The Influence of Exogenous and of Membrane-Bound Phosphatidate Concentration on the Activity of CTP: Phosphatidate Cytidylyltransferase and Phosphatidate Phosphohydrolase

Abstract

Rat liver microsomes were treated with phospholipase D to obtain microsomal membranes with varying amounts of membrane‐bound phosphatidate. This treatment did not impair the activity of two microsomal‐bound enzymes acting with phosphatidate as substrate, i.e. CTP: phosphatidate cytidylyltransferase and phosphatidate phosphohydrolase. The dependency of the activity of these enzymes on the concentration of membrane‐bound phosphatidate was determined. Both enzymes showed a linear increase in activity with membrane‐bound phosphatidate concentrations up to at least 100 nmol phosphatidate/mg microsomal protein. These results indicate that both enzymes have a large reserve capacity and suggest that the enzymes are operating intracellularly, i.e. at phosphatidate concentrations of 5–10 nmol/mg endoplasmic reticulum protein, far below their maximal capacity. The ratio of phosphatidate conversion into CDP‐diglyceride and 1,2‐diglyceride seems to be constant for a large range of membrane‐bound phosphatidate concentrations. The membrane‐bound enzymes cannot utilize phosphatidate substrate present in heat‐denatured membranes, but are active on phosphatidate incorporated into membranes of phospholipid vesicles.