Metabolism and Activation of N-Nitrosodimethylamine by Hamster and Rat Microsomes: Comparative Study with Weanling and Adult Animals

It has been reported that hamster liver preparations are more effective for the metabolic activation of N -nitrosodimethylamine (NDMA) to a mutagen than rat liver preparations. The enzymatic basis for this phenomenon, however, has not been clearly elucidated. The present study was undertaken to examine the enzymology of NDMA metabolism by different hepatic subcellular fractions prepared from hamsters and rats of two different ages, and to investigate the correlation between the metabolism and the activation of NDMA to a mutagen for Chinese hamster V79 cells. The content of cytochrome P-450 was approximately 1.5-fold higher in hamster microsomes than in rat microsomes from both ages (1.19–1.38 versus 0.73–0.83 nmol P-450/mg protein). Weanling hamster microsomes exhibited multiple apparent K m values for NDMA metabolism as did weanling rat microsomes. The apparent K m I value of NDMA demethylase (NDMAd) in hamster microsomes was about onehalf that in rat microsomes (36 versus 83 µm) with corresponding Vmax values of 2.09 and 2.57 nmol/min/nmol P-450. The K m I values for denitrosation did not differ from the corresponding values for NDMAd with Vmax values of 0.17 and 0.22 nmol/min/nmol P-450 for hamster and rat microsomes, respectively. These apparent K m values were affected neither by sonication nor by the presence of cytosolic proteins in S9 fractions. Adult rat liver microsomes showed less than one-half the NDMAd activity in weanling rat liver microsomes, whereas such age difference was not observed in hamster liver microsomes. This result was confirmed by Western blotting showing the levels of P-450ac (an acetoneinducible form of P-450) of these microsomes at comparable levels to their NDMAd activities. NDMAd was highly correlated to the metabolic activation of NDMA to a mutagen for V79 cells in an activation system mediated by microsomes prepared from hamsters and rats of different ages. The results from this study clearly demonstrate the enzymatic basis for the more effective metabolism of NDMA in adult hamsters than in adult rats.