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Table 1.

Kinetic parameters for Na⁺/K⁺-ATPase from foot muscle and hepatopancreas of active and 10-day estivated Otala lactea

	Foot muscle			Hepatopancreas
	Active	Estivated	Active	Estivated
V_max (mU mg⁻¹ soluble protein)	31.0±1.6	20.4±1.6^{^*}	18.0±1.2	11.7±1.0^{^*}
S_0.5 Mg.ATP (mmol 1⁻¹)	0.75±0.05	1.07±0.06^{^*}	0.90±0.11	1.24±0.07^{^*}
K_m Na⁺ (mmol 1⁻¹)	38±4	67±7^{^*}	34±5	42±9
K_m K⁺ (mmol 1⁻¹)	11±2	16±3	14±4	16±2
K_a Mg²⁺ (mmol 1⁻¹)	1.7±0.3	2.7±0.3^{^*}	1.6±0.3	2.4±0.5
I₅₀ Na⁺ (mmol 1⁻¹)	258±33	325±57	194±21	207±20
I₅₀ K⁺ (mmol 1⁻¹)	82±11	167±32	65±9	109±13^{^*}
I₅₀ Mg²⁺ (mmol 1⁻¹)	32±5	45±11	48±8	42±8
I₅₀ Urea (mol 1⁻¹)	3.3±0.4	4.4±0.4^{^*}	4.1±0.4	3.7±0.5
C_m Urea (mol 1⁻¹)	3.9±0.5	3.9±0.4	3.9±0.6	3.4±0.7
E_A (kJ mol⁻¹)	18.6±2.9	27.1±3.1^{^*}	22.2±3.5	31.9±3.1^{^*}

	Foot muscle			Hepatopancreas
	Active	Estivated	Active	Estivated
V_max (mU mg⁻¹ soluble protein)	31.0±1.6	20.4±1.6^{^*}	18.0±1.2	11.7±1.0^{^*}
S_0.5 Mg.ATP (mmol 1⁻¹)	0.75±0.05	1.07±0.06^{^*}	0.90±0.11	1.24±0.07^{^*}
K_m Na⁺ (mmol 1⁻¹)	38±4	67±7^{^*}	34±5	42±9
K_m K⁺ (mmol 1⁻¹)	11±2	16±3	14±4	16±2
K_a Mg²⁺ (mmol 1⁻¹)	1.7±0.3	2.7±0.3^{^*}	1.6±0.3	2.4±0.5
I₅₀ Na⁺ (mmol 1⁻¹)	258±33	325±57	194±21	207±20
I₅₀ K⁺ (mmol 1⁻¹)	82±11	167±32	65±9	109±13^{^*}
I₅₀ Mg²⁺ (mmol 1⁻¹)	32±5	45±11	48±8	42±8
I₅₀ Urea (mol 1⁻¹)	3.3±0.4	4.4±0.4^{^*}	4.1±0.4	3.7±0.5
C_m Urea (mol 1⁻¹)	3.9±0.5	3.9±0.4	3.9±0.6	3.4±0.7
E_A (kJ mol⁻¹)	18.6±2.9	27.1±3.1^{^*}	22.2±3.5	31.9±3.1^{^*}

Assays were conducted at 22°C and data are mean ± s.e.m.(N=4), except for foot muscle V_max and K_m (Mg²⁺-ATP) (N=9) and for I₅₀ (Urea) and C_m (Urea)(N=3).

S_0.5 and K_m are substrate concentrations that produce half-maximal velocity for sigmoidal vshyperbolic relationships, respectively. K_a is the activator concentration that produces half-maximal activation, I₅₀ is the concentration of inhibitor that reduces enzyme activity by 50%, and C_m is the denaturant concentration required to unfold 50% of the protein. E_A is the activation energy calculated from Arrhenius plots.

Kinetic parameters were determined at optimal concentrations of other ions and cosubstrates.

Significantly different from the corresponding value for active snails, P<0.05.

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