Observations on 6-mercaptopurine activity in Saccharomyces cerevisiae

Abstract

Yeast strains carrying the allele su-pur are sensitive to a number of purine analogues including 6-mercaptopurine (6MP), whereas su-pur+-carrying strains are insensitive [1]. Experimentation into 6MP insensitivity revealed that the agent was being utilized as a nutrient source both by the su-pur + strain, and by cells with an adenine requirement. As su-pur + cells reportedly excrete hypoxanthine [1] it is suggested that the biochemistry of 6MP differs in yeast and man, and that degradation of the agent via sulfinic acid to hypoxanthine may occur. INTRODUCTIONDuring a series of experiments to assess the mutagenicity of selected anti-cancer agents in yeast, it was noted that 6MP, a potent anti-metabolite in mammalian cells, caused neither toxicity nor mutagenicity in several test strains. 6MP has been reported to cause growth inhibition in yeast strains containing the su-pur allele [1], however this occurred only under conditions of nutrient limitation. Another purine analogue has also been re-ported to have reduced effectiveness in yeast: 2aminopurine (2AP) was observed to be neither toxic nor mutagenic [2], whilst reported [3] to be mutagenic in an adenine-requiring strain. However, other analogues, including 2,6-diaminopurine which causes mitochondrial mutation [41, and N-6-hydroxyaminopurine [2], are active in yeast. These results suggest that there are qualitative and/or quantitative differences in the biochemistry of 6MP and certain other purine analogues between yeast and mammals. A diagram of the known pathways of 6MP metabolism in man is presented in Fig. 1.The two alleles, su-pur, associated with the inhibition of purine excretion, and believed to be wild-type, and su-pur ÷ (or pur), cells of which genotype excrete purines, including hypoxanthine, were first reported by Armitt and Woods [6]. Analysis, in what was effectively a cis-trans test, of the genetic locus controlling purine nucleotide biosynthesis in yeast [7] has shown that the genotypes su-pur, su-pur ÷ and ade4 are functionally allelic. As ade4 cells lack activity of 5-phosphoribosylpyrophosphate (PRPP) amidotransferase, an enzyme early in the pathway of de novo purine synthesis which is subject to feedback inhibition by excess purines [8], it may be expected 0378-1097/85/$03.30