Detoxification of nitrosamides and nitrosocarbamates in blood plasma and tissue homogenates

Abstract

Nitrosamides and nitrosocarbamates exhibit relatively high mutagenic activity in Salmonella when compared with nitrosoureas. This high activity can be accounted for by activation of nitrosamides and nitrosocarbamates by cellular thiols, predominantly reduced glutathione, that are present intracellularly at concentrations in the millimolar range. In striking contrast to the in vitro mutagenicity tests, a number of studies have indicated that nitrosamides and nitrosocarbamates are less potent than nitrosoureas when tested in vivo in model systems such as the mouse. We extend here previous studies [Aukerman et al, 1983] that demonstrate striking chemical decomposition and inactivation of mutagenic activity of nitrosamides and nitrosocarbamates during exposure to murine blood plasma. Plasma glutathione concentrations are inadequate to account for the rapid inactivations noted. Furthermore, the predominant inactivating species is heat‐sensitive, nondialyzable, and is greater than 25,000 daltons in size as judged by ultrafiltration experiments. Serum albumin has some inactivating capacity at the concentration found in undiluted plasma and could account for the very low but significant inactivating capacity of human plasma. On the other hand, serum albumin lacks the potency necessary to account for the extremely high levels of inactivating activity observed in rodent and rabbit plasma. Elsewhere we present evidence that carboxylesterase activity is the predominant inactivating species in mouse plasma [Aukerman et al, 1983; Aukerman, 1983; Brundrett and Aukerman, 1984]. Mouse liver, large intestine, kidney, and stomach have more activity per milligram protein under the assay conditions used than plasma itself. Rat liver S9 is also active at enhancing the decomposition of nitrosamides and nitrosocarbamates; most of this inactivating capacity resides in the microsomal fraction. The relatively rapid detoxification of these N‐nitroso compounds by plasma and other tissues of rodents has important implications regarding the utility of rodents in assessment of tumorigenicity and/ or antitumor activity of these classes of drugs in other animal species. Tests with Salmonella may be of use in estimating relative levels of protection that vary widely among mammalian species.