Cyclic 3′,5′-nucleotide phosphodiesterase properties of the enzyme of human blood platelets

Abstract

1. 1. A cyclic 3′,5′-nucleotide phosphodiesterase of human blood was mostly localized in the platelets and the leukocytes. The platelets accounted for the majority of the total activity. Most of the activity associated with the erythrocytes could be accounted for by the contamination of leukocytes and platelets. The plasma exhibited no phopshodiesterase activity. 2. 2. After sonication, 80% or more of the phosphodiesterase activity in the platelets remained in a high speed supernatant fluid. The soluble enzyme hydrolyzed cyclic AMP at a rate of 2 to 5 nmoles/mg protein per min at 30°. It also hydrolyzed other cyclic 3′,5′-nucleotides, and the relative rates were: cyclic AMP (100), cyclic UMP (67), cyclic TMP (58), cyclic GMP (56), cyclic IMP (38), cyclic CMP (<0.1) and dibutyryl cyclic AMP (0). The hydrolysis of cyclic AMP was inhibited by cyclic GMP, cyclic IMP, and dibutyryl cyclic AMP, but not by cyclic CMP, cyclic TMP, and cyclic UMP. 3. 3. The platelet enzyme required divalent cations for full activity. Mn 2+, Mg 2+, and Co 2+ were effective ions. It was inhibited by EDTA, ATP and other nucleoside triphosphates. Although the inhibition by EDTA was explainable by metal chelation, the effect of ATP appeared more complicated. The inhibition by caffeine was of a mixed type. 4. 4. The enzyme exhibited two K m values, one of 7·10 −4 M and another of 7·10 −5 M.