Glycerolipid synthetic capacity of rat liver peroxisomes

Intertwined and Finely Balanced: Endoplasmic Reticulum Morphology, Dynamics, Function, and Diseases

The endoplasmic reticulum (ER) is an organelle that is responsible for many essential subcellular processes. Interconnected narrow tubules at the periphery and thicker sheet-like regions in the perinuclear region are linked to the nuclear envelope. It is becoming apparent that the complex morphology and dynamics of the ER are linked to its function. Mutations in the proteins involved in regulating ER structure and movement are implicated in many diseases including neurodegenerative diseases such as Alzheimer’s, Parkinson’s, and amyotrophic lateral sclerosis (ALS). The ER is also hijacked by pathogens to promote their replication. Bacteria such as Legionella pneumophila and Chlamydia trachomatis, as well as the Zika virus, bind to ER morphology and dynamics-regulating proteins to exploit the functions of the ER to their advantage. This review covers our understanding of ER morphology, including the functional subdomains and membrane contact sites that the organelle forms. We also focus on ER dynamics and the current efforts to quantify ER motion and discuss the diseases related to ER morphology and dynamics.

The native molecular size of alkyl-dihydroxyacetonephosphate synthase and dihydroxyacetonephosphate acyltransferase

Dihydroxyacetonephosphate acyltransferase (DHAP-acyltransferase) and alkyl-dihydroxyacetonephosphate synthase (alkyl-DHAP synthase) are the first two enzymes involved in the biosynthesis of ether phospholipids. Both peroxisomal enzymes have recently been purified to homogeneity and their molecular weights under denaturing conditions were reported. To determine the in situ functional size of both enzymes, radiation inactivation experiments were performed. Alkyl-DHAP synthase showed single exponential decays, both when enzymatic activity and when immunoreactive protein levels were measured, from which target sizes of 79+/-2 kDa and 78+/-4 kDa, respectively, were calculated. DHAP-acyltransferase activity increased at lower doses and decayed upon further irradiation with an apparent target size of 62+/-7 kDa. We conclude from these data that the functional unit sizes for both enzymes in situ are represented by their single polypeptide chains.

Purification of guinea pig small intestinal peroxisomes and the subcellular localization of glucose-6-phosphate dehydrogenase

A method for the isolation of highly purified peroxisomes from guinea pig small intestine was developed. This two-stage process involved a rate-dependent banding of a light-mitochondria lambda-fraction followed by a density-dependent banding of the catalase enriched fractions obtained from the first step, using a horizontal rotor. Furthermore, the subcellular localization of glucose-6-phosphate dehydrogenase (NADP+-dependent) activity in guinea pig small intestine was examined. Analysis of density-gradient fractions indicated that approximately 3-4% of the cellular NADP+-dependent glucose-6-phosphate dehydrogenase activity is associated with peroxisomal fractions and that it is localized to the matrix of peroxisomes. It is therefore suggested that a peroxisomal source of NADPH may be utilized by enzyme systems that use NADPH specifically as a reductant.

Localization of enzymes involved in glycero-ether bond formation in rat liver

Studies on the localization of alkyldihydroxyacetone-phosphate synthase in rat liver are described. Less than 5% of the total activity was found in the cytosolic fraction, suggesting that the enzyme is membrane-bound. The ratio of the enzymatic (specific) activity over that of dihydroxyacetone-phosphate acyltransferase, a peroxisomal enzyme, is 10-fold higher in the microsomal fraction, when compared to the peroxisomal fraction. Studying the distribution of the enzyme in a linear density gradient, two activity peaks were found in the peroxisomal and the microsomal fraction indicating a bimodal localization of alkyldihydroxyacetone-phosphate synthase. Rabert et al. (Rabert, U., Völkl, A. and Debuch, H. (1986) Biol. Chem. Hoppe-Seyler 367, 215-222) have presented evidence that the activity in the microsomal fraction was mainly caused by a different enzyme that preferentially converted acyldihydroxyacetone to alkyldihydroxyacetone. We also found a radioactive product, different from alkyldihydroxyacetone-phosphate, upon incubation of microsomal protein in the presence of [14C]hexadecanol. However, it was shown that this product was formed independently of the presence of acyldihydroxyacetone. The product yielded [14C]hexadecanol upon alkaline hydrolysis, clearly showing that it did not contain an ether-bond. Upon incubation of microsomal protein with [14C]palmitic acid and hexadecanol the product was also observed and its chromatographic behaviour resembled that of a synthetically prepared palmitoyl ester of hexadecanol. From these data it was concluded that the product formed is most likely a wax and that the enzyme responsible for this conversion is clearly different from the alkyldihydroxyacetone-phosphate synthase. The implication is that acyldihydroxyacetone-phosphate cannot be replaced by acyldihydroxyacetone in the process of glycero-ether bond formation.

Topography of ether phospholipid biosynthesis

The enzyme, dihydroxyacetone-phosphate acyltransferase (DHAP-AT) is localized at the inner surface of the peroxisomal membrane but shows no latency. The product, i.e., acyl-DHAP, is the first lipidic intermediate in ether lipid biosynthesis that is converted by alkyldihydroxyacetone-phosphate synthase into alkyl-DHAP. This step and the reductions of acyl-DHAP and alkyl-DHAP to the glycerol 3-phosphate analogs can still take place in peroxisomes. The further conversions of these intermediates into ether-linked choline or ethanolamine glycerophospholipids or plasmalogens take place in the endoplasmic reticulum. In this paper we describe studies to localize acyl-DHAP in the transversal plane of the peroxisomal membrane. To enable these studies, the usually employed assay conditions for DHAP-AT were modified by omission of BSA and by lowering the temperature and the palmitoyl-CoA concentration. Using these modified conditions, we were able to label peroxisomes with endogenously generated acyl-DHAP, with retention of catalase within the peroxisomal membrane. Endogenously generated acyl-DHAP was rapidly extractable from the outer surface of peroxisomes with BSA at 0 degree C, suggesting that even at this temperature the product was transported very fast across the membrane. Trypsin treatment of peroxisomes did not affect this behaviour. Only after short incubation periods, an increase in the proportion of non-extractable acyl-DHAP was observed. In large unilamellar vesicles made from peroxisomal phospholipids no transmembrane movement of acyl-DHAP was found. Despite the apparently rapid transbilayer movement of acyl-DHAP in the presence of BSA, it could still serve as a substrate for the enzyme alkyl-DHAP synthase, which is also localized at the inner surface of the peroxisomal membrane. In addition it was shown that endogenously generated substrate is used at a higher efficiency compared to exogenously added acyl-DHAP, suggesting a close interaction of the two enzymes in the peroxisomal membrane.

Dihydroxyacetone phosphate acyltransferase and alkyldihydroxyacetone phosphate synthase activities in rat liver subcellular fractions and human skin fibroblasts

Dihydroxyacetone phosphate acyltransferase (DHAP-AT) and alkyldihydroxyacetone phosphate synthase (DHAP-synthase) activities were examined in subcellular fractions of rat liver. The results indicate that at least 80% of DHAP-AT (assays carried out at pH 5.4) activity in rat liver is in peroxisomes, and the remaining activity is mitochondrial. In contrast to DHAP-AT, DHAP-synthase was detected in all subcellular fractions analyzed but the activity in peroxisomes was 208-fold and 42-fold greater compared to mitochondria and microsomes, respectively. We estimate that at least 70% of the DHAP-synthase activity in rat liver is in peroxisomes. DHAP-AT and DHAP-synthase activities were also examined in homogenates of skin fibroblasts from patients with inherited defects in peroxisomal structure and/or function. Both the enzyme activities were deficient in Zellweger syndrome whereas the activities were only partially deficient in infantile Refsum's disease. Greater reduction in DHAP-synthase activity, but only a partial reduction in DHAP-AT activity was observed in rhizomelic chondrodysplasia punctata. However, both DHAP-AT and DHAP-synthase activities were either normal or near normal in Refsum's disease or X-linked adrenoleukodystrophy. The results reported suggest that various peroxisomal disease states can be identified based on DHAP-AT and DHAP-synthase activities in skin fibroblasts of patients.

Peroxisomal localization of glucose-6-phosphate dehydrogenase and pyrophosphate-stimulated dihydroxyacetone-phosphate acyltransferase in mouse kidney

1. The subcellular localization of dihydroxyacetone-phosphate acyltransferase (DHAPAT) (assayed in the presence of pyrophosphate) and glucose-6-phosphate dehydrogenase (NADP+-dependent) activity in mouse kidney was investigated by density-gradient centrifugation. 2. DHAPAT has a predominantly peroxisomal distribution, and the activity in purified peroxisomes is stimulated by various organic and inorganic phosphate-containing compounds. The pH optimum is acid. 3. Approx. 10% of the cellular NADP+-dependent glucose-6-phosphate dehydrogenase activity is associated with peroxisomal fractions and may provide a source of NADPH for the peroxisomal reduction of acyl-dihydroxyacetone phosphate formed by DHAPAT activity.

Phospholipid metabolism in mouse sciatic nerve in vivo

To probe the activities of various pathways of lipid metabolism in peripheral nerve, six phospholipid-directed precursors were individually injected into the exposed sciatic nerves of adult mice, and their incorporation into phospholipids and proteins was studied over a 2-week period. Tritiated choline, inositol, ethanolamine, serine, and glycerol were mainly used in phospholipid synthesis; in contrast, methyl-labeled methionine was primarily incorporated into protein. Phosphatidylcholine was the main lipid formed from tritiated choline, glycerol, and methionine precursors. Phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol were the main lipids formed from serine, ethanolamine, and inositol, respectively. With time there was a shift in label among phospholipids, with higher proportions of choline appearing in sphingomyelin, glycerol in phosphatidylserine, ethanolamine in phosphatidylethanolamine (plasmalogen), and inositol in polyphosphoinositides, especially phosphatidylinositol 4,5-bisphosphate. We suggest that the delay in formation of these phospholipids, which are concentrated in peripheral nerve myelin, may, at least in part, be due to their formation at a site(s) distant from the sites where the bulk of Schwann cell lipids are made. We propose that separating the synthesis of these myelin-destined lipids to near the Schwann cell's plasma membrane would facilitate their concentration in peripheral nerve myelin sheaths. At earlier labeling times, ethanolamine and glycerol were more actively incorporated into phosphatidylcholine and phosphatidylinositol, respectively, than later. The transient labeling of these phospholipids may reflect some unique role in peripheral nerve function.

Peroxisomal beta-oxidation enzyme proteins in adrenoleukodystrophy: distinction between X-linked adrenoleukodystrophy and neonatal adrenoleukodystrophy

Very long chain fatty acids, which accumulate in plasma and tissues in X-linked adrenoleukodystrophy (ALD), neonatal ALD, and the Zellweger cerebrohepatorenal syndrome, are degraded by the peroxisomal beta-oxidation pathway, consisting of acyl-CoA oxidase, the bifunctional enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase, and beta-ketothiolase. A marked deficiency of all three enzyme proteins was reported in livers from patients with the Zellweger syndrome, a disorder in which peroxisomes are decreased or absent. Peroxisomes are not as markedly decreased in neonatal ALD and appear normal in X-linked ALD. Immunoblot analysis of the peroxisomal beta-oxidation enzymes revealed an almost complete lack of the bifunctional enzyme in neonatal ALD liver, similar to the finding in Zellweger tissue. In contrast, acyl-CoA oxidase and beta-ketothiolase were present in neonatal ALD liver, although the thiolase appeared to be in precursor form (2-3 kDa larger than the mature enzyme) in neonatal ALD. Unlike either neonatal ALD or Zellweger syndrome, all three peroxisomal beta-oxidation enzymes were present in X-linked ALD liver. Despite the absence in neonatal ALD liver of bifunctional enzyme protein, its mRNA was detected by RNA blot analysis in fibroblasts from these patients. These observations suggest that lack of bifunctional enzyme protein in neonatal ALD results from either abnormal translation of the mRNA or degradation of the enzyme prior to its entry into peroxisomes.

The role of peroxisomes in mammalian cellular metabolism

Peroxisomes, which are widely distributed in mammalian tissues, carry out several important functions in cellular metabolism. Production of alkylglycerol-3-phosphate, a key intermediate in the synthesis of plasmalogens and other ether lipids, occurs in the peroxisome. A fatty acid beta-oxidation system with significant differences from mitochondrial beta-oxidation is also found in the peroxisomes; the acetyl-CoA produced is used for synthetic reactions. This pathway has a particularly important physiological role in the oxidation of very long chain fatty acids and the side chain of cholesterol. Peroxisomes also possess a number of oxidases that produce H2O2 which is decomposed by peroxisomal catalase. The function of this peroxisomal respiratory pathway is disposal of excess reducing equivalents, protection of the cell against H2O2 and possibly a role in thermogenesis in brown adipose tissue. Other peroxisomal functions include a role in gluconeogenesis and in purine and polyamine catabolism. Some enzymes of peroxisomes can be induced by dietary, hormonal and other physiological changes. The entire organelle proliferates under certain of these conditions.

Distribution of alkylglycerone-phosphate synthase in subcellular fractions of rat liver

Subcellular fractions of rat liver were isolated by density-gradient centrifugation on a linear Metrizamide gradient and were assayed for marker enzymes of peroxisomes, lysosomes, microsomes and mitochondria. Alkylglycerone-phosphate synthase catalysing the formation of the ether bond in glycerolipids was also determined along the gradient. The enzyme was found to be enriched in the peroxisomal and the microsomal fractions thus, displaying a bimodal distribution pattern. Two reaction-products each, alkylglycerone phosphate and alkylglycerone were obtained in the enzymic assays performed, the ratio of which was clearly dependent upon the fraction employed. Alkylglycerone phosphate was mainly synthesized by the 'peroxisomal synthase', whereas an inverse proportion was observed assaying the microsomal counterpart. Furthermore, comparing the mean specific activities of both the enzymes the microsomal one was shown to be roughly twice as active in metabolizing 1-O-palmitoylglycerone 3-phosphate, simultaneously displaying a somewhat different sensitivity to NaF. These findings provide a first line of evidence, that two separate synthases, one in microsomes and another one in peroxisomes might be engaged in the biosynthesis of 1-O-alkyl-glycerolipids in rat liver.