Cytoplasmic crystalline inclusions are found in some larval haemocytes of Drosophila melanogaster. Blackening can be experimentally induced in these cells, and previously it was suggested that either the substrate or enzyme for the tyrosine-tyrosinase system leading to melanin production in Drosophila larvae is found in these inclusions in the crystal cells. The present report is an attempt to further localize the enzyme and substrate.

Larvae have been fed on food containing α-C14-tyrosine and autoradiographs of the blood cells taken from these larvae subsequently prepared. The C14 activity in the crystal cells is restricted to the crystal inclusions in the cells and is significantly higher than that found in the other type of haemocytes, the plasmatocytes. When samples of the blood cells are incubated in DOPA solution, the extra-crystalline cytoplasm becomes blackened while the crystals themselves remain colorless. These observations are consistent with the hypothesis that the substrate is localized in the crystal inclusions whereas enzyme is found in the surrounding cytoplasm. An in vivo structural isolation would serve to separate enzyme and substrate rather than an inhibition by dehydrogenases as postulated by previous authors. In vitro examination with the phase microscope has shown that the crystal cells rupture easily and the crystals dissolve in the haemolymph. Therefore any treatment which tends to disrupt the structural integrity of the cell will allow the enzyme and substrate to come together. Humoral factors preceding metamorphosis might account for the in vivo release of the enzymatic reaction by initially altering the permeability of the cell.

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