Subcellular localization of xanthine oxidase in rat hepatocytes: high-resolution immunoelectron microscopic study combined with biochemical analysis.

Xanthine oxidase (XO), a molybdo-flavoprotein enzyme involved in purine degradation, was localized immunocytochemically in rat hepatocytes by high-resolution immunoelectron microscopy. XO was isolated from rat liver and a 150 KD polypeptide was purified. Antibodies were raised in rabbits. Small pieces of fresh liver were quickly frozen by contact with a copper block pre-cooled with liquid helium and were freeze-substituted with either 2.5% OsO4 or 0.2% glutaraldehyde in acetone. They were then warmed and embedded in Epon-Araldite or Araldite 6005. Resin sections were treated by indirect immunostaining using anti-rat liver XO antibody and protein A-gold. The labeling pattern was clearly over the cytosol and not on cell organelles. A few gold particles were found over the mitochondrial matrix, but not over the endoplasmic reticulum, Golgi apparatus, lysosomes, or peroxisomes, including their crystalloid core. These results are consistent with those of the biochemical assay of XO in this study. The significance of the occasional immunolabeling of the mitochondrial matrix remains obscure, since biochemical determinations in this study indicate no XO activity in the mitochondrial fraction.