Transferrin (Tf) and ceruloplasmin (Cp) in human serum are found to strongly inhibit lipid peroxidation by rat liver microsomes, and a study was made of this inhibition. Apo-Tf strongly inhibited NADPH dependent and ascorbic acid dependent lipid peroxidations of microsome. However, the inhibition of apo-Tf almost disappeared by adding iron ion to the system of the reaction. Inhibition of holo-Tf formed by adding Fe3+ to apo-Tf was mar kedly decreased. As a result of this, it was inferred that inhibition of apo-Tf is due to bon ding with iron ion necessary for lipid peroxidation in microsome. Cp showed a stronger in hibition than apo-Tf. It was confirmed that inhibition of Cp is due to oxidation of Fe2+, which is necessary for lipid peroxidation of microsome, into Fe3+ by ferroxidase action of Cp.
