CHOLINE BASE-EXCHANGE ACTIVITY IN RAT HEPATOCYTE NUCLEI AND NUCLEAR-MEMBRANES

Previous investigations have demonstrated the presence of phospholipid s as a component of chromatin; however the mechanism of their synthesi s, namely if they are synthesized in the nuclei or in the cytoplasm (m icrosomal fraction), from where they may eventually be transported to the nucleus, has not yet been clarified. The phosphatidylcholine, for example, can be formed, albeit in a limited amount, by an interconvers ion reaction between bases. The aim of the present research was to asc ertain the presence of the enzyme complex responsible for this reactio n in hepatocyte nuclei and in isolated nuclear membrane. The incorpora tion of [C-14]-choline in phosphatidylcholine was assayed in microsome s, hepatocyte nuclei, liver nuclei and nuclear membranes of rat liver. The reaction was Ca2+-dependent and the specific activity was higher in microsomes but was present, albeit at a low level, also in nuclei a nd in nuclear membranes. Possible contaminations were excluded by spec ific microsomal markers and by the reaction time course. In fact, the nuclear reaction reached the maximum level slowly with respect to micr osomes. Since the phosphatidylcholine extracted from the nuclei show a n enrichment in unsaturated fatty acids of monoenoic fraction, such as oleic acid, the difference in reaction kinetics has been tentatively explained as due to the phosphatidylcholine fatty acid content. The pr esence of this base exchange enzyme complex may allow a fast change in chromatin phospholipid composition. (C) 1997 Academic Press Limited.