Lowering extracellular calcium in cultures of human diploid fibroblast-like cells caused a rapid depletion of NAD pools. This loss of NAD was reversed by restoring extracellular Ca2+ and was inhibited by 3-aminobenzamide, an inhibitor of ADP-ribosyl transfer reactions. The concentrations of 3-aminobenzamide needed to inhibit the loss of NAD were consistent with those required to inhibit cellular mono(ADP-ribosyl) rather than poly(ADP-ribosyl) reactions. Calcium depletion did not inhibit the biosynthesis of NAD. These results suggest that mono(ADP-ribosyl)ation is involved in the regulation of cellular Ca2+ levels.

Incorporation of [3H]thymidine into the DNA of fresh human lymphocytes, treated with various chemical mutagens, was measured and correlated with cellular NAD levels before and after treatment. NAD levels in lymphocytes were significantly reduced following treatment with mutagenic chemicals. Reduction of cellular NAD pools was directly correlated with [3H]thymidine incorporation. As NAD levels decreased, [3H]thymidine incorporation increased. Theophylline, a known inhibitor of poly(ADP-ribose)polymerase, inhibited both the NAD reduction in cells treated with DNA damaging agents and the incorporation of [3H]thymidine into DNA. The inhibitory effect of theophylline on NAD depletion and on [3H]thymidine incorporation was dose and cell number dependent. Near normal responses to carcinogen exposure could be restored to theophylline-treated cells following the removal of theophylline. These data suggest that conversion of NAD to poly(ADP-ribose) may be necessary, or at least closely associated with, DNA repair in human lymphocytes.

A component capable of binding benzo(a)pyrene was measured in plasma from cigarette smokers and nonsmokers. This plasma fraction was found to have a high specificity of binding to benzo(a)pyrene, bound benzanthracene competitively with benzo(a)pyrene, and was positively correlated (r = 0.861, p less than 0.001) with the capacity of the individual subject's lymphocytes to be induced for AHH activity in culture. An inverse correlation (r = -0.957, p less than 0.001) between the presence of the plasma component in lung cancer patients and the capacity of lung cancer patients' lymphocytes to be induced in culture is unexplained at this time. A benzo(a)pyrene-binding fraction was not found in induced or uninduced cultured lymphocytes from smokers or nonsmokers, or in homogenates of lung excisional tissue from smokers with or without primary lung cancer.