Apparent maintenance of cytochrome P 450 by nicotinamide in primary cultures of rat hepatocytes.

The sole addition of a high, unphysiological, concentration of nicotinamide (25 mM) to a cell culture medium was found to maintain the cytochrome P 450 concentration of rat hepatocytes cultured for 24 hours at 71% of the level found in intact liver, whilst hepatocytes cultured without nicotinamide contained only 20% of their initial cytochrome P 450. Furthermore the P 450 concentration of hepatocytes cultured for 24 hours in the presence of 25 mM nicotinamide could be increased to the same level as found in intact rat liver by the inclusion of 1 mM nicotinamide into the medium used for cell isolation. Although the mechanism of action of nicotinamide is unknown this simple system for the maintenance of cytochrome P 450 in hepatocyte culture could provide the opportunity to study, under defined conditions $\text{in vitro}$, the factors that regulate cytochrome P 450 and hence determine hepatotoxicity and hepatocarcinogenesis.