement of 1-nitro-2-(p-hydroxyphenyl)ethane or more likely its aci-nitro tautomer as an intermediate between N-hydroxytyrosine and p-hydroxyphenylacetaldehyde oxime. The conversion of (E)-p-hydroxyphenylacetaldehydeoxime to p-hydroxymandelonitrile requires a single oxygen molecule. The oxygen molecule is utilized for hydroxylation of p-hydroxyphenylacetonitrile into p-hydroxymandelonitrile. This indicates that the conversion of p-hydroxyphenylacetaldehyde oxime into p-hydroxyphenylacetonitrile proceeds by a simple dehydration reaction.

• Authors: B A Halkier, B L Møller
• Journal: The Journal of biological chemistry (J Biol Chem) Vol. 265 Issue 34 Pg. 21114-21 (Dec 05 1990) ISSN: 0021-9258 [Print] United States
• PMID: 2250015 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't)

Chemical References

• Carbon Radioisotopes
• Glucosides
• Nitriles
• Nitroparaffins
• Tritium
• 1-nitro-2-(4-hydroxyphenyl)ethane
• Tyrosine
• dhurrin

Topics

• Carbon Radioisotopes
• Glucosides (biosynthesis)
• Hydrogen-Ion Concentration
• Hydroxylation
• Kinetics
• Microsomes (metabolism)
• Nitriles (metabolism)
• Nitroparaffins (metabolism)
• Oxygen Consumption
• Plants (metabolism)
• Tritium
• Tyrosine (metabolism)