Sphingomyelin synthase in rat liver nuclear membrane and chromatin

The presence of phospholipids in chromatin has been demonstrated, as well a s the difference in composition and turnover compared to those present in t he nuclear membrane. Recently, some enzymes were also evidenced in chromati n: the base exchange protein complex and neutral sphingomyelinase. The latt er has a particular relevance, since sphingomyelin is one of the phospholip ids more represented in chromatin, We therefore decided to study the synthe sis of sphingomyelin in chromatin and in nuclear membrane isolated from liv er nuclei. The evaluation of tbe enzyme was made (i) using [H-3]phosphatidy lcholine as donor of radioactive phosphorylcholine and (ii) by identify inn the product isolated by thin layer chromatograph!, In both fractions the e nzyme phosphatidylcholine :ceramide phosphocholine transferase or sphingomy elin synthase was present, although with higher activity in nuclear membran e, The enzyme present in the chromatin differs in pH optimum and K-m, showi ng a higher affinity for the substrates than that of nuclear membrane. The results presented show that sphingomyelin synthase is present not only in t he cytoplasm at the level of the Golgi apparatus, but also in the nuclei, a t the level of either the nuclear membrane or the chromatin, (C) 1999 Feder ation of European Biochemical Societies.