The hydrogen peroxide produced during photolysis of melanin pigments has been measured using an oxidase electrode. The photooxidation has been shown to occur via the superoxide intermediate. In the presence of superoxide dismutase the rate of photo‐induced production of hydrogen peroxide is increased, reflecting the ability of melanin to scavenge superoxide radicals. Evidence for metal‐ion dependent formation of hydroxyl radicals during photooxidation of melanin pigments was obtained using electron spin resonance‐spin trapping procedures. Superoxide dismutase increased the rate of formation of hydroxyl radicals in the system. Mechanisms of metal ion‐induced production of hydroxyl radicals during photolysis of melanin pigments are discussed.

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Cells of the moth immune system are derived from organs that loosely envelop the four wing imaginal discs. The immune response in these insects is believed to depend on the activities of two main classes of hemocytes: plasmatocytes and granular cells. The fates of cells that arise from these hematopoietic organs have been followed by immunolabeling with plasmatocyte-specific and granular-cell-specific antibodies. Cells within each hematopoietic organ differ in their coherence and in their expression of two plasmatocyte-specific surface proteins, integrin and neuroglian. Within an organ there is no overlap in the expression of these two surface proteins; neuroglian is found on the surfaces of the coherent cells while integrin is expressed on cells that are losing coherence, rounding up, and dispersing. A granular-cell-specific marker for the protein lacunin labels the basal lamina that delimits each organ but only a small number of granular cells that lie on or near the periphery of the hematopoietic organ. When organs are cultured in the absence of hemolymph, all cells derived from hematopoietic organs turn out to immunolabel with the plasmatocyte-specific antibody MS13. The circulating plasmatocytes derived from hematopoietic organs have higher ploidy levels than the granular cells and represent a separate lineage of hemocytes.

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Laser flash photolysis and electron spin resonance as well as conventional absorption and spectrofluorimetry have been used to study the interaction of anionic, neutral and cationic dyes with two synthetic melanins obtained by polymerising dihydroxyphenylalanine (dopa) and 5-S-cysteinyldopa. Cationic dyes were shown to complex strongly to both melanins via ionictype interactions and this binding was monitored as a broadening of the absorption band, fluorescence quenching, triplet-state quenching and, using e.s.r. oximetry, via oxygen consumption rates which are reduced on adding melanin. These triplet-state results imply that ca. 5-10 monomer units of synthetic melanins are involved in binding a positively charged porphyrin. For the anionic dyes there is no binding and consequently melanin has little or no effect on the absorption, fluorescence or triplet states of the dyes. For these dyes there is a linear relationship between the oxygen consumption rate and the added melanin concentration. Singlet oxygen was shown to be involved using D 2 0 and, as quencher, azide ion. The results are discussed in terms of the possible use of such porphyrins for the photochemotherapy of pigmented tumours.