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 i nvolved in these reactions in different tissues. Hydroxy-nitrile-lyase (HNLyase) which catalysed hydrolysis of acetone cyanohydrin to cyanid e at pH 5.5 was present in leaf, rind and tuber and showed maximum act ivity in leaves. It followed a complex saturation kinetics and has a h igh K-m value (30-50 mM). While rhodanese and thiocyanate levels are v ery low, significant activity of different enzymes of beta-cyanoalanin e 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. Sig nificantly lower activity of asparaginase in tuber and its high K-m, v alue (500-625 mM) suggest that asparagine may accumulate in tuber whil e in leaf and rind it gets hydrolysed to ammonia. While the degradatio n of linamarin to cyanide occurs at acidic pH 5-6, the beta-cyanoalani ne pathway operates at alkaline pH 8-9 suggesting a compartmentalisati on of these two processes. (C) 1997 Elsevier Science Ireland Ltd.