HETEROGENEITY OF PEROXISOMES IN HUMAN HEPATOBLASTOMA CELL-LINE HEPG2 - EVIDENCE OF DISTINCT SUBPOPULATIONS

Peroxisomes in human hepatoblastoma cell line HepG2 have been studied using immunocytochemical, ultrastructural and biochemical techniques. By immunofluorescence, small spherical peroxisomes were seen next to r od-shaped and elongated Forms. By electron microscopy and catalase cyt ochemistry, small particles with a diameter of 90 to 100 nm were found next to larger ones measuring up to 300 nm and some exhibiting tail l ike extensions. Both the intensity of DAB-staining and the immunolabel ing density for catalase were heterogenons in different peroxisomes. C ontrary to a recent biochemical study, the enzyme alanine glyoxylate a minotransferase was found by double immunofluorescence and immunoelect ron microscopy to be localized exclusively in peroxisomes of HepG2 cel ls. By Metrizamide density gradient centrifugation two populations of peroxisomes were isolated: a regular fraction banding at 1.20 g/cm(3) with a mean diameter of 222 nm and a lighter peroxisome fraction bandi ng at 1.14 g/cm(3). The ratio of lipid beta-oxidation enzymes to catal ase was significantly higher in peroxisomes with lower density than in the regular ones. These observations show clearly the existence of he terogenous populations of peroxisomes in HepG2 cells which may provide a useful model system for the investigation of biogenesis and metabol ism of peroxisomes of human origin.