The Origin of Mitochondrial Phosphatidylcholine within the Liver Cell

Abstract

The incorporation of choline into the phosphatidylcholine of subcellular fractions of rat liver has been examined. The mitochondrial fraction shows a minimal phosphatidylcholine synthesis compared with that produced by adding microsomes providing an incubation medium which allows adequate phospholipid synthesis is used. The small mitochondrial incorporation can be partly if not wholly explained by microsomal contamination or a non-energy dependent exchange process. It is concluded that the appreciable turnover of mitochondrial phosphatidylcholine observed in vivo can be explained most satisfactorily by assuming a transfer of newly synthesized phospholipid molecules from the endoplasmic reticulum.Although phospholipids play an essential role in many reactions of the mitochondrial electron transport system [l] their site of synthesis within the cell is a subject of some controversy. The phospholipid present in highest concentration in liver mitochondria is phosphatidylcholine and from experiments in vivo this appears to be turned over a t a rate which is only slightly slower than the microsomal phosphatidylcholine [2]. Early experiments suggested that liver mitochondria are devoid of the phosphatidylcholine synthesizing enzyme CDP-choline : 1,2 diglyceride cholinephosphotransferase which was found mainly in the microsomes [3,4]. Later studies of Stoffel and Schiefer [5] indicated that some of the enzyme was present in the outer mitochondrial membranes of rat liver, but these studies have been criticised on the grounds that no recovery of total activity or degree of microsomal contamination was provided [2,6,7 ]. Recently experiments were performed in this laboratory which indicated that the limited phosphatidylcholine synthesis from [32P]phosphate or CDP-[W]choline carried out by isolated liver mitochondria was largely due to microsomal contamination of these organelles [2]. An exchange of phosphatidylcholine between isolated microsomes and mitochondria was demonstrated and it was suggested that this was the process by which the dynamic turnover of phosphatidylcholine in the mitochondria is maintained. Contrasting markedly with these conclusions, Bygrave and Kaiser [8] chondria by an energy-dependent process with little or no contribution from contaminating or added microsomes. The experiments reported in this paper were performed in an attempt to resolve these differences. EXPERIMENTAL PROCEDUREIsolation of Mitochondria, Microsomes and Cell Supernatant The procedure followed was essentially the same as described by McMurray and Dawson [2]. Female rats of the Wistar albino strain, 200-250 gin weight were decapitated and the livers rapidly excised and chilled. All subsequent operations were performed a t 0"-4". The livers, usually pooled from two animals, were minced with scissors and homogenized in a motor-driven homogenizer with 0.008 in clearance [ l l ] for 1 min in 10 volumes of cold 0.25 M sucrose-0.1 mM EDTA, pH 7.4. The homogenate was centrifuged a t 2500 rev./min (800 xg) for 10 min in a Servall RC-2 centr...

Endoplasmic Reticulum as the Site of Lecithin Formation in Castor Bean Endosperm

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Intracellular Distribution of Enzymes of the Cytidine Diphosphate Choline Pathway in Castor Bean Endosperm

“…It has been reported that isolated ratliver mitochondria can synthesize lecithin via the CDPcholine pathway (13)(14)(15), but this is probably due to contamination of mitochondrial preparations (9,16). The general role of the endoplasmic reticulum in lecithin biosynthesis in animal (9) and plant (4.5) tissue is supported by the demonstration that the enzymes PChol-cytidyl transferase and PChol-glyceride transferase are found together in cell components sedimenting between 10,000 X g and 100,-000 X g ("microsomes").…”

New Insights into the Role of Mitochondria-Associated Endoplasmic Reticulum Membrane

“…The Structure of the MAM 3.1. Biochemical studies in the 1970s also suggested the physical contact by demonstrating that microsomes from liver contain enzymes required for the synthesis of mitochondrial phospholipids (Jungalwala and Dawson, 1970;McMurray and Dawson, 1969;Sauner and Levy, 1971). Elongated ER tubules are shown to express a precise orientation with respect to the mitochondria in cells of the pseudobranch gland (Copeland and Dalton, 1959).…”