Changes in membrane surface properties of hepatic peroxisomes of rats under several conditions as determined by partition in aqueous polymer two-phase systems

Quality control: from molecules to organelles

There is a vast body of literature on the quality control of protein folding and assembly into multisubunit complexes. Such control takes place everywhere in the cell. The correcting mechanisms involve cytosolic and organellar proteases; the result of such control is individual molecules with proper structure and individual complexes both with proper stoichiometry and proper structure. Obviously, the formation of organelles as such requires some additional criteria of correctness and some new mechanisms of their implementation. It is proposed in this article that the ability to carry out an integral (key) function may serve as a criterion of correct organelle assembly and that autophagy can be accepted as a mechanism eliminating the assembly mistakes.

Alterations in the integrity of peroxisomal membranes in livers of mice treated with peroxisome proliferators

Catalase leakage from its particulate compartment within the light mitochondrial fraction of liver was used as an index of the integrity of peroxisomes in untreated mice and in mice treated with the peroxisome proliferators clofibrate(ethyl-p-chlorophenoxyisobutyrate), Wy-14,643(4-chloro-6[2,3-xylidino)-2-pyrimidinylthio]acetic acid) and DEHP(di-(2-ethylhexyl)phthalate). Catalase leakage represented about 2% of the total catalase activity when fractions from untreated mice were incubated at 4 degrees C, increasing to about 5% during 60 min incubation at 37 degrees C. In fractions from livers of mice treated with peroxisome proliferators, catalase leakage was significantly higher, being 7-11% at 4 degrees C and increasing to approximately 20% after 60 min incubation at 37 degrees C. The pattern of release was similar for all proliferators. Parallel data were obtained for catalase latency in these fractions, i.e. following 60 min incubation at 37 degrees C, free (non-latent) catalase activity was 18% in control mice and 65, 67, and 83% in fractions from clofibrate-, Wy-14,643- and DEHP-treated mice, respectively. Differences in catalase leakage from peroxisomes in fractions from untreated mice and clofibrate-treated mice were also apparent following treatments designed to effect membrane permeabilization, as in freeze-thawing, osmotic rupture, and extraction with Triton X-100 and lysophosphatidylcholine. These data are consistent with a significant alteration in the integrity of the membranes of peroxisomes in livers of mice which have been treated with peroxisome proliferators, and furthermore indicate a commonality of effect of these agents.

Changes in the activities of dihydroxyacetone phosphate and glycerol-3-phosphate acyltransferases in rat liver under various conditions

Activities of enzymes relating to the acyl dihydroxyacetone phosphate (acyl DHAP) pathway were determined in rat liver under conditions known to elevate the peroxisomal beta-oxidation activity. In fasted and streptozotocin-induced diabetic rats, DHAP acyltransferase activity showed a small but significant increase, though the activities of glycerol-3-phosphate (GP) acyltransferase and alkyl DHAP synthase were not changed. After 2 weeks, feeding of 20% partially hydrogenated marine oil, the activity of DHAP acyltransferase also increased to 140% of the control. The feeding of 0.25% clofibrate and 2% di(2-ethylhexyl)phthalate (DEHP) increased the activities of both DHAP and GP acyltransferases by 2- to 3-fold, whereas alkyl DHAP synthase activity decreased under the same conditions. A fractionation study showed that the increases in the activities of DHAP acyltransferase and acyl/alkyl DHAP reductase in the liver of rats treated with DEHP occurred mainly in peroxisomes and microsomes, respectively. The phospholipid contents per mg protein of the isolated hepatic peroxisomes from rats were as follows (percent of the control): fasting, 62%; diabetic, 69%; high fat-diet, 89%; clofibrate-treated, 126%; DEHP-treated, 119%. These results suggest that glycerophospholipid metabolism might also be controlled by peroxisomal enzymes under physiological and pathological conditions.

Turnover of fatty acyl-CoA oxidase in the liver of rats fed on a partially hydrogenated marine oil

The change in turnover of fatty acyl-CoA oxidase (FAO), the rate-limiting enzyme of the peroxisomal beta-oxidation system, was investigated in rats fed a 30% (w/w) partially hydrogenated marine oil (PHMO) diet. The FAO activity increased five-fold after two weeks of PHMO feeding, and decreased after withdrawal of the diet. Based on in vivo experiments using L-[4,5-3H]leucine and an immunoprecipitation technique, the increase in the activity of FAO could be accounted for by a 1.6-fold higher rate of FAO synthesis and a 3.4-fold slower rate of FAO degradation as compared to controls. In the same PHMO-fed rats, the rates of synthesis and degradation of carnitine palmitoyltransferase were 1.8-fold higher and 2.0-fold slower, respectively, as compared to controls. The results indicate that the observed increase in the activity of the enzymes of peroxisomal beta-oxidation is mainly due to a reduced rate of FAO degradation in the liver of rats fed the PHMO diet.

The effect of clofibrate on the phospholipid composition of the peroxisomal membranes in mouse liver

Membranes were prepared from peroxisomes which had been isolated from the livers of normal mice and from mice treated with clofibrate (a hypolipidemic drug and peroxisome proliferator). Phospholipid analysis of these membranes revealed that clofibrate treatment resulted in a decrease in the membrane content of phosphatidylcholine, the most abundant phospholipid, and a concomitant increase in the amount of lysophosphatidylcholine, this latter component reaching a level of almost 6% of the total membrane phospholipid. The concentrations of other phospholipids in these membranes were not significantly altered. The parallel analysis of microsomal membranes demonstrated an analogous increase in the level of lysophosphatidylcholine following clofibrate treatment. In control experiments with microsomal membranes employing quinacrine, an inhibitor of phospholipase A2, the increased lysophosphatidylcholine concentration was still observed in clofibrate-treated animals. As well, a decrease in the proportion of microsomal phosphatidylcholine with clofibrate treatment was seen when quinacrine was used. Fatty acid analysis of the phosphatidylcholines from peroxisomal membranes showed some minor changes, including an increase in one component tentatively identified as docosahexaenoic acid, in clofibrate-treated animals. Overall, these data demonstrate that clofibrate causes a marked perturbation of the phospholipid composition of peroxisomal membranes, and are interpreted as indicating that the main site of action of the drug is the deacylation-reacylation cycle between phosphatidylcholine and lysophosphatidylcholine.