Biochemistry - Chemical Reactions in Living Cells, Volume 3 - D. Metzler 1980

Biochemical Genetics and the Synthesis of Nucleic Acids and Proteins
Mutations, Cancer, and Genetic Engineering
Chemical Mutagens

One of the most striking properties of living organisms is the exceptionally high degree of Gene mutability. Harmful Mutations claim many human lives at an early age, and it is widely believed that the exceptionally high incidence of Cancer in older individuals is attributable, at least in part, to the accumulation of somatic mutations. Many Mutations arise as a consequence of METABOLISM/36.html">DNA Replication errors, as well as repair and recombination processes. Mutation rates increase in the presence of Chemical Mutagens, under The Influence of physical factors such as ultraviolet and X-ray irradiation, and through the occasional insertion of viral DNA into Chromosomes.

Mutations such as GC to AT base pair transitions can be induced by a simple chemical Treatment, namely with nitrous acid (HNО₂), which deaminates amino groups to hydroxyl groups. Consequently, cytosine is converted into uracil, which pairs with A rather than G. This essentially constitutes a simple substitution or transition (Section D, 1). Under the action of nitrous acid, adenine is converted into hypoxanthine, which (similar to guanine) tends to pair with C rather than T. (Guanine can also be converted into xanthine, though this substitution apparently has no significant effect on base pairing.) Many other chemical modifications of bases are also mutagenic. For instance, hydroxylamine, which exhibits weak mutagenic properties, can add to the carbon atom at position 6 in Pyrimidines. Among the most potent mutagens are alkylating agents. Regardless of whether they act via an S_N1 or S_N2 mechanism, these compounds tend to interact selectively with the nitrogen atom at position 7 of guanine residues. For various reasons, guanine alkylation leads to an increased frequency of pairing errors¹⁾.

Among the most toxic and potent alkylating agents are mustard gas and its sulfur-containing analogues, such as bis(2-chloroethyl) sulfide.

¹⁾ The generation of a positive charge within the purine ring resulting from the methylation of the nitrogen atom at position 7 facilitates the Hydrolysis of the N-glycosidic bond and apurinization. However, this process may lead to Cell death rather than mutation. It is likely that methylation of the oxygen atom at position 6 of guanine is more critical for mutagenesis.

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Bifunctional compounds of this type are extremely toxic and cause numerous lethal cross-links between DNA strands. Monofunctional "half-mustards" possess mutagenic activity, yet they are less toxic. Another class of potent mutagenic alkylating agents comprises nitrosamines:

In laboratory research, N-methyl-N'-nitro-N-nitrosoguanidine is frequently employed as one of the most effective known;

chemical mutagens. Nitrosamines rank among the most powerful carcinogenic agents and are believed to play a significant role in The Development of human cancer [245]. Nitrosamines can be formed through the reaction of any secondary amine with nitrous acid [equation (15-11)]. This reaction readily occurs in The Stomach, allowing the resulting nitrosamines to be absorbed and promote carcinogenesis at various sites. Since all plants contain certain amounts of nitrates—levels that are quite substantial in some vegetables such as beets and spinach—it is entirely plausible that these nitrates can be reduced to nitrites and react in the stomach with secondary amines According to the equation

Bacon and other cured meats contain both nitrites and nitrates. The presence of secondary amines in many pharmaceutical preparations and natural food products suggests that they may play an important role in human carcinogenesis. The Significance of this issue is further underscored by the fact that quaternary amines can undergo a similar reaction (by losing one of their alkyl groups).

Another mechanism by which chemical compounds can induce base-substitution mutations involves the direct incorporation of the compound into the DNA molecule itself. For instance, 5-bromodeoxyuridine (or bromouracil), a potent mutagenic agent, can substitute for thymidine in DNA. Less efficient agents presumed to act in a similar manner include 2-aminopurine and 2,6-diaminopurine.

Frame-shift mutations (Section D, 1) are considerably less common than base-pair substitutions. Unlike base-substitution mutants, such mutants do not readily revert, and reversion is not induced by agents that cause base substitutions.

At the same time, the reversion of frame-shift mutations is induced by acridines and other planar compounds that act as intercalating agents, "slipping between" the Base Pairs of the DNA helix (Chapter 2, Section D, 9). These same intercalating substances promote frame-shift mutations and are particularly effective at inducing them in regions containing long runs of a single repeated base, such as AAAAAAAAA. The reversion of a Histidine-auxotrophic Salmonella mutant (—1) can be induced by 2-nitrosofluorene, which causes the deletion of two base pairs within a "hot spot" (a site with a high mutation frequency) of the histidine Operon [246]:

Simple intercalating agents often fail to display strong mutagenic activity, whereas compounds combining The properties of both intercalating and alkylating agents prove to be exceptionally effective. An example of such a compound is a potent mutagen containing an intercalating ring and a half-mustard side chain:

When the CH₂Cl group in the side chain is replaced by a CH₂OH group, the mutagenic activity of this compound drops 100-fold.