Assay and subcellular localization of H₂O₂ generating mannitol oxidase in the terrestrial slug Arion ater

An assay is described that allows the measurement of H₂O₂‐generating mannitol oxidase activity in sucrose‐containing homogenates and subcellular fractions. Using this assay high activity was found in digestive gland and crop of the slug Arion ater. The subcellular localization of mannitol oxidase in these tissues was examined using sucrose density gradient centrifugation in a B‐XIV zonal rotor. Mannitol oxidase was found to reside in a lowdensity membrane tentatively identified as golgi or plasma membrane, rather than in mitochondria or peroxisomes as had previously been supposed.