Monoamine oxidase activity in different density gradient fractions of human platelets.

Monoamine oxidase (MAO) activity measured in human platelets is reportedly altered by such drugs as epinephrine, lithium carbonate, and imipramine, and also reduced in a number of clinical disorders. To evaluate whether MAO activity might differ in platelet subpopulations, density gradient centrifugation with arabino-galactan was used to prepare four platelet fractions that differed in weight and volume. MAO activity in the lightest and smallest platelet subpopulation was approximately one-half that in the heaviest and largest subpopulation. Because platelet weight and size are thought to be related to platelet age, it is possible that some drug effects on platelet MAO activity might represent changes in platelet turnover. Factors other than platelet turnover rates may contribute to individual differences in platelet MAO activity, however, since one group of individuals with markedly reduced platelet MAO activity exhibited no shift in the proportion of lighter versus heavier platelets nor in the relative amount of MAO activity in each density gradient subfraction.