Phosphatidylcholine biosynthesis in rabbit kidney tubule suspensions. Effect of metabolic substrates on precursor incorporation.

To gain insight in the mechanisms that regulate phosphatidylcholine (PC) biosynthesis we studied the incorporation of [14C]choline and 14C-labelled fatty acids into PC of tubule suspensions of rabbit kidney cortex and outer medulla, in vitro. Moreover, the influence of renal substrates on these incorporation rates were examined. When incubated with saturating concentration of 1 mM [14C]choline tubules incorporated 4.40 +/- 0.36 mumol choline per g protein per h. Addition of fatty acids such as palmitate and oleate led to a significant increase in [14C]choline incorporation into PC. Further addition of renal substrates such as glucose, propionate, proline, glutamine and glutamate further enhanced this stimulated incorporation significantly. Maximal rates of [14C]choline incorporation were found to be 9.94 mumol per g protein per h in presence of choline, oleate and glucose. In tubules of the outer medulla only glucose showed a stimulatory effect. On the other hand, 14C-labelled fatty acids were incorporated into tubular PC to a similar extent as was found for [14C]choline incorporation (4.54 +/- 0.62 and 5.52 +/- 0.77 mumol/g protein per h for oleate and palmitate, respectively). Addition of choline or renal substrates increased fatty acid incorporation in an additive manner. Maximal rates of 14C-labelled fatty acid incorporation were found to be 17.7 mumol per g protein per h in presence of oleate, choline and propionate. In contrast to isotope studies, net PC content decreased rather than increased under all experimental conditions, indicating that results may represent membrane lipid turnover. The observed rates are sufficient to replace total renal membrane PC in 1 day.