Metabolism of phospholipids VIII. Biosynthesis of phosphatidylcholine in the intestinal mucosa

Abstract

1. 1. The occurrence of the enzyme cholinephosphotransferase (CDPcholine: 1,2-diglyceride cholinephosphotransferase, EC 2.7.8.2) has been demonstrated in the intestinal mucosa of the cat, guinea pig and rabbit. 2. 2. The characteristics of the enzyme with respect to substrate concentrations, pH optimum, Mg 2+ requirement, inhibition by Tween 20 and stability to freezing and thawing were studied, using a mixed particulate subcellular fraction of cat intestinal mucosa as source of the enzyme and 1,2-diglyceride and [ 32P]CMPcholine as substrates. 3. 3. A fractionation of homogenates of the small intestinal mucosa of the cat and guinea pig showed that the enzyme is largely concentrated in the microsomal fraction, though comparison using enzyme markers indicated that it is also a true mitochondrial constituent. 4. 4. Further subfractionation of the microsomal fraction from cat small intestinal mucosa into two smooth- vesicle and one rough-vesicle fractions revealed that the cholinephosphotransferase was ahnost exactly divided between the rough-vesicle fraction and one of the smooth-vesicle fractions, whereas the second smooth-vesicle fraction contained negligible amounts of this enzyme. 5. 5. The enzyme cholinephosphate cytidyltransferase (CTP: phosphorylcholine cytidyltransferase, EC 2.7.7.15) also occurs in the intestinal mucosa of the cat indicating that the CDPcholine-dependent biosynthetic pathway for phosphatidylcholine is operating in the intestinal mucosa. 6. 6. The possibility of alternative pathways for the biosynthesis of phosphatidylcholine in the small intestinal mucosa were examined in studies in vivo using cats injected with inorganic 32P. From activity-time curves of the individual phospholipids it was concluded that neither the methylation of phosphatidylethanolamine nor the Ca 2+-activated exchange reaction of free choline with preformed phospholipid could be a major pathway for the formation of phosphatidylcholine. 7. 7. The results are discussed in relation to fat absorption and formation of chylomicrons.