Properties of Some Heart Sarcolemmal-bound Enzymes

Abstract

Both Ca²⁺ and Mg²⁺ stimulated ATP hydrolysis by the dog heart sarcolemmal fraction. The mean K_m values were 0.90 and 0.95mM and the mean V_max values were 17.2 and 16.0μmoles P₁/mg per hr for the membrane Ca²⁺ ATPase [EC 3. 6. 1. 3] and Mg²⁺ ATPase respectively. Other divalent cations such as Mn²⁺, Co²⁺, and Ni²⁺ were also found to stimulate ATP hydrolysis in this fraction. Excess of ATP was inhibitory to the ATP hydrolysis due to various divalent cations. Ni²⁺, Co²⁺, Mg²⁺ and Mn²⁺ were shown to depress the ATP hydrolysis due to Ca²⁺. The Ca²⁺ ATPase activity in the heart membranes was also inhibited by Na⁺ whereas K⁺ had no effect. The adenylate cyclase activity of the heart membranes was increased by about 35% and 4 fold by epinephrine and NaF respectively. These agents increased V_max (286 pmoles cyclic AMP/mg per min) without any changes in the K_m value (0.08mM) for ATP. The activation of adenylate cyclase [EC 4. 6. 1. 1] due to epinephrine was blocked by a well known β-adrenergic blocking agent, propranolol.