The antimalarial drug, chloroquine, enhances both the rate and extent of enzymatic charging of tryptophan to transfer ribonucleic acid of Escherichia coli. The effect occurs throughout a 230-fold purification of tryptophanyl transfer ribonucleic acid synthetase. Chloroquine does not affect the rate of tryptophan-dependent adenosine triphosphate-[32P]inorganic pyrophosphate exchange. Although the synthetase forms tryptophanyladenosine triphosphate ester in addition to tryptophanyl transfer ribonucleic acid, chloroquine does not significantly affect formation of the tryptophanyladenosine triphosphate. Chloroquine changes neither the Km nor the Vmax for the active form of tryptophan transfer ribonucleic acid. The effect of chloroquine on reaction rate can be attributed entirely to its conversion of the inactive form of tryptophan transfer ribonucleic acid into the active form when the latter is at suboptimal concentration. The inactive form of tryptophan transfer ribonucleic acid is not an inhibitor of the enzyme.
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