Synthesis of Phosphatidylcholines in Ozone-Exposed Alveolar Type II Cells Isolated from Adult Rat Lung: Is Glycerolphosphate Acyltransferase a Rate-Limiting Enzyme?

Abstract

Type II cells were exposed to ozone by gas diffusion through the thin Teflon bottom of culture dishes. The rate of phosphatidylcholine synthesis by type II cells, monitored by the incorporation of [Me-14C]choline, was impaired by ozone at concentrations that did not affect other cellular parameters. The enzymes choline kinase and cholinephosphate cytidylyltransferase were not susceptible to inactivation by ozone at concentrations at which the activity of glycerolphosphate acyltransferase was decreased. The enzyme activity of lactate dehydrogenase increased after ozone exposure. The specific activity of choline kinase in the cytosolic fraction of type II cells was fivefold that in whole lung. The metabolism of [Me-14C]choline was studied as a function of the choline concentration. Maximal rates of phosphatidylcholine synthesis were already attained at a concentration of 20 microM choline. Exposure of type II cells to ozone did not affect the recovery of label from [Me-14C]choline in choline phosphate and CDP choline. However, the maximal rate of phosphatidylcholine synthesis decreased after ozone exposure, which indicates that the decreased apparent activity of glycerolphosphate acyltransferase limits the supply of diacylglycerols and thereby the rate of phosphatidylcholine synthesis. If the flux through the diacylglycerol pathway was stimulated by the addition of palmitic acid, a higher maximal rate of phosphatidylcholine synthesis was observed. The uptake of [Me-14C]choline and the recovery of label in CDPcholine were not altered by the addition of different concentrations of palmitate. It is concluded that type II cells take up choline very efficiently, probably due to the high specific activity of choline kinase. At low choline concentrations the rate of phosphatidylcholine synthesis is determined by the supply of CDPcholine. At concentrations of choline in the upper physiological range, the rate of phosphatidylcholine synthesis is determined by the availability of diacylglycerols, which in turn is limited by the apparent activity of glycerolphosphate acyltransferase.

Lung lipid composition in zinc‐deficient rats

“…Adult type II cells may be different from other cells because of their exceptionally high rate of uptake and choline phosphorylation and their high rate of PC synthesis, even at low substrate concentration (22). In our experiments, the rate of PC synthesis from [Me-14 C]choline appeared to be sensitive to ZD because the radioactivity increased 12% in PC, but in SPH it decreased 73% in relation to the control group (Fig.…”

“…CTP:cholinephosphate CYT assay. Lungs of control and zinc-deficient rats were used for the separation of cytosol and microsomes for enzymatic determination (22). Assay mixtures contained 20 mM Tris/succinate pH 7.8, 6 mM MgCl 2 , 8mM CTP, and 4 mM [Me-14 C]phosphorylcholine (specific radioactivity 1 µCi/assay).…”

Stress oxidativo na lesão pulmonar neonatal

“…Este último leva à produção de radicais hidroxil, altamente reactivos. A enzima glicerol-3-fosfatase acetiltransferase, que catalisa a primeira reacção na síntese de todos os fosfoglicerídeos necessários para a formação de surfactante, é extremamente sensível ao stress oxidativo 60 . Todos estes eventos afectam o sistema de produção e de funcionamento do surfactante, com compromisso da função pulmonar e diminuição da resistência à infecção 58 .…”

Effect of air pollutants on the pulmonary surfactant system

“…It was shown that incorporation of a number of precursors into PC was clearly reduced after O 3 . In parallel it was found that the activity of the glycerol‐3‐phosphate acyltransferase, which catalyses the first reaction in phosphoglyceride synthesis, was significantly decreased, whereas other enzymes remained unaltered [ 124, 125]. Also, after exposure to 100% hyperoxia (64 h), the activity of this enzyme was decreased.…”