Mechanisms of nicotinamide and thymidine protection from alloxan and streptozocin toxicity.

A common mechanism has been proposed for the beta-cell toxins alloxan (ALX) and streptozocin (STZ) involving the formation of single-strand breaks in DNA that lead to the overactivation of the enzyme poly(ADP-ribose) synthetase and the critical depletion of its substrate NAD. If the toxins act via this common mechanism, the poly(ADP-ribose) synthetase inhibitors nicotinamide and thymidine would be expected to affect the formation of DNA single-strand breaks in a similar fashion. To test the effects of these inhibitors, the formation of single-strand breaks in the DNA of insulin-secreting RINr cells was monitored by assessing changes in the supercoiling of nucleoids after exposure to STZ, ALX, or methylnitrosourea (MNU). With the inclusion of nicotinamide or thymidine and STZ or MNU, more single-strand breaks in RINr cell DNA were detected. These results would be expected if nicotinamide and thymidine acted through inhibition of poly(ADP-ribose) synthetase. However, when the inhibitors were used in combination with ALX, fewer single-strand breaks were present. This suggests a reduction in ALX-induced hydroxyl radicals available to interact with DNA. Because nicotinamide has been demonstrated to be a hydroxyl-radical scavenger, the ability of thymidine to scavenge hydroxyl radicals was investigated. Thymidine, like nicotinamide, was found to be a potent scavenger of hydroxyl radicals. Thus, the mechanisms by which nicotinamide and thymidine protect against the toxic effects of STZ or ALX appear different. These findings suggest that the actions of beta-cell toxins are more complex than simply the overactivation of a single enzyme.