Rat Liver Glutamyl Ribonucleic Acid Synthetase

The properties of glutamyl ribonucleic acid synthetase purified from rat liver were investigated. The pH optimum for glutamyl-RNA formation was about 6.8. The apparent Km values for glutamate, adenosine triphosphate, Mg++, and soluble RNA in the formation of glutamyl-RNA were 1.3 x 10-4 m , 2.1 x 10-4 m , 1.25 x 10-3 m , and 2.9 x 10-7 m , respectively. The enzyme also displayed some activity in the presence of Mn++ or Co++. Deoxy-ATP could partially replace ATP as the energy source. The enzyme was very specific for the glutamic acid structure, with many closely related analogues being unable to inhibit glutamyl-RNA formation. The purified liver enzyme was unable to utilize soluble RNA from yeast, Escherichia coli, or Bacillus subtilis. p-Mercuribenzoate, at very low concentrations, completely inhibited the enzyme, and the substrates did not protect against this inhibition. The enzyme catalyzed an anomalously low ATP-PPi exchange reaction under the usual conditions of assay, but this could be stimulated by the addition of high concentrations of glutamate or soluble RNA. The addition of soluble RNA lowered the apparent Km for glutamate in the exchange reaction from 0.2 m to 6.7 x 10-4 m and also increased the Vmax. The pH optima for the exchange in the presence of soluble RNA or high glutamate concentrations were different. The results are discussed in relation to a concerted mechanism of glutamate activation and transfer to soluble RNA and to the possibility of soluble RNA-PPi intermediates.