Enzymatic Release and Metabolism of Hydrogen Cyanide in Sorghum Infected by Gloeocercospora sorghi

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Abstract
The activities of enzymes that may be important in the release or metabolism of hydrogen cyanide (HCN) during disease development were determined in extracts of sorghum infected by G. sorghi. The activity of .beta.-glucosidase increased 20-fold during the first 24 h after inoculation and then remained at that level. This enzyme hydrolyzes dhurrin, the cyanogenic glucoside in sorghum, to glucose and p-hydroxymandelonitrile. The increase in .beta.-glucosidase activity preceded the first detectable decline in HCN potential of infected leaves. The activity of oxynitrilase, an enzyme that releases HCN from p-hydroxymandelonitrile, remained constant. Of the HCN-metabolizing enzymes examined, activity of formamide hydro-lyase was the highest. No formamide hydro-lyase activity was detected earlier than 12 h after inoculation, but activity increased at least 200-fold 18-48 h after inoculation. This increase preceded the decline in HCN potential in diseased leaves. The product of formamide hydro-lyase activity, formamide, did not accumulate in diseased sorghum. No formamide hydro-lyase was detected in extracts of diseased older leaves in which no HCN was detected. Activities of .beta.-cyanoalanine synthase and rhodanese (2 other HCN-metabolizing enzymes) were detected at low levels in healthy sorghum but were not detected in the pathogen. These activities did not increase during disease development. These data are consistent with the hypotheses that G. sorghi is exposed to HCN upon pathogenesis of sorghum leaves and that formamide hydro-lyase may be involved in HCN detoxification.