CLEARANCE OF N-NITROSODIMETHYLAMINE AND N-NITROSODIETHYLAMINE BY THE PERFUSED-RAT-LIVER - RELATIONSHIP TO THE K(M) AND V(MAX) FOR NITROSAMINE METABOLISM

The first-pass clearance of dietary N-nitrosodimethylamine (NDMA) by the liver is the most important factor in the pharmacokinetics of this carcinogen in the rat, but is less important in the pharmacokinetics of N-nitrosodiethylamine (NDEA). The reason for the difference in clearance of these two nitrosamines is not known. These experiments were carried out to see whether the general characteristics of the clearance of these two carcinogens in vivo could be reproduced in the perfused liver, and whether the clearance could be correlated with the Michaelis-Menten parameters K(m) and V(max) for their metabolism. If this could be done one would be able to predict the possible extent of first-pass clearance of nitrosamines in man from measurement of K(m) and V(max) for nitrosamine metabolism by the human liver. The K(m) (22 muM) and V(max) (10.2 and 13.4 nmol/g liver/min) for the metabolism of NDMA by slices from two human livers, the inhibition of that metabolism by ethanol (K(i) 0.5 muM), and the Tate of N-7 methylation of DNA when slices are incubated with NDMA, were measured. These results are similar to those reported previously with rat liver. The K(m) (27 muM) for the metabolism of NDEA by rat liver slices and the inhibition of that metabolism by ethanol (K(i) 1 muM) were estimated from the rate of ethylation of the DNA of the slices. The clearance of both these nitrosamines by the perfused rat liver was measured, and the results appeared to parallel those in vivo with a striking difference between the clearance of NDMA and NDEA. The maximal rate of clearance of NDMA was 11.2 nmol/g liver/min and of NDEA 8.9 nmol/g liver/min, similar to the V(max) for metabolism of NDMA by liver slices and to the estimated maximal rate of liver metabolism of both nitrosamines in the living rat. However, although the K(m) for metabolism of these two nitrosamines by liver slices is similar (about 25 muM), the logarithmic mean sinusoidal concentration [see Bass and Keiding, Biochem Pharmacol 37: 1425-1431, 1988] giving half maximal clearance during perfusion (the equivalent to K(m)) was 2.3 muM for NDMA and 10.6 muM for NDEA. The almost 5-fold difference between these two values is the basis for the difference between the clearance of the two nitrosamines. These values could not be correlated with the K(m) for metabolism of these nitrosamines by liver slices or microsomes, suggesting that it will be impossible to predict the clearance of nitrosamines in man from measurements of the characteristics of the metabolizing enzymes in vitro.