Table 4.

Specificity of GEK1 towards the amino acid esterifed to tRNA

Substrate	Initial rate (s−1)
d-Tyr-tRNATyr	25
l-Tyr-tRNATyr	<10−3
Diacetyl-L-Lys-tRNALys	<1.6 × 10−4

Initial rates of hydrolysis of d-[³H]Tyr-tRNA^Tyr, l-[¹⁴C]Tyr-tRNA^Tyr or diacetyl-l-[¹⁴C]Lys-tRNA^Lys catalyzed by purified GEK1 (3–6 pM) were measured at 28°C for 5 min in the presence of 20 mM Tris-HCl (pH 7.8), 100 nM substrate, 5 mM MgCl₂, 40 μM zinc acetate, 50 μg/ml BSA and 2.5 mM 2-mercaptoethanol. Prior to the assay, GEK1 was diluted in 20 mM Tris-HCl (pH 7.8) containing 160 μM zinc acetate and 200 μg/ml BSA. Shown values are within ± 15%.