The biochemical mechanisms by which deoxyadenosine (in the prsence of an adenosine deaminase inhibitor), 2-chlorodeoxyadenosine and 2-bromodeoxyadenosine exert their cytotoxic effects on cells will be further investigated and compared. These nucleosides are markedly cytotoxic to human T-lymphoblastoid, B-lymphoblastoid and myeloid cell lines in cutlure and produc therapeutic responses against murine tumors and against human hemaologic malignancies. One part of the project will be aimed at determining whether inhibition of ribonucleoside diphosphate reductase by tripyhospahtes of the nucleosides is the main mechanism of cytotoxicity. Some of these experiments will involved work with intact cells, and for this purpose the human T-lymphoblastoid cell line CCRF-CEM will be used. Other studies will employ partially purified reductase from L1210 cells and highly purified reductase from calf thymus. In the second part of the project evidence will be obtained as to whether inhibition of DNA synthesis by these necleosides involves another signifiicant mechanism. Particular attention will be given to the possibility that DNA polymerase Alpha is inhibited, or that incorporation of analogue into DNA inhibits chain elongation. Purified DNA polymerase Alpha will be used for these experiments. The use of cells selected for resistance to the analogues will also be used in both parts of the project.