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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
ActiveEstivatedActiveEstivated
Vmax (mU mg−1 soluble protein) 31.0±1.6 20.4±1.6* 18.0±1.2 11.7±1.0* 
S0.5 Mg.ATP (mmol 1−10.75±0.05 1.07±0.06* 0.90±0.11 1.24±0.07* 
Km Na+ (mmol 1−138±4 67±7* 34±5 42±9 
Km K+ (mmol 1−111±2 16±3 14±4 16±2 
Ka Mg2+ (mmol 1−11.7±0.3 2.7±0.3* 1.6±0.3 2.4±0.5 
I50 Na+ (mmol 1−1258±33 325±57 194±21 207±20 
I50 K+ (mmol 1−182±11 167±32 65±9 109±13* 
I50 Mg2+ (mmol 1−132±5 45±11 48±8 42±8 
I50 Urea (mol 1−13.3±0.4 4.4±0.4* 4.1±0.4 3.7±0.5 
Cm Urea (mol 1−13.9±0.5 3.9±0.4 3.9±0.6 3.4±0.7 
EA (kJ mol−118.6±2.9 27.1±3.1* 22.2±3.5 31.9±3.1* 
Foot muscle
Hepatopancreas
ActiveEstivatedActiveEstivated
Vmax (mU mg−1 soluble protein) 31.0±1.6 20.4±1.6* 18.0±1.2 11.7±1.0* 
S0.5 Mg.ATP (mmol 1−10.75±0.05 1.07±0.06* 0.90±0.11 1.24±0.07* 
Km Na+ (mmol 1−138±4 67±7* 34±5 42±9 
Km K+ (mmol 1−111±2 16±3 14±4 16±2 
Ka Mg2+ (mmol 1−11.7±0.3 2.7±0.3* 1.6±0.3 2.4±0.5 
I50 Na+ (mmol 1−1258±33 325±57 194±21 207±20 
I50 K+ (mmol 1−182±11 167±32 65±9 109±13* 
I50 Mg2+ (mmol 1−132±5 45±11 48±8 42±8 
I50 Urea (mol 1−13.3±0.4 4.4±0.4* 4.1±0.4 3.7±0.5 
Cm Urea (mol 1−13.9±0.5 3.9±0.4 3.9±0.6 3.4±0.7 
EA (kJ mol−118.6±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 Vmax and Km (Mg2+-ATP) (N=9) and for I50 (Urea) and Cm (Urea)(N=3).

S0.5 and Km are substrate concentrations that produce half-maximal velocity for sigmoidal vshyperbolic relationships, respectively. Ka is the activator concentration that produces half-maximal activation, I50 is the concentration of inhibitor that reduces enzyme activity by 50%, and Cm is the denaturant concentration required to unfold 50% of the protein. EA 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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