Enzymatic Hydrogen Cyanide Release from Mandelonitrile Esters in Oribatid Mites

Abstract

Arthropods use chemical defenses against predators by releasing toxic or repellent compounds from specialized exocrine glands. Oribatulid mites (Oribatida: Oribatulidae) contain mandelonitrile hexanoate (MNH); when they are subjected to physical stimuli, MNH decomposes to release hydrogen cyanide (HCN), suggesting that cyanogenesis is an effective defense strategy in these mites. However, the underlying process remains unclear. Here, we investigated the defensive secretions of the oribatid mites, Phauloppia adjecta and Oribatula sakamorii, and identified MNH and mandelonitrile octanoate (MNO), along with the sesquiterpenes β-selinene and calarene. Chiral gas chromatography analysis determined the absolute configurations of MNH and MNO as the (R)-isomers, consistent with the configuration of mandelonitrilea known defensive compound in polydesmid millipedes. (R)-MNO did not undergo spontaneous hydrolysis in water at pH 3−8, suggesting that a mite-derived enzyme is required for HCN production. In vitro experiments showed that a crude mite enzyme extract incubated with (R)-MNO generated over 200 nmol of HCN, whereas negligible production occurred in the controls. These findings support an enzyme-dependent cyanogenic defense mechanism wherein mandelonitrile esters stored in the oil glands release HCN upon enzyme interaction during disturbance, potentially protecting conspecifics from predation.