Various vitamins and vitamin-like substances inhibited the activity of poly(ADP-ribose) synthetase in vitro. The most potent were essential fatty acids, i.e. arachidonic acid, linoleic acid, and linolenic acid; their 50% inhibitory concentrations (IC50) were 44–110 μM, indicating a higher potency than nicotinamide, a well-known vitamin inhibitor (IC50 = 210 μM). Vitamins K3, K1, and retinal were the next strongest inhibitors, followed by α-lipoic acid, coenzyme Q10, and pyridoxal 5-phosphate. Nicotinamide and vitamin K3, exhibited mixed-type inhibition with respect to NAD+,while arachidonic acid exhibited dual inhibitions, competitive at 50 μM and mixed-type at 100 μM.
