The Activity of Ovine Luteal Cholesterol Esterase during Several Experimental Conditions1

Abstract

An assay for ovine luteal cholesterol esterase is described which measures the liberation of tritiated cholesterol from cholesterol oleate. The enzyme displayed a pH optimum near 7.0 and was sensitive to ionic strength. When luteal tissue was fractionated by differential centrifugation, 64% of the activity was recovered from the soluble fraction while 23% of the activity was in the microsomal fraction. The Michaelis constant for the enzyme was estimated at 3.0 x 10^-4 M. The activity of the enzyme was enhanced 2-fold by adenosine triphosphate (ATP), Mg⁺⁺, and N⁶,O²'-dibutyryl adenosine 3':5'-cyclic monophosphate [(Bt)₂cAMP]. A phosphodiesterase inhibitor, 1-methyl-3-isobutylxanthine (MIX), increased the activity of the enzyme 4-fold. In addition, luteal slices stimulated by luteinizing hormone (LH) or (Bt)₂cAMP responded with an increased enzyme activity and progesterone secretion.