The biological effects of three furocoumarins on the proliferation of human normal peripheral blood lymphocytes have been investigated. Mitogen-stimulated human lymphocytes were assayed "in vitro" by measuring 3H-thymidine (3H-TdR) incorporation in the presence and in the absence of 15-30 μM 3-carbethoxypsoralen (3-CPs), trimethylangelicin (TMA) and psoralen (PSR) with and without UV-A irradiation (365 nm). The three furocoumarins differ in their ability to form mono- and bi-functional adducts with DNA pyrimidine bases and in producing reactive species of oxygen. At low furocoumarin doses and short times of UV-A irradiation (15-30 sec) used in the present study, 3-CPs did not affect 3H-TdR incorporation in PHA-stimulated human lymphocytes, TMA strongly inhibited 3H-TdR incorporation, while unexpectedly, PSR increased 3H-TdR incorporation in the absence of irradiation, likely acting, under these experimental conditions, as a co-mitogen.
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