Purification and kinetic properties of the soluble Mn 2+-dependent adenylyl cyclase of the rat testis

Abstract

The soluble Mn 2+-dependent adenylyl cyclase (AC) of the rat testis was purified 1500-fold with some 19% yield of the initial activity. These results were accomplished by conventional separation techniques including (NH 4) 2SO 4 precipitation of testis cytosol (106 000 × g), gel filtration (Sephadex G-200) and ion-exchange chromatography (Sephadex DEAE-A50) followed by Sephadex G-100 gel filtration and isoelectric focusing. Analysis by polyacrylamide-gel electrophoresis (PAGE) of aliquots from each purification step revealed the following. (a) The Mn 2+-dependent AC migrated with a R f value of 0.40 irrespective of the degree of purification. (2) The AC peak from the isoelectric focusing column separated into 2 major protein bands; however, only one band ( R f 0.40) had AC activity. The molecular weight emerging from the position of migration on the Sephadex G-200 and G-100 columns appeared to be consistent at 47 000–48 000 D, as estimated from the relationship log MW versus elution volume. The purified enzyme fulfilled the requirements for a simple Michaelis-Menten kinetics with an apparent Km for Mn 2+ and MnATP 2− of 6.7 and 2.5 mM, respectively. Varying the concentrations of ATP or Mn 2+ separately did not alter the apparent affinity ( K m app ) for the other parameter. These and previous data from our laboratory show that the physico-chemical and kinetic properties (molecular weight and K mapp for Mn 2+ and MnATP 2−) do not alter during purification. Furthermore, the additional step of affinity chromatography seems obligatory if a homogeneous AC preparation is to be obtained.