Analytical subcellular fractionation of cultivated mouse resident peritoneal macrophages

Resident peritoneal macrophages of the mouse, cultivated for 3 d, have been studied by quantitative subcellular fractionation using differential centrifugation and density equilibration in linear gradients of sucrose. Density equilibration experiments were carried out on untreated cytoplasmic extracts, on cytoplasmic extracts treated with digitonin or sodium pyrophosphate, and on cytoplasmic extracts derived from cells cultivated for 24 h in the presence of Triton WR-1339. The enzyme distributions obtained distinguished six typical behaviors characteristic of distinct subcellular entities. Acid α-galactosidase and other acid hydrolases displayed the highest average velocity of sedimentation and equilibrium density. Culturing in a medium that contained Triton WR-1339 markedly decreased their density, most likely as a result of Triton WR-1339 accumulation within lysosomes. Cytochrome c oxidase and the sedimentable activity of malate dehydrogenase showed a narrow density distribution centered around 1.17, very similar under all the experimental situations; their rate of sedimentation fell within the range expected for mitochondria. Catalase was particle-bound and exhibited structure-linked latency (80 percent); it was released in soluble and fully active form by digitonin, but this required a much higher concentration than in the case of lysosomal enzymes. Differences relative to all the other enzymes studied suggest the existence of a particular species of organelles, distinctly smaller than mitochondria, and possibly related to peroxisomes. Many enzymes were microsomal in the sense that the specific activities, but not the yields, were greater in microsomes than in other fractions obtained by differential centrifugation. These enzymes were distinguished in three groups by their properties in density equilibration experiments. NAD glycohydrolase, alkaline phosphodiesterase I, and 5’-nucleotidase had low equilibrium densities but became noticeably more dense after addition of digitonin. The other microsomal enzymes were not shifted by digitonin, in particular N-acetylglucosaminyltransferase and galactosyltransferase, which otherwise equilibrated at the same position in the gradient. We assign the digitonin-sensitive enzymes to plasma membranes and possibly to related endomembranes of the cells, and the two glycosyltransferases to elements derived from the Golgi apparatus. Finally, α-glucosidase, sulphatase C, NADH cytochrome c reductase, NADPH cytochrome c reductase, and mannosyltransferase, equilibrated at a relatively high density but were shifted to lower density values after addition of sodium pyrophosphate. These properties support their association with elements derived from the endoplasmic reticulum

Quantitative assay and subcellular distribution of enzymes acting on dolichyl phosphate in rat liver

To establish on a quantitative basis the subcellular distribution of the enzymes that glycosylate dolichyl phosphate in rat liver, preliminary kinetic studies on the transfer of mannose, glucose, and N-acetylglucosamine-1-phosphate from the respective (14)C- labeled nucleotide sugars to exogenous dolichyl phosphate were conducted in liver microsomes. Mannosyltransferase, glucosyltransferase, and, to a lesser extent, N- acetylglucosamine-phosphotransferase were found to be very unstable at 37 degrees C in the presence of Triton X-100, which was nevertheless required to disperse the membranes and the lipid acceptor in the aqueous reaction medium. The enzymes became fairly stable in the range of 10-17 degrees C and the reactions then proceeded at a constant velocity for at least 15 min. Conditions under which the reaction products are formed in amount proportional to that of microsomes added are described. For N- acetylglucosaminephosphotransferase it was necessary to supplement the incubation medium with microsomal lipids. Subsequently, liver homogenates were fractionated by differential centrifugation, and the microsome fraction, which contained the bulk of the enzymes glycosylating dolichyl phosphate, was analyzed by isopycnic centrifugation in a sucrose gradient without any previous treatment, or after addition of digitonin. The centrifugation behavior of these enzymes was compared to that of a number of reference enzymes for the endoplasmic reticulum, the golgi complex, the plasma membranes, and mitochondria. It was very simily to that of enzymes of the endoplasmic reticulum, especially glucose-6-phosphatase. Subcellular preparations enriched in golgi complex elements, plasma membranes, outer membranes of mitochondira, or mitoplasts showed for the transferases acting on dolichyl phosphate relative activities similar to that of glucose- 6-phosphatase. It is concluded that glycosylations of dolichyl phosphate into mannose, glucose, and N-acetylglucosamine-1-phosphate derivatives is restricted to the endoplasmic reticulum in liver cells, and that the enzymes involved are similarly active in the smooth and in the rough elements