For an early review of plant cyanogenic glucosides, see Poulton (1980). For a more recent review, see Celenza (2001).

Dhurrin (derived from the amino acid tyrosine) is the major cyanogenic glucoside in Sorghum bicolor (sorghum), and can represent 30% of the dry weight of shoot tips of sorghum seedlings. It may play a role in defense against herbivore attack.

Dhurrin is synthesized via a multifunctional P450TYR (CYP79) that catalyzes two initial hydroxylations on tyrosine, generating N,N-dihydroxytyrosine. Sequential dehydration and decarboxylation reactions on this labile intermediate, probably proceeding nonenzymatically, generate p-hydroxyphenylacetaldehyde oxime. The cytochrome P450 catalysing the conversion of tyrosine to p-hydroxyphenylacetaldoxime in the biosynthesis of the cyanogenic glucoside dhurrin in sorghum has been designated CYP79A1 (Bak et al, 1999).

p-Hydroxyphenylacetaldoxime is subsequently converted to p-hydroxymandelonitrile by a second multifunctional P450 monooxygenase (P450ox) (Schuler, 1996; Kahn et al, 1997). P450ox (now known as CYP71E1 (Bak et al, 2000)) catalyzes dehydration of p-hydroxyphenylacetaldehyde oxime to p-hydroxyphenylacetonitrile and C-hydroxylation of p-hydroxyphenylacetonitrile to p-hydroxymandelonitrile. In vitro reconstitution of the entire dhurrin biosynthetic pathway from tyrosine has been accomplished by the insertion of CYP79 (tyrosine N-hydroxylase), P450ox, and NADPH-P450 oxidoreductase in lipid micelles in the presence of uridine diphosphate glucose glucosyltransferase (Kahn et al, 1997).

The final step in the biosynthesis of the cyanogenic glucoside dhurrin in sorghum is the transformation of the labile cyanohydrin into a stable storage form by O-glucosylation of (S)-p-hydroxymandelonitrile at the cyanohydrin function (Jones et al, 1999). The UDP-glucose:p-hydroxymandelonitrile-O-glucosyltransferase has been isolated and its cDNA cloned (Jones et al, 1999). The deduced translation product shows high identity to Zea mays flavonoid-glucosyltransferase.

Note that conversion of an amino acid to an aldoxime by N-hydroxylation and decarboxylation, is also the first step in glucosinolate biosynthesis (see: Glucosinolates under Sulfate uptake and assimilation). It is hypothesized that the glucosinolate pathway evolved from the cyanogenic glucoside pathway (Bak et al, 1998; Celenza, 2001). Expression of CYP79A1 in Arabidopsis thaliana results in the production of high levels of the tyrosine-derived glucosinolate p-hydroxybenzylglucosinolate, which is not a natural constituent of Arabidopsis (Bak et al, 1999). This provides further evidence that the enzymes of the glucosinolate pathway in Arabidopsis must have low substrate specificity with respect to aldoxime substrates (Bak et al, 1999; Petersen et al, 2001).

When both the two multifunctional sorghum cytochrome P450 enzymes CYP79A1 and CYP71E1 are constitutively expressed in tobacco and Arabidopsis p-hydroxymandelonitrile is formed. This compound is labile and dissociates into p-hydroxybenzaldehyde and hydrogen cyanide, the same products released from dhurrin upon cell disruption as a result of pest or herbivore attack (Bak et al, 2000).

The entire pathway for synthesis of the tyrosine-derived cyanogenic glucoside dhurrin has been transferred from sorghum to Arabidopsis thaliana by expressing CYP79A1, CYP71E1 and UDP-glucose:p-hydroxymandelonitrile-O-glucosyltransferase (Tattersall et al, 2001). The accumulation of dhurrin in the transgenic plants confers resistance to the flea beetle Phyllotreta nemorum; a natural pest of other members of the crucifer group (Tattersall et al, 2001). This clearly demonstrates the importance of cyanogenic glucosides in plant defense against insect herbivores.

In Triglochin maritima (seaside arrow grass) the two cyanogenic glucosides taxiphyllin and triglochinin are derived from tyrosine in a pathway that is very similar to that of the dhurrin biosynthesis pathway in sorghum (Nielsen and Moller, 1999; 2000). Triglochin maritima has two cytochrome P450 enzymes of the CYP79 family (CYP79E1 and CYP79E2) that are functionally identical to that of sorghum CYP79A1 (Nielsen and Moller, 2000).

Bak S, Nielsen HL, Halkier BA 1998 The presence of CYP79 homologues in glucosinolate-producing plants shows evolutionary conservation of the enzymes in the conversion of amino acid to aldoxime in the biosynthesis of cyanogenic glucosides and glucosinolates. Plant Mol. Biol. 38: 725-734.

Bak S, Olsen CE, Halkier BA, Moller BL 2000 Transgenic tobacco and Arabidopsis plants expressing the two multifunctional sorghum cytochrome P450 enzymes, CYP79A1 and CYP71E1, are cyanogenic and accumulate metabolites derived from intermediates in dhurrin biosynthesis. Plant Physiol. 123: 1437-1448.

Bak S, Olsen CE, Petersen BL, Moller BL, Halkier BA 1999 Metabolic engineering of p-hydroxybenzylglucosinolate in Arabidopsis by expression of the cyanogenic CYP79A1 from Sorghum bicolor. Plant J. 20: 663-671.

Celenza JL 2001 Metabolism of tyrosine and tryptophan - new genes for old pathways. Curr. Opin. Plant Biol. 4: 234-240.

Jones PR, Moller BL, Hoj PB 1999 The UDP-glucose:p-hydroxymandelonitrile-O-glucosyltransferase that catalyzes the last step in synthesis of the cyanogenic glucoside dhurrin in Sorghum bicolor. Isolation, cloning, heterologous expression, and substrate specificity. J. Biol. Chem. 274: 35483-35491.

Kahn RA, Bak S, Svendsen I, Halkier A, Moller BL 1997 Isolation and reconstitution of cytochrome P450ox and in vitro reconsitution of the entire biosynthetic pathway of the cyanogenic glucoside dhurrin from sorghum. Plant Physiol. 115: 1661-1670.

Nielsen JS, Moller BL 1999 Biosynthesis of cyanogenic glucosides in Triglochin maritima and the involvement of cytochrome P450 enzymes. Arch. Biochem. Biophys. 368: 121-130.

Nielsen JS, Moller BL 2000 Cloning and expression of cytochrome P450 enzymes catalyzing the conversion of tyrosine to p-hydroxyphenylacetaldoxime in the biosynthesis of cyanogenic glucosides in Triglochin maritima. Plant Physiol. 122: 1311-1322.

Petersen BL, Andreasson E, Bak S, Agerbirk N, Halkier BA 2001 Characterization of transgenic Arabidopsis thaliana with metabolically engineered high levels of p-hydroxybenzylglucosinolate. Planta 212: 612-618.

Poulton JE 1990 Cyanogenesis in plants. Plant Physiol. 94: 401-405.

Schuler MA 1996 The role of cytochrome P450 monooxygenases in plant-insect interactions. Plant Physiol. 112: 1411-1419.

Tattersall DB, Bak S, Jones PR, Olsen CE, Nielsen JK, Hansen ML, Hoj PB, Moller BL 2001 Resistance to an herbivore through engineered cyanogenic glucoside synthesis. Science 293: 1826-1828.