Inhibition of aminoacyl-transfer RNA formation by low-molecular substances from melanoma extract

Two partially purified fractions of the ethanol precipitate (70-95%) of the water extract of Harding-Passey mouse melanoma, which inhibit protein and DNA syntheses of B-16 melanoma cells in culture, also inhibit protein synthesis in various cell-free systems. By examining their inhibitory effects on limited reactions of protein synthesis, it was found that one of them (ME II) inhibits protein synthesis by blocking aminoacyl-tRNA formation, while the other (ME IV) does not. This inhibition of aminoacyl-tRNA formation was not limited to specific amino acids. Since the amino acid-dependent pyrophosphate (PPi)-ATP exchange reaction catalyzed by aminoacyl-tRNA synthetases was not inhibited, it was concluded that some factor(s) in ME II inhibits amino acid transfer from aminoacyl-AMP to tRNA. ME II contains more than 20 proteins from 10,000 to 90,000 daltons. EDTA treatment of this fraction caused the release of low-molecular substances with inhibitory activity from the proteins. The molecular weights of the active substances are less than 5,000 daltons. The active low-molecular substances are apparently not peptides or nucleotides.