AZT treatment dramatically alters the pattern of glycosphingolipid biosynthesis, nearly abolishing ganglioside synthesis at clinically relevant concentrations (1-5 M), and suppresses the incorporation of both sialic acid and galactose into proteins. Control experiments demonstrate that these changes do not result from nonspecific effects on either the secretory apparatus or protein synthesis. On the other hand, studies using isolated nuclei as a model system for chromosomal DNA replication show that AZTTP is a very weak inhibitor of DNA synthesis. These observations strongly suggest that the myelosuppressive effects of AZT in vivo are due to inhibition of protein and/or lipid glycosylation and not to effects on chromosomal DNA replication. 3Ј-Azidothymidine (AZT)1 is one of the primary chemotherapeutic agents used in the treatment of HIV infection (1). This drug is effective because the triphosphate form of AZT, AZTTP, is a potent and somewhat selective inhibitor of HIV reverse transcriptase (2). Unfortunately, AZT therapy is often accompanied by side effects such as severe anemia and neutropenia due to inhibition of the maturation of blood stem cells, especially in the late stages of the disease (3).The current paradigm to explain AZT's hematologic toxicity focuses on DNA replication. AZT is proposed to impede growth or development of stem cells through incorporation of the analog into chromosomal DNA (3). This hypothesis is consistent with the rapid proliferation of blood stem cells and their general sensitivity toward inhibitors of DNA replication (for example cancer chemotherapeutics). However, AZTTP is a remarkably weak inhibitor of the three nuclear replicative DNA polymerases, ␣, ␦, and ⑀ (4, 5). Under physiological nucleotide concentrations, the amount of AZTTP needed to inhibit these enzymes is much higher than the concentrations that accumulate in treated cells (6) and raises the possibility that the myelosuppressive effects of AZT are not related to inhibition of chromosomal DNA replication.We recently demonstrated that the primary intracellular metabolite of AZT, AZTMP, is a potent competitive inhibitor of pyrimidine nucleotide sugar import into Golgi-enriched membrane fractions (7). Consequently, the glycosylation reactions that occur within the Golgi lumen were almost completely inhibited. Since AZTMP is known to accumulate to millimolar levels in several cell types (8), these observations suggested a novel mechanism for AZT toxicity, namely selective inhibition of lipid and protein glycosylation.Several lines of evidence indicate that inhibition of glycosylation could indeed lead to cytotoxicity. Small changes in glycosphingolipid synthesis can profoundly affect signal transduction, differentiation, and cell-cell interactions. Ganglioside synthesis varies in a characteristic manner during growth and differentiation (9, 10), and subtle changes in glycolipid composition dramatically alter the properties of many receptors and enzymes (10). For example, variations in ganglioside composition as small a...