Subcellular location of sulphite reductase in plant tissues.

Sulphite redutctase (E.C. 1.8.1.2) is able to cause the reduction of inorganic sulphite to sulphide. This enzyme has been studied in a number of micro organisms (1, 2, 3, 4, 5). Although the enzyme from Alliutmi, odoruzt has been purified to homogeneity (6, 7) its characterisation is still incomplete. Recently Asada & Bandtirski (8) and Asada et al. (9) have described some of the properties of a partially purified suilphite reductase from spinach and showed that the enzyme catalyses not only the reduiction of sullphite but also that of nitrite and h droxylamine. These findings threw dotibt on the specificity of this enzyme, which in certain cases seems to be identical with enzymes involvedI in the reduction of nitrate to ammonia (6). Futrther (loubts about the natture of sulphite reductase arise from the observation of Torii & Banduirski (5, 10) that free suilphite is not a true intermediate in the reduiction of suilphate. Fuirthermore, it is not clear at present in which part of the plant, root, stem or leaves, suilphite is reduced. Since sulphite redutctase appears to be a very electro-negative reduictase, it is possible that the enzyme, as uisuially isolated, is part of an electron transport system. Stlch systems are uisuially associated with the partictilate fraction of the cell. It therefore appeared important to determine in which suibcelltular fraction of the cell sulphite re(Itictase of the higher plant is located and whether