Abstract
A cytochemical technique was devised for demonstrating a lysosomal enzyme capable of hydrolyzing histone. The test for histonase involved incubation of peripheral blood leukocytes in a mixture of calf thymus histone (substrate) and Fe(NH4)2(SO4)2·6H2O. Combination of this iron salt occurred with products of histone cleavage (arginine, lysine, glycine). The iron-amino acid complex was made visible in the cytoplasm of the cell by the Prussian blue stain. Histonase enzyme was found to have a narrow substrate specificity and to react only with histone and not with a variety of biologic and synthetic substrates for protease activity. The role of histonase in the transfer of information from nucleus to cytoplasm is discussed.