Ethionine is the toxic S-ethyl analog of the essential amino acid methionine. Whereas in prokaryotes the ethionine just competes with the methionine, in eukaryotes it can also be transformed into S-adenosyl-ethionine (Ado-Eth), competing with the S-adenosyl-methionine (Ado-Met). When the Ado-Met synthetase activity was studied in strains defective in either of the two isoenzymes, the one coded by the SAM1 gene was totally unable to convert ethionine into Ado-Eth and was inhibited by the analog, whereas the enzyme coded by the SAM2 gene was able to bind ethionine and was not inhibited by it. This has allowed the development of a procedure to measure Ado-Met synthetase and differentiate between the two isoenzymes present in Saccharomyces cerevisiae.