Stabilization of methionine-enkephalin in human and rat blood

Methods of preventing the degradation of 3H-methionine-enkephalin (3H-ME) in human blood both at 37 degrees C and under conditions of immediate cooling were examined. We found that, contrary to previous suggestions, use of aprotinin (with or without immediate cooling) was ineffective in preventing the degradation of 3H-ME in blood. Thus, previous reports on the circulating levels of ME which relied on such procedures to stabilize the ME may have reported artifactually low values. However, we found that citric acid effectively prevents 3H-ME breakdown in both human and rat blood. Thus, we propose the use of citric acid, mixed with blood immediately upon collection, as an effective method for the stabilization of ME in blood.