A study of the effects of exposure to a carcinogen, N-methyl-N'-nitro-N-nitrosoguandine, in bacteria and mammalian cells

It remains to discuss the results of the present study in the context of the known biological activity of MNNG. The potency of the compound as a cytotoxic agent, over other monofunctional agents, is evident from Table 17. This presumably results from the reactivity of the guanido group - most probably with thiol or amino groups as discussed already - in addition to the alkylating properties of the methyl group, both derived from MNNG. Lawley (1966), in reviewing the alkylation of compounds by carcinogens and other agents, adopts molecular size as the criterion of probability that alkylation of a given molecule takes place within a cell. Thus, DNA is alkylated more extensively than RNA, and RNA more than protein (mole for mole). Although methylation of nucleic acids by MNNG is expected in vivo, by analogy with the in vitro results, it is possible that guanidination of groups plays an equal, or greater part in the biological activity of MNNG. From the results of the present study, and the reports of others, the influence of surrounding ionic conditions (pH, thiol, amino), in addition to the locality of MNNG within the cell (structures and/or organelles), would be important in the determination of the groups of the cell which are affected. Thus, the analysis for the-significant target molecule for reaction with MNNG assvunes a qualitative, as opposed to quantitative, nature. It is not surprising that for an agent with such reactivity, there are cells at different stages of the life cycle (G1, S and G2) which are sensitive to MNNG. The sensitivity of cells in S phase was examined further, although only from the standpoint of the gross inhibition of precursor incorporation into DNA and not from kinetic cell cycle analysis. The metabolic inhibition in cells following treatment with MNNG is similar to that following treatment with other alkylating agents. However, in a more detailed examination of the inhibition of precursor incorporation into DNA, it was demonstrated that MNNG inhibited in vitro DNA polymerase activity. Yet, there are several objections to the collation of the in vitro results with the in vivo observation, some of which have already been mentioned in the discussion of the enzyme inhibition. Other features which require to be explained are the persistence of extractable DNA polymerase activity in cells previously treated with MNNG, and also the requirement for preincubation of extract with MNNG in order to demonstrate, satisfactorily, inhibition of in vitro enzyme activity. Since the relationship between the in vitro and in vivo DNA polymerase activities is not known, these anomalies do not negate the probability that the reaction of in vitro inhibition occurs also in vivo. Nevertheless, the general nature of the metabolic inhibition must again be emphasised. In respect of incorporation of precursor into RNA or protein, other enzyme pathways may be shown to be inhibited by MNNG but cannot be commented upon in the absence of experimental results. The two most significant biological activities of MNNG are as a mutagen and as a carcinogen. Bacterial mutagenesis by MNNG is demonstrated in the experiments of Part II, and it may be surmised that here, also, interference with DNA polymerase activity is of importance. The results of the present study may be thought to invoke somatic mutation as the event responsible for MNNG carcinogenesis. However, such an interpretation is a gross oversimplication of events and fails to take into account the observed inhibition of precursor incorporation into RNA or protein (Fig. 40). For instance, MNNG may inhibit mammalian DNA-dependent RNA polymerase; guanidination of thiol or amino groups of proteins is also likely to occur within the cell, with possible consequences to functional activity. Furthermore, Barnes (1968) draws attention to the delay, between administration and toxic effect, with a number of poisons, such as the nitrosamlnes. The target molecule which reacts with the poison, or carcinogen, may have a very slow metabolic turnover and the consequences only become apparent after a number of cell generations. (Abstract shortened by ProQuest.)