The rate-limiting reaction in the biosynthesis of surfactant phosphatidylcholine by type II cells isolated from adult rat lung was examined. 2. Studies on the uptake of [Me-14C]choline and its incorporation into its metabolites over a 5 h period indicated that in these cells the cholinephosphate pool was much larger than both the choline and CDPcholine pool. This is consistent with the idea that the rate-limiting reaction is that catalyzed by cholinephosphate cytidylyltransferase. 3. Evidence that cholinephosphate cytidylyltransferase is the slowest of the three enzymes incorporating choline into phosphatidylcholine was also obtained from pulse-chase experiments. [Me-14C]Choline taken up by the cells was rapidly converted into cholinephosphate during the pulse period. As the radioactivity disappeared from cholinephosphate during the chase period, the label was incorporated immediately into phosphatidylcholine, without much change in the labelling of CDPcholine. This indicates that the cholinephosphotransferase is at least as fast as the cholinephosphate cytidylyltransferase. 4. Inclusion of palmitate in the chase medium accelerated the conversion of labelled cholinephosphate into phosphatidylcholine and decreased the radioactivity associated with CDPcholine. This indicates that under these conditions the cholinephosphate cytidylyltransferase reaction cannot keep up with increased utilization of the CDPcholine in the terminal step of the CDPcholine pathway.
