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Journal of Lipid Research, Vol. 7, 501-510, July 1966
Copyright © 1966 by Lipid Research, Inc.

Preparation and properties of a cell-free system from rat skin that catalyzes sterol biosynthesis

J. L. Gaylor , C. V. Delwiche , D. R. Brady , and A. J. Green

Graduate School of Nutrition and the Division of Biological Sciences, Cornell University, Ithaca, New York

Homogenates of epidermis from rat skin were centrifuged at 10,000 X g for 20 min. The supernatant fraction ("whole homogenate") catalyzed the demethylation of lanosterol (C₃₀) to yield C₂₇-sterols. The rate of reaction was measured by the rate of release of ¹⁴CO₂ from the 4-methyl group of lanosterol. Conditions for maximal rates of demethylation were established.

Addition of increasing amounts of washed microsomes to a constant amount of substrate resulted in additional release of ¹⁴CO₂, but the release was not proportional to the amount of microsomes. Incubation with increasing amounts of microsomes treated with Triton WR-1339 yielded a proportional rate of release of ¹⁴CO₂. The Triton-treated microsomes were frozen and stored without loss of activity. The rate of formation of ¹⁴CO₂ was constant up to 1 hr of incubation with both Triton-treated microsomes and whole homogenate, for which the K_m for lanosterol was 5.0 and 3.0 X 10^-5 m, respectively. Other 4-gem-dimethyl sterols were competitive inhibitors, K_i', 2.0 and 5.5 X 10^-5 m. The enzyme system was inhibited by arsenite.

24,25-Dihydrolanosterol, 24,25-dihydrolanostenone, and squalene were demethylated by the homogenate. The whole homogenate catalyzed the incorporation of mevalonic acid, but not acetic acid, into squalene and sterols. The enzymatic properties of the sterol synthetic system from skin resemble those of similar preparations from rat liver.

Supplementary key words skin • rat • homogenate • demethylation • lanosterol • biosynthesis • cholesterol • microsomes • Triton-treated • inhibitors • sterols

Submitted on August 31, 1965
Accepted on March 21, 1966

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