Phenylalanine transfer ribonucleic acid synthetase from Drosophila melanogaster: Purification and properties

Phenylalanine transfer ribonucleic acid synthetase from Drosophila melanogaster has been purified 1400-fold over a crude 230,000 g supernatant fraction. The optimum activity of the enzyme occurs at magnesium concentrations above 10 m m at 37 °C and pH 7.5. At a 50 m m Mg 2+ concentration, NH 4 + stimulates the ATP-PP 1 exchange reaction as much as 2-fold. Ammonium chloride causes an increase in the V with no change in the K m with phenylalanine as substrate. Homologous ( Drosophila) tRNA, in the presence of NH 4 +, further stimulates the ATP-PP i, exchange reaction but inhibits the reaction in the absence of NH 4 +. In the presence of its substrates the enzyme is inactivated by NEM to varying degrees depending upon the substrate or combinations of substrates used. In the presence of phenylalanine the enzyme is partially protected but both ATP and tRNA make the enzyme more susceptible to inactivation. NEM together with ATP and tRNA or all three substrates results in near-total inactivation.