Catabolism of linamarin in cassava (Manihot esculenta crantz)

The catabolism of cyanoglucoside linamarin in cassava was examined by studying the activity and kinetic characteristics of various enzymes involved in these reactions in different tissues. Hydroxy-nitrile-lyase (HNLyase) which catalysed hydrolysis of acetone cyanohydrin to cyanide at pH 5.5 was present in leaf, rind and tuber and showed maximum activity in leaves. It followed a complex saturation kinetics and has a high K-m value (30-50 mM). While rhodanese and thiocyanate levels are very low, significant activity of different enzymes of beta-cyanoalanine pathway for utilisation of cyanide was found in leaf, rind and tuber tissues. beta-cyanoalanine hydrolase which converts beta-cyanoalanine to asparagine showed maximum activity in tuber when compared to leaf and rind, similar to that reported for beta-cyanoalanine synthase. Significantly lower activity of asparaginase in tuber and its high K-m, value (500-625 mM) suggest that asparagine may accumulate in tuber while in leaf and rind it gets hydrolysed to ammonia. While the degradation of linamarin to cyanide occurs at acidic pH 5-6, the beta-cyanoalanine pathway operates at alkaline pH 8-9 suggesting a compartmentalisation of these two processes. (C) 1997 Elsevier Science Ireland Ltd.