Table 1. Apparent K_i values for inhibition of Pb²⁺-induced cleavage of pATSerCGSyn and yeast tRNA^Phe.

Divalent Me2+	Apparent Ki/mM pATSerCGSyn	Apparent Ki/mM yeast tRNAPhe	Ionic radius (Å)a	Coordination numbera	Hardness absolutea	Exchange rates for H2O (s–1)a
Mg2+	9.3 ± 2.2	13 ± 1	0.72	6	47.59	105
Ca2+	6.7 ± 3.1	17 ± 1.6	1.12	8	19.52	108
Sr2+	16 ± 3.0	41 ± 0.1	1.18	6	27.30	108–109
Mn2+	3.4 ± 0.75	5.6 ± 0.4	0.67	6	9.02	107
Zn2+	2.8 ± 0.8	51 ± 0.8	0.74	6	10.88	107
Co(NH3)63+	1.0 ± 0.6	ND	0.61	6	8.90	1

The K_i values were determined as outlined in Materials and Methods and each number is an average of several independent experiments.

ND, not determined.

^aNumbers taken from Feig and Uhlenbeck (27) and Cowan (43).