Mechanism of D-amphetamine inhibition of protein synthesis

At 1 h after intraperitoneal administration of D-amphetamine sulphate (15 mg/kg), rat brain polyribosomes show disaggregation accompanied by reduced capacity for in vitro peptide chain elongation. The direct action of amphetamine on cell-fine protein-synthesizing systems was therefore explored. When brain or liver polyribosomes from untreated rats were incubated with pH 5 enzyme, peptide chain elongation was not inhibited by the addition 4 mM amphetamine to the medium. On the other hand, an initiation-dependent system consisting of rat liver of brain mRNA and wheat germ S-30 fraction showed inhibition of [3H]leucine incorporation by 50% when 4 mM amphetamine were added. The metabolites of amphetamine, p-hydroxyamphetamine and p-hydroxynorephedrine, had no inhibitory action in either system, but the potent neurotoxin p-chloroamphetamine was a more powerful inhibitor of initiation than amphetamine. By using [3H]amphetamine, it was shown that amphetamine binds to the 80-S ribosomes of the wheat germ system. This binding depended on the presence in the system of natural liver or brain mRNA or several synthetic mRNAs, but was not promoted by polyuridylic acid as the messenger. Significantly, polyuridylic acid-dependent polyphenylalanine synthesis by the wheat germ system was not inhibited by amphetamine or p-chloroamphetamine. Therefore, it was concluded that amphetamine inhibits protein synthesis by interfering with initiation through a step related to formation of the mRNA ribosome complex.