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Summary Benzamide and m -aminobenzamide, the most potent inhibitors of poly-(adenosine diphosphate ribose) polymerase known, enhanced unscheduled DNA synthesis after ultraviolet irradiation in human lymphocytes. A positive correlation was found between the inhibitory activities and enhancing effects on unscheduled DNA synthesis of inhibitors related to benzamide and nicotinamide. Lymphocytes of a patient with xeroderma pigmentosum did not show enhanced unscheduled DNA synthesis with these inhibitors after ultraviolet irradiation, but like normal lymphocytes, showed enhanced unscheduled DNA synthesis after treatment with N-methyl-N′-nitro-N-nitrosoguanidine.

Poly(ADP-ribose) metabolism during differentiation of HL-60 human promyelocytic leukemia cells induced by dimethylsulfoxide was studied. Endogenous poly(ADP-ribose) decreased to 15% of the uninduced level at 14 hours after addition of dimethylsulfoxide, when the morphology, phagocytic activity and DNA synthesis of the cells had not yet changed and myeloperoxidase activity had decreased only slightly. Endogenous poly(ADP-ribose) remained at a low level until day 3, but increased sharply to 800% of the uninduced level on day 5 when most of the differentiated phenotypes became obvious. This is the first report of measurement of the endogenous level of poly(ADP-ribose) during cellular differentiation.