Effect of pH and Chloride Ions on Plasma Kininase Activity

Abstract

PREVIOUS experiments on the influence of metal ions on plasma kininase activity were performed by incubating the enzyme with synthetic bradykinin in magnesium-free Tyrode solution at pH 7.4–7.6 ¹. However, we wondered whether these conditions were optimal for testing the activity of kininase preparations from different origins. Edery and Lewis² found in their experiments a strong influence of pH on kininase activity of pseudoglobulin prepared from dog or ox plasma. Therefore, we examined the bradykinin-destroying activity of our kininase preparation from guinea pig serum¹ in 0.1 M sodium phosphate buffers at different pH value. Above pH 8 the buffer capacity of phosphate solutions is very low, and no other more suitable buffers were found for higher pH values, as these buffers interfere with the isolated guinea pig ileum used for the estimation of bradykinin. Kininase activity was expressed as the reciprocal of the time (min) in which 50 per cent of the initial amount of bradykinin had been inactivated. This half-life was determined as follows: 2.5 ml. of phosphate buffer containing 1 µg bradykinin was incubated at 34° C with 1 mg or 0.5 mg crude kininase preparation. At various times of incubation 0.2 ml. of this mixture was tested for bradykinin alternately with 0.2 ml. of a bradykinin solution (200 ng/ml.) in the same buffer and kept under the same circumstances.