α-Hydroxynitrile lyase in Hevea brasiliensis and its significance for rapid cyanogenesis

The hydroxynitrile lyase (HNL, EC 4.2.1.-) of Hevea brasiliensis (Muell.-Arg.) catalyzes the dissociation of acetone cyanohydrin and mandelonitrile, but shows higher activity towards the natural substrate acetone cyanohydrin. The ratio between the activities of linamarase (β-glycosidase, EC 3.2.1.21) to HNL was screened for more than 30 Hevea plants. In mixed-enzyme incubations various ratios of HNL to β-glucosidase were analyzed for the rapidity of HCN liberation. Addition of HNL increased the rate of HCN liberation up to 20-fold, thus demonstrating the significance of the HNL for rapid cyanogenesis. Its physiological importance is shown by the fact that only plants possessing high HNL activity are able to liberate HCN efficiently. Cyanogenic plants have been described as being weakly or strongly cyanogenic depending on the total amount of HCN which is potentially liberated. The data presented in this paper suggest that cyanogenic plants should also be differentiated as fast or slow cyanogenic according to the observed velocity of HCN liberation. Thus, for evaluating the repellent action of cyanogenic plants not only the final level of the HCN liberated is important but rather the rate with which this level is reached.