Summary. The preparation of a purified fraction of ram sperm plasma membranes is described and validated in this paper. Lipid analyses were performed on both the membrane preparation and whole spermatozoa; the main differences were that plasma membranes showed a significantly higher cholesterol:phospholipid molar ratio and a higher sphingomyelin content than did whole spermatozoa, but a lower proportional content of phosphatidylethanolamine. Enzymic assays revealed the presence of two distinct adenosine triphosphatases (ATPases) in the membrane fraction, activated independently by calcium and sodium ions. Arrhenius plots of the calcium-stimulated ATPase activity demonstrated that a change in energy of activation occurred in the region of 23°C; it is believed that this is evidence for the occurrence of a thermal phase transition in the lipid environment of the enzyme molecules.
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