Metabolic activation of cyanamide to an inhibitor of aldehyde dehydrogenase in vitro.

The inhibition of aldehyde dehydrogenase (AIDH) by cyanamide is dependent on the conversion of the latter to an active metabolite. This accounts for the in vivo activity of cyanamide in raising ethanol-derived blood acetaldehyde levels to the mM range in the rat (ED50 for cyanamide = 0.11 mmole/kg) and its lack of inhibitory activity in vitro with purified AIDH enzymes. Liver mitochondria were shown to catalyze this activation. The Low Km mitochondrial AIDH isozyme was strongly inhibited by cyanamide when measured in intact rat liver mitochondria (I50 = 2.0 microM). Cyanamide also inhibited yeast AIDH when incubated in the presence, but not in the absence, of rat liver mitochondria (I50 = 7.8 microM). Using yeast AIDH activity as a measure of cyanamide activation, the subcellular distribution of the cyanamide-activating system was assessed. Microsomes plus an NADPH generating system were equally active as mitochondria in activating cyanamide. In the absence of NADPH, microsomal activity was about half that of mitochondria. Little or no activity was found in the cytosolic fraction. A series of cyanamide analogs and derivatives were screened for their ability to inhibit the low Km AIDH isozyme measured in intact mitochondria. Only monoalkylcyanamides exemplified by n-butylcyanamide showed significant inhibition. Other cyanamide analogs and derivatives including N-acetylcyanamide, the major urinary metabolite of cyanamide, were inactive in this system.