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Improved, largely automated methods are described for the purification and analysis o peroxisomes, lysosomes, and mitochondria from the livers of rats injected with Triton WR-1339. With these new methods, it has become possible to obtain, in less than 6 hr and with reliable reproducibility, mitochondria practically free of contaminants, as well as the rarer cytoplasmic particles in amounts (about 100 mg of protein) and in a state of purity (95%) that make them suitable for detailed biochemical studies. The results obtained so far on these preparations have made more conclusive and precise previous estimates of the biochemical and morphological properties of the three groups of cytoplasmic particles. In addition, peroxisomes were found to contain essentially all the L-a-hydroxy acid oxidase of the liver, as well as a small, but significant fraction of its NADP-linked isocitrate dehydrogenase activity. Another small fraction of the latter enzyme is present in the mitochondria, the remainder being associated with the cell sap. The mitochondrial localization of the metabolically active cytoplasmic DNA could be verified. The relative content of the fractions in mitochondria, whole peroxisomes, peroxisome cores, lysosomes, and endoplasmic reticulum was estimated independently by direct measurements on electron micrographs, and by linear programming (based on the assumption that the particles are biochemically homogeneous) of the results of enzyme assays. The two types of estimates agreed very well, except for one fraction in which low cytochrome oxidase activity was associated with mitochondrial damage.

Conflicting reports exist in the literature with respect to the intracellular localization of catalase, D-amino acid oxidase and monoamine oxidase in liver. Studies performed by a number of different authors indicate that catalase is associated partly with cytoplasmic particles and partly with the supernatant fraction, the ratio of particulate to soluble activity depending on the species and sex of the aniimal and on the nature of the medium adopted for preparing the homogenate (Ludewig &

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We prepared homogeneous populations of colloidal gold particles of various sizes. These were analyzed for size distribution and number of particles per unit volume. On exposure to increasing concentrations of insulin, myoglobin, protein A, peroxidase, serum albumin, galactosylated serum albumin, lactoferrin, transferrin, catalase, low-density lipoprotein, ferritin, and polymeric IgA, protein binding was a saturable process. Using serum albumin, we verified that a reversible equilibrium was reached within 15 minutes. Scatchard analysis of the interactions between all of these proteins and the gold particles resulted in a single component, linear relation. For a given particle size, the number of binding sites for various proteins was inversely proportional to their molecular weight. Conversely, when the size of particles was varied, the number of binding sites was directly proportional to the average area of each gold particle. All results are compatible with a monomolecular shell of protein surrounding the particle at saturation, the binding capacity being inversely proportional to the projection area of the protein. We present direct morphological evidence for this model. The affinity of the various proteins for the colloid also increased with molecular weight, and was not related to the protein isoelectric point. For globular proteins, the monomolecular shell model makes possible prediction of the number of molecules that will saturate a gold particle, if the average diameter of the gold particles and the molecular weight of the protein are known.

Trypanosoma brucei glycosomes (microbodies containing nine enzymes involved in glycolysis) have been purified to near homogeneity from bloodstream-form trypomastigotes for the purpose of morphologic and biochemical analysis . Differential centrifugation followed by two isopycnic centrifugations in an isotonic Percoll and in a sucrose gradient, respectively, resulted in 12-to 13-fold purified glycosomes with an overall yield of 31% . These glycosomes appeared to be highly pure and contained <1% mitochondrial contamination as judged by morphometric and biochemical analyses. In intact cells, glycosomes displayed a remarkably homogeneous size distribution centered on an average diameter of 0.27 um with a standard deviation of 0.03 um . The size distribution of isolated glycosomes differed only slightly from that measured in intact cells. One T. brucei cell contained on average 230 glycosomes, representing 4.3% of the total cell volume . The glycosomes were surrounded by a single membrane and contained as phospholipids only phosphatidyl choline and phosphatidyl ethanolamine in a ratio of 2:1 . The purified glycosomal fraction had a very low DNA content of 0.18 hg/mg protein . No DNA molecules were observed that could not have been derived from contaminating mitochondrial or nuclear debris.

The uptake of various host plasma proteins by the bloodstream form of Trypanosoma brucei was studied both biochemically, using radiolabeled proteins, and with the electron microscope, using colloidal gold particles as molecular tracers onto which plasma proteins had been adsorbed. Total plasma proteins and serum albumin were taken up by a mechanism of fluid endocytosis with low clearance (0.1 pl [mg cell protein]-l h -I), while low-density lipoprotein (LDL) and transfemn were taken up by a receptormediated process with a clearance of two to three orders of magnitude higher than that of serum albumin. Binding prior to uptake of LDL and transfenin was saturable, depended on the presence of Ca2+ , and the labeled ligand could be displaced by the homologous but not by heterologous protein. Binding of gold-labeled proteins was seen only to the membrane of the flagellar pocket and not elsewhere on the plasma membrane. After 1 h of incubation at 30°C with gold-labeled LDL and transfemn, labeled cellular structures represented respectively half and one-third of the total volume of all single-membrane bounded endocytotic and electron-dense vacuoles within the cell FRICAN trypanosomes of the Trypanosoma brucei com-