When cyanogenic plants are damaged, HCN is released into the environment. This phenomenon, designated as cyanogenesis, is caused by the catabolism of cyanogenic glycosides (such as dhurrin, in sorghum). Cyanogenesis is initiated by B-glucosidases which hydrolyze the cyanogenic glycoside to cyanohydrin (alpha-hydroxynitrile) and a saccharide. Subsequently the unstable cyanohydrin decomposes spontaneously or enzymatically by the action of a hydroxynitrile lyase to cyanide and a carbonyl compound (Wajant et al, 1995).

In sorghum, the B-glucosidase responsible for hydrolyzing dhurrin is called dhurrinase; this enzyme exists in two isoforms, which show organ specific expression. The gene (dhr1) encoding dhurrinase-1 has been cloned from sorghum (Cicek and Esen, 1998), and shows 70% sequence identity to the two maize B-glucosidases.

All hydroxynitrile lyases characterized thus far fall into two broad groups; FAD-containing and FAD-lacking (Hu and Poulton, 1997). The hydroxynitrile lyases of species such as sorghum, cassava and linen flax, lack FAD. The FAD-containing lyases have been isolated from members of the Prunoideae and Maloideae subfamilies of the Rosaceae, and are major seed storage proteins. The flavoprotein (R)-(+)-mandelonitrile lyase (MDL) [EC 4.1.2.10] which plays a key role in cyanogenesis in rosaceous stone fruits, has recently been cloned and characterized from black cherry (Prunus serotina) (Hu and Poulton, 1997).

Cicek M, Esen A 1998 Structure and expression of a dhurrinase (B-glucosidase) from sorghum. Plant Physiol. 116: 1469-1478.

Halkier BA, Moller BL 1989 Biosynthesis of the cyanogenic glucoside dhurrin in seedlings of Sorghum bicolor (L.) (Moench) and partial purification of the enzyme system involved. Plant Physiol. 90: 1552-1559.

Hu Z, Poulton JE 1997 Sequencing, genomic organization, and preliminary promoter analysis of a black cherry (R)-(+)-mandelonitrile lyase gene. Plant Physiol. 115: 1359-1369.

Wajant H, Forster S, Selmar D, Effenberger F, Pfizenmaier K 1995 Purification and characterization of a novel (R)-mandelonitrile lyase from the fern Phlebodium aureum. Plant Physiol. 109: 1231-1238.