Photooxidation of Diaminobenzidine (dab) by Chloroplast Lamellae

The introduction of diaminobenzidine (DAB) into ultrastructural cytochemistry in 1966 by Graham and Karnovsky (1) for demonstrating horseradish peroxidase activity was an important milestone. Its usefulness has been extended to demonstration of mitochondrial cytochrome oxidase via cytochrome c by Seligman et al . (2) in 1968, and to demonstration of peroxisomes of kidney (3-5) and peroxisomes of the leaf (6) . It has been shown that DAB is oxidized to a waterinsoluble, lipid-insoluble, osmiophilic polymer which reliably localizes the sites of enzymatic activity (1, 2) . Since DAB is a good electron donor in the mitochondrial succinoxidase system of animal tissue, it was of interest to see whether it would also act as an electron donor in some part of the chain of electron transport in photosynthesis, making it possible to study the localization of this reaction in ultrastructural preparations . The publication in 1969 of papers (7, 8) reporting that benzidine may be used as an electron donor in photoreactions supported our experiments . These have consisted of showing that DAB is oxidized by illuminated chloroplasts and that following osmication, the sites of oxidation on the membranes of the chloroplasts may be studied in