The effects of Triton X-100 and chlorpromazine on the Mg2+-dependent and Mg2+-independent phosphatidate phosphohydrolase activities of rat lung.
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.
Walton
P A
PA
Department of Biochemistry, University of Western Ontario, London, Canada.
Possmayer
F
F
D000818:Animals; D002746:Chlorpromazine / Q000494:pharmacology*; D003600:Cytosol / Q000187:drug effects / Q000201:enzymology; D004789:Enzyme Activation; D006358:Hot Temperature; D008168:Lung / Q000187:drug effects / Q000201:enzymology*; D008274:Magnesium / Q000378:metabolism*; D008861:Microsomes / Q000187:drug effects / Q000201:enzymology; D017830:Octoxynol; D010711:Phosphatidate Phosphatase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism*; D011092:Polyethylene Glycols / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains
D011092:Polyethylene Glycols; D017830:Octoxynol; D010744:Phosphoric Monoester Hydrolases; D010711:Phosphatidate Phosphatase; D008274:Magnesium; D002746:Chlorpromazine