Microplate quantification of plant leaf superoxide dismutases

Abstract

Superoxide dismutases (SODs) catalyze the dismutation of superoxide radicals in a broad range of organisms, including plants. Quantification of SOD activity in crude plant extracts has been problematic due to the presence of compounds that interfere with the dose–response of the assay. Although strategies exist to partially purify SODs from plant extracts, the requirement for purification limits the rapidity and practical number of assays that can be conducted. In this article, we describe modification of a procedure using o-dianisidine as substrate that permits relatively rapid quantification of SOD activity in crude leaf extracts in a microplate format. The method employs the use of a commercial apparatus that permits lysis of 12 tissue samples at once and the use of Pipes buffer to reduce interference from compounds present in crude leaf extracts. The assay provided a linear response from 1 to 50 units of SOD. The utility of the assay was demonstrated using tissue extracts prepared from a group of taxonomically diverse plants. Reaction rates with tissue extracts from two grasses were linear for at least 60 min. Tissues of certain species contained interfering compounds, most of which could be removed by ultrafiltration. The presence of plant catalases, peroxidases, and ascorbate in physiological quantities did not interfere with the assay. This approach provides a means to quantify SOD activity in relatively large numbers of plant samples provided that the possibility for the presence of interfering compounds is considered. The presence of interfering compounds in certain plant tissues necessitates caution in interpreting the effects of plant stresses on SOD.