Sources of Error in the Determination of Platelet Monoamine Oxidase: A Review of Methods

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Abstract
The methods used in the determination of platelet monoamine oxidase (MAO) activity in 26 studies of chronic schizophrenic patients and normal subjects (reviewed by Wyatt, Potkin, and Murphy 1979) are examined. The most commonly used substrate was tryptamine (64 percent) followed by tyramine, benzylamine, and β-phenylethylamine. When the relationship between the MAO activity and substrate concentration was examined, it was found that as many as 75 percent of the studies used suboptimal substrate concentrations. It appears that some chronic schizophrenics have a lower Michaelis constant (Km) and maximal velocity (Vmax). Thus, the use of suboptimal substrate concentrations could lead not only to a low estimate of the MAO activity, but also to erroneous comparisons between schizophrenic and normal groups. Because platelets are markedly heterogeneous in size and density, the differential centrifugation methods used for platelet preparation can produce up to an eight-fold error in the estimation of the MAO activity. We describe the use of a new method for the preparation of platelets that avoids many of the problems of the differential centrifugation methods. The new method shows substantially less intrasubject variance in MAO activity and is significantly different from the differential centrifugation procedures (by X2 of pooled intraclass correlations, p <.005).

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