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A new large scale purification procedure was developed for isolation from rat liver of the protein originally called squalene and sterol carrier protein (SCP).Homogeneous SCP was obtained by gel filtration of the liver soluble proteins on Sephadex G-75 followed by ion exchange chromatography on DEAE-cellulose at pH 9.0.A striking finding was that SCP represents at least 8% of the soluble proteins in the liver extract.SCP functional activity was determined by a new spectroscopic assay, measuring activation of membrane-bound A7-sterol AS-dehydrogenase.Structural studies indicated that SCP is a single polypeptide chain (Mr = 16,000; PI, 7.0).SCP has one free sulfhydryl group, partially buried in the native protein.No NHz-terminal residue was detected by Edman degradation, dansylation, or by Edman degradation after digestion of SCP by pyroglutaminase.The COOH-terminal sequence of SCP determined by carboxypeptidase Y digestion is -Thr-(Glu,Lys)-(Thr,Val)-Met-COOH.SCP does not con-
Dempsey et al. (Sun,) studied this question.