Optimal conditions necessary for the cytochemical demonstration of megakaryocyte (Mk) and platelet acid phosphatase (AP) were determined. Methanol, a constituent of fixatives commonly used in AP cytochemistry, was found to inhibit MkAP, and the degree of inhibition varied with Mk maturity. Immature Mk contained predominantly methanol-resistant AP, and mature Mk, predominantly methanol-sensitive AP. Platelets contained methanol-sensitive AP similar to mature Mk, suggesting that this enzyme provides an index of platelet formation by Mk. Soluble platelet AP showed three bands on polyacrylamide gel electrophoresis, visualized by the same reactions applied cytochemically. Two bands, accounting for 98% of the platelet AP activity, were slow moving and methanol sensitive; and one fast moving band accounting for 2% of activity was methanol resistant. Measurement of Mk and platelet AP isoenzymes may prove to have applications in evaluating Mk function.

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