A review of the enzyme activities of plasma membranes reveals that none of them characterize plasma membranes universally. Potentially, adenyl cyclase is the best plasma membrane marker enzyme. Species, tissue and sex differences complicate the sole use of enzymes as markers in the characterization of plasma membrane preparations. It is necessary to establish specific enzymological criteria for plasma membrane enzymes to distinguish them from isoenzymes in other subcellular membranes. Standard assays have been suggested for comparative purposes.

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