NAD + loading of mammalian cells by electrotransfection leads to increased poly(ADP-ribosyl)ation capacity

Many cellular enzymes use NAD + as coenzyme or substrate, depending on the nature of the enzymatic reaction. Under certain conditions the cellular NAD + concentration may become rate-limiting for such enzymes. For instance, when eucaryotic cells are exposed to high concentrations of DNA-damaging agents, the resulting DNA strand breaks may stimulate the nuclear enzyme poly(ADP-ribose) polymerase (PARP) to such an extent that the cellular pool of NAD +, which is the substrate for this enzyme, is severely depleted, possibly leading to acute cell death. Here we show that NAD + concentrations in CV-1 monkey and CO60 hamster cells can be raised 3- to 4-fold by electrotransfection of NAD +. This additional NAD + is indeed available for PARP to synthesize higher-than-normal amounts of p poly(ADP-ribose) after treatment with the alkylating agent N-methyl- N′-nitro- N-nitrosoguanidine. NAD + loading of cells by electrotransfection may be useful also for the study of other cellular reactions in which NAD + is involved.