Characterization of subcellular components in synchronized hepatoma cells as a function of the cell cycle

Abstract

The specific activity and subcellular distribution of marker enzymes for the main subcellular components were analysed in homogenates of synchronized hepatoma cells (Morris 7288c), obtained by selective detachment at mitosis combined with a metaphase block with Colcemid. Markers for lysosomes, mitochondrial outer membrane, plasma membrane and cytosol are synthesized throughout the cycle at the same rate as the bulk of cellular protein. Larger variations are observed for a Golgi marker; after a decrease around mitosis, the specific activity of galactosyltransferase increases steadily from middle G₁-phase on, and at the end of G₂-phase it is nearly twice that observed at the beginning of G₁-phase. Our results show that synthesis of cytochrome oxidase may occur preferentially in G₂-phase. Large modifications of the density distribution of lysosomes are observed during the cell cycle; the median equilibrium density of lysosomal markers decreases in G₁-phase, and some increase in soluble activity occurs at the same time. Reverse changes occur progressively during S- and G₂-phases. At mitosis, Golgi galactosyltransferase shows a more dispersed distribution, and modifications in the density distribution of endoplasmic-reticulum NADPH–cytochrome c reductase are observed. The latter can be most easily explained by a detachment of ribosomes from endoplasmic-reticulum membranes. No significant modifications occur in mitochondrial and plasma-membrane markers.