Table 2.

Effect of ionic strength on hybridase activity and T_m of the double-helical substrate

Ionic strength (mM)	V0/[E] (s−1 × 106) with		Tm (°C)
	hRNase-1	RNase A
30	178 000 ± 12 000	390 ± 40	58.1
90	185 000 ± 19 000	270 ± 10
150	1900 ± 100	10 ± 1	61.8
210	1200 ± 100	5.4 ± 0.4
270	28 ± 2	ND

Hybridase assays were performed at 37°C in 40 mM Tris–HCl (pH 7.4) containing 20 µg/ml BSA, 10 nM substrate and variable concentrations of NaCl. Enzyme concentrations were 0.02–100 nM and 0.01–1.2 µM for the human and bovine enzyme, respectively. Activity values are the mean ± SEM of three to four measurements. Values of T_m were obtained from thermal transition profiles obtained by raising the temperature of undigested reaction mixtures from 22 to 82°C over a 90 min period. Each T_m is the mean of two values deviating from the mean by <0.6%. ND, not detectable.