Bone marrow, purine biosynthesis

Besides this salvage role, hypoxanthine-guanine phosphoribosyltransferase is probably important also for the transfer of purines from liver to other tissues. Purine biosynthesis de novo is especially active in the liver, and extrahe-patic cells that have a low capacity for the synthesis of purines de novo, such as erythrocytes and bone marrow cells, depend on uptake of hypoxanthine and xanthine from the... 

T There are several distinct types of inhibitors of nucleotide biosynthesis, each type acting at different points in the pathways to purine or pyrimidine nucleotides. All these inhibitors are very toxic to cells, especially rapidly growing cells, such as those of tumors or bacteria, because interruption of the supply of nucleotides seriously limits the cell s capacity to synthesize the nucleic acids necessary for protein synthesis and cell replication. In some cases, the toxic effect of such inhibitors makes them useful in cancer chemotherapy or in the treatment of bacterial infections. However, some of these agents can also damage the rapidly replicating cells of the intestinal tract and bone marrow. This danger imposes limits on the doses that can be used safely. 

Methotrexate acts by inhibition of dihydrofolate reductase, the enzyme requisite for the reduction of dihydrofolic acid (3) to 5,6,7,8-tetrahydrofolic acid (4). In turn, (4) is a precursor to a series of enzyme cofactors (5-7) essential for the transfer of one carbon unit necessary for the biosynthesis of purines and pyrimidines and hence, ultimately, DNA. As an inhibitor of dihydrofolate reductase, methotrexate kills cells during the S phase of the cell cycle, when the cells are in the log phase of growth. Unfortunately, this cytotoxicity is non-selective, and rapidly proliferating normal cells, e.g., gastrointestinal epithelium cells and bone marrow, are dramatically affected as well. In addition, recent use of high dose methotrexate therapy with leucovorin rescue has led to additional clinical problems arising from a dose-related nephrotoxic metabolite, 7-hydroxy methotrexate (8). Finally, the very polar nature of methotrexate renders it virtually impenetrable to the blood-brain barrier, which can necessitate direct intrathecal injection in order to achieve therapeutic doses for the treatment of CNS tumours. 

B Mycophenoiate mofetil and other antiproiiferative agents fre-quentiy cause leukopenia and thrombocytopenia. Mycophenoiic acid (active form of mycophenoiate mofetil) inhibits the enzyme inosine monophosphate dehydrogenase, the key enzyme in the de novo pathway for purine biosynthesis. Activated T-lymphocytes cannot use the salvage pathway for purine biosynthesis, and therefore are sensitive to the effects of mycophenoiic acid. The caicineurin inhibitors, IL-2 receptor antagonists, and corticosteroids do not cause bone marrow suppression. 

This reaction is the first committed step in the conversion of IMP to AMP and represents a branch point in the synthesis of IMP, the end product of tie novo purine biosynthesis. Although it had been known for some time that nitrogen from dietary components is rapidly incorporated into cellular AMP, especially into the 6-amino group 1, 2), specific s3mthesis of AMP from IMP was not demonstrated until 1955. Abrams and Bentley (3) showed that the conversion occurred in rabbit bone marrow while Lieberman (4) demonstrated it in Escherichia coli. Carter and Cohen (5, 6) isolated and characterized adenylosuccinate from yeast extracts. Lieberman (4) demonstrated that adenylosuccinate could be formed reversibly from IMP and aspartate by a partially purified enzyme fraction fromE. coli. 

With aminopterin, the reductase forms a complex with a low dissociation constant. Thus, in the presence of aminopterin many of the reductase molecules are trapped in an inactive form. The coenzymes necessary for purine biosynthesis are not formed, and that pathway is blocked. In this manner aminopterin interferes with the progress of leukemia and with the proliferation of normal bone marrow. Unfortunately, the cells of individuals treated for leukemia overcome the metabolic block by building up a resistance to the antimetabolites by increasing the level of the reductase. We will now consider the mechanism of action of each of these coenzymes separately. 

The relative absence of pmine biosynthesis de novo in bone marrow has been demonstrated with glycine-I-C in rabbit marrow slices (4 2), with formate-C in cell suspensions of rabbit marrow 3S5), and with formate-C in human marrow in tissue culture 453). In a comparative study of elementary precursors and purines, it was found that glycine and formate were incorporated into the purines of bone marrow nucleic acids... 

---