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Regulation of the translocation of phosphatidate phosphohydrolase between the cytosol and the endoplasmic reticulum of rat liver. Effects of unsaturated fatty acids, spermine, nucleotides, albumin and chlorpromazine.

ABSTRACT: The translocation of phosphatidate phosphohydrolase between the cytosol and the microsomal membranes was investigated by using a cell-free system from rat liver. Linoleate, alpha-linolenate, arachidonate and eicosapentenoate promoted the translocation to membranes with a similar potency to that of oleate. The phosphohydrolase that associated with the membranes in the presence of [14C]oleate or 1mM-spermine coincided on Percoll gradients with the peak of rotenone-insensitive NADH-cytochrome c reductase, and in the former case with a peak of 14C. Microsomal membranes were enriched with the phosphohydrolase activity by incubation with [14C]oleate or spermine and then incubated with albumin. The phosphohydrolase activity was displaced from the membranes by albumin, and this paralleled the removal of [14C]oleate from the membranes when this acid was present. Chlorpromazine also displaced phosphatidate phosphohydrolase from the membranes, but it did not displace [14C]oleate. The effects of spermine in promoting the association of the phosphohydrolase with the membranes was inhibited by ATP, GTP, CTP, AMP and phosphate. ATP at the same concentration did not antagonize the translocating effect of oleate. From these results and previous work, it was concluded that the binding of long-chain fatty acids and their CoA esters to the endoplasmic reticulum acts as a signal for more phosphatidate phosphohydrolase to associate with these membranes and thereby to enhance the synthesis of glycerolipids, especially triacylglycerol. The translocation of the phosphohydrolase probably depends on the increased negative charge on the membranes, which could also be donated by the accumulation of phosphatidate. Chlorpromazine could oppose the translocation by donating a positive charge to the membranes.

SUBMITTER: Hopewell R

PROVIDER: S-EPMC1152906 | biostudies-other | 1985 Dec

REPOSITORIES: biostudies-other

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Project description:Lung contains both Mg2+-dependent and Mg2+-independent phosphatidate phosphohydrolase activities. Addition of Triton X-100 (0.5%) or chlorpromazine (1 mM) leads to a marked increase in the total phosphatidate phosphohydrolase activity in rat lung microsomes (microsomal fractions), but a decrease in the Mg2+-dependent activity. These observations suggest that the Mg2+-independent activity is stimulated, whereas the Mg2+-dependent activity is inhibited. However, the possibility exists that Triton X-100 could stimulate the Mg2+-dependent enzymic activity in an Mg2+-independent manner. In addition, the positively charged amphiphilic drug could be replacing the enzyme's requirement for Mg2+. These two possibilities were examined by using subcellular fractions in which the Mg2+-dependent phosphatidate phosphohydrolase had been abolished by heat treatment at 55 degrees C for 15 min. Heat treatment does not affect the microsomal Mg2+-independent phosphohydrolase to any great extent. Since the 6-8-fold stimulations due to Triton X-100 and chlorpromazine are retained after heat treatment of this fraction, the Mg2+-independent activity must be involved. Addition of Triton X-100 and chlorpromazine to cytosol virtually abolishes the Mg2+-dependent phosphatidate phosphohydrolase activity and decreases the Mg2+-independent activity by half. Heat treatment also abolishes the Mg2+-dependent activity and decreases the Mg2+-independent activity by over half. The Mg2+-independent phosphatidate phosphohydrolase activity remaining after heat treatment was not affected by Triton X-100 or chlorpromazine. These studies demonstrate that Triton X-100 and chlorpromazine specifically stimulate the heat-stable Mg2+-independent phosphatidate phosphohydrolase activity in rat lung microsomes. In contrast, the heat-labile Mg2+-independent phosphatidate phosphohydrolase activities in cytosol are inhibited by these reagents. Triton X-100 and chlorpromazine inhibit the Mg2+-dependent phosphatidate phosphohydrolase activities in both rat lung microsomes and cytosol. These results are consistent with the view that a single Mg2+-dependent phosphatidate phosphohydrolase present in both microsomes and cytosol is specifically involved in glycerolipid metabolism.