Table 1.

Concentration-dependent effects (K_d) of thapsigargin and analogs on SERCA ATPase activity and apparent Ca²⁺ dissociation rates from Ca₂E1–Inh (k₁) in the first step after binding of inhibitor and from CaE–Inh in the second step (k₂)

K_d values were calculated as described previously (23) and based on data from Fig. 2.

Compound	Inhibition	Ca2+ dissociation rate
	Kda	k1b	k2b
	nm	min−1
Tgc	0.2	2.5 ± 0.5c	2.5 ± 0.5
Leu-8ADT	7 ± 1	2.0 ± 0.3	0.55 ± 0.05
βAsp-8ADT	5 ± 1.5	2.3 ± 0.3	0.16 ± 0.01
Boc-8ADTc	4 ± 0.4	0.0215 ± 0.0001	0.0215 ± 0.0001
EpoTg	13 ± 1.5	0.0	0.0

^a Concentration of free inhibitor required for 50% inhibition of Ca²⁺-ATPase activity in 10 mm TES (pH 7.5), 100 mm KCl, 0.1 mm Ca²⁺, 5 mm MgATP, 1 mm Mg²⁺ at 23 ± 0.2 °C. Based on data from Fig. 2.

^b Apparent rate constants for the dissociation constants estimated from the release of the first and second bound Ca²⁺ in the presence of 12 μm Tg or Tg analogs. These rate constants ±S.D. were evaluated from double-exponential analysis of three experiments with well-resolved logarithmic data.

^c Note that for Boc-8ADT and Tg, removal of both Ca²⁺ ions has been evaluated on the basis of a single reaction because kinetically k₁ and k₂ are not sufficiently well resolved to allow estimation of the individual apparent rate constants. For Boc-8ADT, K_d refers to the value obtained after 24-h preincubation.