Sphingomyelin synthase in rat liver nuclear membrane and chromatin

The presence of phospholipids in chromatin has been demonstrated, as well as the difference in composition and turnover compared to those present in the nuclear membrane. Recently, some enzymes were also evidenced in chromatin: the base exchange protein complex and neutral sphingomyelinase. The latter has a particular relevance, since sphingomyelin is one of the phospholipids more represented in chromatin. We therefore decided to study the synthesis of sphingomyelin in chromatin and in nuclear membrane isolated from liver nuclei. The evaluation of the enzyme was made (i) using [(3)H]phosphatidylcholine as donor of radioactive phosphorylcholine and (ii) by identifying the product isolated by thin layer chromatography. In both fractions the enzyme phosphatidylcholine:ceramide phosphocholine transferase or sphingomyelin synthase was present, although with higher activity in nuclear membrane. The enzyme present in the chromatin differs in pH optimum and K(m), showing a higher affinity for the substrates than that of nuclear membrane. The results presented show that sphingomyelin synthase is present not only in the cytoplasm at the level of the Golgi apparatus, but also in the nuclei, at the level of either the nuclear membrane or the chromatin.