Is dipalmitoylphosphatidylcholine a substrate for convertase?
Convertase has homology with carboxylesterases, but its substrate(s) is not known. Accordingly, we determined whether dipalmitoylphosphatidylcholine (DPPC), the major phospholipid in surfactant, was a substrate for convertase. We measured [(3)H]choline release during cycling of the heavy subtype containing [(3)H]choline-labeled DPPC with convertase, phospholipases A(2), B, C, and D, liver esterase, and elastase. Cycling with liver esterase or peanut or cabbage phospholipase D produced the characteristic profile of heavy and light peaks observed on cycling with convertase. In contrast, phospholipases A(2), B, and C and yeast phospholipase D produced a broad band of radioactivity across the gradient without distinct peaks. [(3)H]choline was released when natural surfactant containing [(3)H]choline-labeled DPPC was cycled with yeast phospholipase D but not with convertase or peanut and cabbage phospholipases D. Similarly, yeast phospholipase D hydrolyzed [(3)H]choline from [(3)H]choline-labeled DPPC after incubation in vitro, whereas convertase, liver esterase, or peanut and cabbage phospholipases D did not. Thus convertase, liver esterase, and plant phospholipases D did not hydrolyze choline from DPPC either on cycling or during incubation with enzyme in vitro. In conclusion, conversion of heavy to light subtype of surfactant by convertase may require a phospholipase D type hydrolysis of phospholipids, but the substrate in this reaction is not DPPC.
Dhand
R
R
Division of Pulmonary and Critical Care Medicine, Edward Hines Jr. Veterans Affairs Hospital and Loyola University of Chicago Stritch School of Medicine, Hines, Illinois 60141, USA. dhand@research.hines.med.va.gov
Young
J
J
Teng
A
A
Krishnasamy
S
S
Gross
N J
NJ
D015060:1,2-Dipalmitoylphosphatidylcholine / Q000378:metabolism*; D000818:Animals; D002265:Carboxylic Ester Hydrolases / Q000096:biosynthesis*; D002794:Choline / Q000378:metabolism; D004950:Esterases / Q000378:metabolism; D008099:Liver / Q000201:enzymology; D051379:Mice; D010739:Phospholipase D / Q000378:metabolism; D011663:Pulmonary Surfactants / Q000378:metabolism; D051381:Rats; D013379:Substrate Specificity
D011663:Pulmonary Surfactants; D015060:1,2-Dipalmitoylphosphatidylcholine; D004950:Esterases; D002265:Carboxylic Ester Hydrolases; D010739:Phospholipase D; D002794:Choline