Journal of Lipid Research, Vol 19, 467-477, Copyright © 1978 by Lipid Research, Inc.
Pulmonary surfactant synthesis. A highly active microsomal phosphatidate phosphohydrolase in the lung
RD Mavis, JN Finkelstein and BP Hall
Lung cell-free homogenate, which contains about twice the units of
phosphatidate phosphohydrolase per mg of protein compared to liver, was
fractionated by differential centrifugation and the fractions were assayed
for phosphatidate phosphohydrolase and marker enzymes of endoplasmic
reticulum, mitochondria, and lysosomes. Over 60% of the lung phosphatidate
phosphohydrolase was associated with the endoplasmic reticulum, compared to
50% of the total liver enzyme. Thus a major portion of the more active
lung enzyme is potentially involved in lipid biosynthesis by the
endoplasmic reticulum. Less than 0.2% of the total lung enzyme was found in
a lamellar body fraction, consistent with previous findings. The lung
microsomal phosphohydrolase was specific for lipid substrates, showing
equal activity towards phosphatidic acid or lysophosphatidic acid and
relatively low activities towards glycerophosphates. It had a neutral pH
optimum, similar to the liver enzyme, but differed somewhat in its relative
activity at extremes of pH. Stability at 65 degrees C was greater for the
lung enzyme. Fluroide inhibited lung (or liver) microsomal phosphatidate
phosphohydrolase, while tartrate, MgCl2, or EDTA had no effect. The
presence of a high activity of phosphatidate phosphohydrolase in lung
endoplasmic reticulum is consistent with the rapid synthesis of pulmonary
surfactant phosphatidylcholine.
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