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Methioninyl adenylate is a specific and potent inhibitor of the enzyme methionyl-tRNA synthetase and, consequently, of protein biosynthesis. In cultures of chick embryo fibroblasts infected with Rous sarcoma virus, incubation for a 2-day period with 1 to 3 mM concentrations of this inhibitor, as late as 4 days after infection, irreversibly prevented subsequent formation of foci of transformed cells. Later addition could also elicit the irreversible disappearance of already existing foci, by phenotypic reversion and/or cell killing. Virus production in transformed cells and replication in newly infected cells were also inhibited but to a lesser degree. Under the same conditions, the same concentrations of methioninyl adenylate caused only a reversible growth arrest of normal cells. The selective toxicity of the inhibitor for transformed cells is not due to a greater affinity for the target enzyme, but it may be due to the fact that inhibition of protein biosynthesis is only partially reversible in these cells, whereas it is fully reversible in normal cells.

Treatment of Rous Sarcoma virus transformed chick embryo fibroblasts with 1 mM 5'-deoxy-5'-S-isobutyladenosine for 24 hrs. leads to the inhibition of transforming virus production. A kinetic analysis of the inhibition of active virion production revealed that the effect of the drug was time and concentration dependent. After 24 hrs. with 1 mM SIBA, the production of transforming virus was inhibited 165 fold. However, under these conditions there was only a 2 fold inhibition in viral particle production. Thus, these viral particles were either non infective (non adsorbed on cell membrane) or non transforming. The majority of viral particles produced by cells cultured with the drug have a decreased density. Analysis of these virions showed a decrease of protein P19 and an accumulation of proteins with high molecular weight.

5' -Deoxy-5' -S-isobutylthioadenosine (iBuS)5' Ado has been shown to be rapidly degraded to 5-deoxy-5-S-isobutylthioribose and adenine in procaryotes. In chick embryo fibroblasts there are two metabolic pathways for (iBuS)5' Ado degradation: (a) oxidative deamination into 5' -deoxy-5'-S-isobutylthioinosine (the main product) and (b) hydrolysis into 5-deoxy-5-S-isobutylthioribose plus adenine. The latter reaction is not due to bacterial contamination, since the same results were obtained under sterile conditions and in chick embryo fibroblasts in culture. The inhibition of the virus-induced cell transformation reported by us previously was due to (iBuS)5' Ado rather than to the main metabolic product of this molecule in chick embryo fibroblasts.

The rate of deamination of 5'-deoxy-5'-S-isobutylthioadenosine [(iBuS5'Ado] in chick embryo fibroblasts was substantially reduced after their infection and morphological transformation by Rous sarcoma virus. Concomitant with the reduction in rate of (iBuS)5'Ado deamination there was a decrease in adenosine deaminase and 5'-adenylic acid deaminase activities. The drop of these activities was related to infection and not to the expression of the src gene. (iBuS)5'Ado was deaminated by at least three enzymes or isoenzymes whose apparent molecular weights have been estimated to be 295000, 121000 and 37000 respectively. Two of these enzymes have been characterized as 5'-adenylic acid deaminase and the heavy form of adenosine deaminase, respectively.