4.1. Sterol and Oxysterol Reagents

Cholesterol (CHOL) and 7-ketocholesterol (7kCHOL) were purchased from Sigma-Aldrich Chemical Corp. (Saint Louis, MO, USA) and stored in solid form in source bottles at −20 °C, desiccated and protected from light. The oxysterol 5,9-endoperoxycholest-7-ene-3β,6α-diol (EPCD) was custom synthesized through directed oxidation of 7DHC and purified by previously described methods [22,97,241], and it was stored as a lyophilizate of known mass, in an argon atmosphere, desiccated, and protected from light at −80 °C, until just before use. Structures of CHOL and the two oxysterols used here, and of 7DHC from which the latter are known to be biologically derived, are given in Supplementary Materials Figure S1. 5 mM and 10 mM stocks of CHOL and EPCD, respectively, were made by dissolving these two compounds in dimethyl sulfoxide (DMSO; Sigma-Aldrich); after Ar purging, these solutions were stored desiccated and protected from light, at −20 °C, and used within one week.