Activity of β-glucosidase in plants with high content of cyanogenic glycosides as important factor determining their toxicity

Background: The acute toxicity of cyanide after its enzymatic release from cyanogenic glycosides in plants (e.g. bitter almonds) is triggered by the peak level of cyanide in the body. In this regard, not only the content of bound cyanide in foods is relevant, but also the velocity of the release after chewing of the food caused by the β-glucosidase and the following surge of free cyanide in the body. In a human cross-over study (n = 12) with consumption of persipan, linseed, bitter apricot kernels and cassava (dose of bound cyanide: 6.8 mg each), mean peak levels of cyanide in blood were found to be 1.3, 5.7, 14.3 and 15.4 μM, respectively. Additional in-vitro investigations were performed to confirm the velocity of the enzymatic release of cyanide (by plant β-glucosidase) as important factor determining the toxicity of cyanogenic foods. Methods: Food samples containing 1 mg of bound cyanide were homogenized and suspended in 100 ml PBS buffer (pH 4 and 7). Free cyanide was determined at different times in samples taken from the suspension gently stirred at 37 °C (GC–MS method, internal standard: K13C15N). Results: Mean proportion of the total amount released within 10 min after tissue destruction was 0% for persipan, 15% (pH 7) for linseed, 62% (pH 4) for bitter apricot kernels and 78% (pH 7) for cassava. Conclusion: The toxicity of cyanogenic foods is determined not only by its content of bound cyanide, but also by the activity of the β-glucosidase as a plant-specific property.