INCORPORATION OF 14C‐CHOLINE INTO PHOSPHOLIPIDS OF RAT LIVER MICROSOME SUBFRACTIONS: EVIDENCE AGAINST FORMATION OF ENDOPLASMIC RETICULUM FROM THE NUCLEAR MEMBRANE

Methyl‐labelled 14 C‐choline was injected intraperitoneally into rats, and at various times from 10 to 60 min. thereafter the amount of radioactivity in the phospholipids of liver cell nuclei, granular (rough‐surfaced) endoplasmic reticulum and agranular (smooth‐surfaced) endoplasmic reticulum was examined. The phospholipids of all subcellular fractions exhibited significant labelling within 10 min. after injection; incorporation appears to take place through de novo phospholipid synthesis via phosphorylcholine. There was no evidence of precursor‐product relationships between the phospholipids in the different fractions. Consequently, these findings do not support the hypothesis that the membranes of the endoplasmic reticulum arise from the nuclear membrane or that the agranular reticulum is derived from the membrane of the granular reticulum. The phospholipids in each membrane thus appear to be metabolically independent.