Microsomal CTP: choline phosphate cytidylyltransferase: kinetic mechanism of fatty acid stimulation

Fatty acids are known to cause an increase in the incorporation of radioactive choline into phosphatidylcholine. A coincident increase in membrane cytidylyltransferase activity is well documented. The purpose of the present studies was to determine the direct effects of oleic acid on the kinetic properties of membrane cytidylyltransferase. An examination of the reaction characteristics of membrane cytidylyltransferase revealed that membranes from adult rat lung contained high CTPase activity. This activity prevented the determination of reaction velocities at low CTP concentrations. The CTPase activity was blocked by the addition of ADP or ATP to the reaction. The addition of 6.0 mM ADP to the assay mixture enabled us to determine the effect of oleate on the CTP K,,,. Oleate (122 p'.:) caused a significant decrease in CTP K m for microsomal cytidylyltransferase (0.99 mM to 0.33 raM) and H-Form ¢ytidylyltransferase (1.04 mM to 0.27 raM). Oleate did not decrease the CTP K~ for L-Form ¢ytidylyltransferase. Oleate had no effect on the choline phosphate K m in microsomal, H-Form or L-Form cytidylyitransferase. Oleate also increased the Vmx for cytidylyltransferase. The increase was dependent upon the concentration of oleate with a maximal increase of 50-60% at 100-130 /zM oleate. We conclude that oleate has a direct stimulatory effect on cytidylyltransferase when it is in the active form (membrane bound or H-Form lipoprotein complex). We suggest that the kinetic cffccts operate synergistically witlt oi|te,' i egttlaiory tnechal|i.~tt|Is such as translocation or conversion of inactive to active species. The direct effect of oleate on the cytidylyltransferase may be an important regulatory mechanism when CTP concentrations are limiting.