Abstract
The characterization of human lipofuscin pigment granules as lysosomes is strengthened by the cytochemical demonstration of β-glucuronidase and glucosaminidase activities in hepatocyte pigment granules which also have acid phosphatase activity. Lysosomes of parenchymal cells and macrophages are seen to differ in their content of cytochemically demonstrable hydrolase activities. Reduced diphosphopyridine nucleotide tetrazolium reductase and peroxidase activities that are remarkably resistant to heat (90°C for 15 min) and prolonged fixation are also visualized in lipofuscin granules. It is suggested that these oxidative activities reflect the presence of heme compounds that accumulate during the degradation of cytoplasmic constituents in autophagic vacuoles or dense bodies. The presence of metabolites with peroxidase activity within lysosomes may be responsible for the oxidation of melanin precursors to melanin and unsaturated fat to the alcohol-insoluble lipid of lipofuscin pigment. Cytoplasmic iron (hemosiderin) that is stained by Perls' ferrocyanide or Quinke's sulfide procedures is also visualized by incubating sections for peroxidase activity in Karnovsky's 3,3'-diaminobenzidine medium. This activity disappears following the extraction of stainable iron in Lillie's dithionite medium. However, the heme-associated peroxidase activity is unaffected.