Biosynthesis of pulmonary surfactant: Comparison of 1-palmitoyl- sn-glycero-3-phosphocholine and palmitate as precursors of dipalmitoyl- sn-glycero-3-phosphocholine in adenoma alveolar type II cells

Abstract

Urethan-induced pulmonary adenomas of mice are composed of cells that appear to be morphologically identical to alveolar type II cells and synthesize disaturated diacyl- sn-glycero-3-phosphocholine, the major component of pulmonary surfactant. 1-[1- 14C]Palmitoyl- sn-glycero-3-phosphocholine and [1- 14C]palmitic acid were compared as precursors of disaturated diacyl- sn-glycero-3-phosphocholine in the adenoma type II cells by incubating both substrates with whole adenomas. When the precursors were compared at equal concentrations (100 μ m) in the presence of albumin (1 mg/ml), the rates of incorporation of 1-[1- 14C]palmitoyl- sn-glycero-3-phosphocholine and [1- 14C]palmitic acid into diacyl- sn-glycero-3-phosphocholine were 5.2 and 2.9 nmol/min · g tissue, respectively. The concentration of monoacyl- sn-glycero-3-phosphocholine (lysolecithin) in the blood plasma of BALB/c mice was 150 μ m. In short-term labeling experiments, the label in disaturated diacyl- sn-glycero-3-phosphocholine was equally distributed between the sn-1 and sn-2 positions when 1-[1- 14C]palmitoyl- sn-glycero-3-phosphocholine was the precursor, whereas 75 to 80% was in the sn-2 position when [1- 14C]palmitic acid was the precursor. The ratios are consistent with incorporation of 1-palmitoyl- sn-glycero-3-phosphocholine via the lysolecithin:lysolecithin transacylase reaction and incorporation of palmitate via acylation of 1-palmitoyl- sn-glycero-3-phosphocholine by acyl-CoA:lysolecithin acyltransferase. 1-[1- 14C]Palmitoyl- sn-glycero-3-phospho-[ 3H- methyl]choline was incorporated into total cellular diacyl- sn-glycero-3-phosphocholine with an isotope ratio similar to that of the precursor; the disaturated species was more enriched in 14C. These findings indicate the cells take up intact monoacyl- sn-glycero-3-phosphocholine and incorporate it into diacyl- sn-glycero-3-phosphocholine. The ability of the cells to utilize intact lysophosphoglycerides for synthesis of cellular lipids was further demonstrated by showing that ether analogs, 1-alkyl- sn-glycero-3-phosphocholine and 1-alkyl- sn-glycero-3-phosphoethanolamine, are taken up and acylated by the cells. Activities of lysolecithin:lysolecithin transacylase and acyl-CoA:lysolecithin acyltransferase were measured in subcellular fractions of the adenoma type II cells; the specific activities of the enzymes were 2.1 nmol/min · mg soluble protein and 21 nmol/min · mg microsomal protein, respectively. The total activity of the acyltransferase in the cell fractions was about four-fold higher than the activity of the transacylase. Characteristics of the two enzymes were studied and are discussed. The findings indicate that exogenous 1-palmitoyl- sn-glycero-3-phosphocholine and palmitic acid both serve as efficient precursors of disaturated diacyl- sn-glycero-3-phosphocholine in the adenoma alveolar type II cells.