The relation between reaction kinetics and mutagenic action of mono-functional alkylating agents in higher eukaryotic systems. I. Recessive lethal mutations and translocations in Drosophila

Abstract

The relationship in Drosophila males between chemical reaction pattern of mono-functional alkylating agents (AA), described in terms of primary alkylation pattern with DNA and proteins as well as the Swain--Scott s factor, and their biological effectiveness were investigated. The agents chosen for comparative analysis were the nitrosamides ENU and MNU, the methanesulfonic esters iPMS, EMS and MMS, the dialkylsulfate DMS, and the nitrosamines DEN and DMN. Parameters of their biological activity were mortality (LC50) of treated adult males, induction in post-meiotic stages of X-chromosomal recessive lethal mutations and 2--3 translocations after either adult feeding or injection. Induced frequencies of recessive lethals, determined for each AA with a range of concentrations, served as biological dosimeter for interaction with target DNA in the germ line. The results are interpreted as indicating for these AA a causal connection between the pattern of primary alkylation of DNA and the quality of genetic damage observed. 1. The agent with the lowest s value, ENU, and its pendant DEN, failed to produce translocations at mutation frequencies that reached 44% for ENU. The highest chromosome-breaking activity was attributed to AA with high s, MMS and DMS. For MMS, the proportions of translocations (T) to mutations (M) approximately reached a 1 : 1 ratio in stored spermatozoa, at a recessive-lethal frequency of 14%. Ability to break chromosomes, as indicated by the T : M ratios, decreased in the sequence MMS greater than or equal to DMS, MNU greater than DMN greater than EMS greater than iPMS greater than ENU = DEN. 2. Nearly the reversed sequence in relative mutagenci effectivenss was obtained when the (directly acting) AA were arranged on the basis of their CM4/LC50 ratios (CM4, the exposure condition producing 4% recessive lethals after injection): ENU greater than EMS greater than iPMS, MNU greater than MMS = DMS. 3. Among the AA, EMS had a somewhat unique position, in that it was slightly less effective in the translocation test, and also less cytotoxic but more mutagenic in the recessive-lethal test than one would expect from its s value. This is taken as an indication of the influence on biological effectiveness of factors other than the s value, e.g. methylation versus ethylation and the lipid/water partition ratio. An example of the latter was also provided by DMS which, although having the same s as MMS, with its 5-fold higher lipid/water partition ratio, was more toxic than MMS. 4. For those AA that were clearly active in the translocation tests--MMS, DMS, MNU, DMN and EMS--delayed formation of exchanges was observed. Only in 17 out of 555 translocation tests with positive response translocations were already found in progeny from unstored spermatozoa. Consequently, it was concluded that performance of storage experiments in Drosophila is an absolute necessity for the detection of this type of rearrangement by AA. 5...