Studies on the microsomal sodium-plus-potassium ion-stimulated adenosine triphosphatase system in rat ventral prostate

Abstract

A Mg²⁺+Na⁺+K⁺-stimulated adenosine triphosphatase (ATPase) preparation was isolated from rat ventral prostate by flotation of microsomal membranes in high-density sucrose solutions. The reaction medium for optimum Na⁺+K⁺-stimulated ATPase activity was found to be: Na⁺, 115mm; K⁺, 7–10mm; Mg²⁺, 3mm; ATP, 3mm; tris buffer, pH7·4 at 38°, 20mm. The average ΔP_i (Mg²⁺+Na⁺+K⁺ minus Mg²⁺+Na⁺) was 9μmoles/mg. of protein/hr., representing a 30% increase over the Mg²⁺+Na⁺-stimulated ATPase activity. At high concentrations, K⁺ was inhibitory to the enzyme activity. Half-maximal inhibition of Na⁺+K⁺-stimulated ATPase activity was elicited by ouabain at 0·1mm. The preparation exhibited phosphatase activity towards ribonucleoside triphosphates other than ATP. However, stimulation of P_i release by Na⁺+K⁺ was observed only with ATP as substrate. The apparent K_m for ATP for Na⁺+K⁺-stimulated activity was about 0·3×10⁻³m. Ca²⁺ inhibited only the Na⁺+K⁺-stimulated ATPase activity. Mg²⁺ could be replaced by Ca²⁺ but then no Na⁺+K⁺ stimulation of ATPase activity was noticed. The addition of testosterone or dihydrotestosterone (17β-hydroxy-5α-androstan-3-one) in vitro at 0·1–10μm under a variety of experimental conditions did not significantly increase the Na⁺+K⁺-stimulated ATPase activity. The enzyme preparations from prostates of orchidectomized rats, however, exhibited a drastic decrease in the specific activity of Na⁺+K⁺-stimulated ATPase; these changes were prevented in the orchidectomized rats by injection of testosterone propionate.