Heterogeneity of Human Platelets. VII. Platelet Monoamine Oxidase Activity in Normals and Patients With Autoimmune Thrombocytopenic Purpura and Reactive Thrombocytosis: Its Relationship to Platelet Protein Density

Abstract

The level of platelet monoamine oxidase (MAO) activity has been used as a genetic marker for certain mental disorders. The purpose of this study was to investigate the influence of several nongenetic factors on platelet MAO activity. Platelet MAO, platelet count, mean platelet volume, per cent megathrombocytes, platelet protein, and platelet protein density were determined in 19 normal subjects and in patients with two clinical conditions associated with extremes in platelet count and mean platelet volume: 6 patients with autoimmune thrombocytopenic purpura (ATP), characterized by low platelet count and increased mean platelet volume; and 6 patients with reactive thrombocytosis (RT), characterized by high platelet count and decreased mean platelet volume. Platelet protein density was expressed as total platelet protein/total platelet volume. Patients with ATP, when compared to normal subjects, had 2.3-fold lower platelet protein density and 2.1-fold lower specific activity of MAO. Patients with RT, when compared to normal subjects, had 1.9-fold higher platelet protein density and 1.5-fold higher specific activity of MAO. Platelets from RT patients, when compared to ATP patients, had 4.3-fold greater protein density and 3.2-fold higher specific activity of MAO. Platelets from normal subjects were separated into five different platelet density fractions. Platelet MAO and protein content were found to be heterogeneously dispersed. The extreme heavy platelet fraction had 15-fold greater specific activity of MAO and 18-fold greater protein content, compared to the extreme light platelet fraction. We conclude that high specific activity of MAO is associated with platelets densely packed with protein. Accordingly, nongenetic factors such as changes in platelet number, volume, and protein content, which could be induced by stress, blood loss, iron deficiency, or other conditions, could contribute to changes in protein density, and consequently affect the specific activity of platelet MAO.