Convertibility of a saponifiable lipoidal derivative of 18-hydroxycorticosterone

Metabolic properties and subcellular localization of the biosynthesis of SM, a saponifiable 18-OH-B (18-Hydroxycorticosterone) derivative, were investigated. Homogenates biosynthesized SM at a nearly constant rate of 463 pmol/50 mg tissue during 30 min. This biosynthesis was more efficient at pH 7.4 than at pH 4.8. Not only 18-OH-B but also its less polar anhydride 18-DAL (18-Deoxyaldosterone) were good precursors. SM was reverted to these precursors both enzymatically and spontaneously, 4.8 being a more suitable pH for this reversion than 7.4. Trapping experiments demonstrated a sequence comprising, in this order, the following echelons: SM, 18-OH-B, 18-DAL, Aldosterone. The first two steps are reversible and the last two ones depend on proton concentrations. It is postulated that SM could be on a dead-end to which 18-OH-B could be deviated if Aldosterone biosynthesis became temporarily unnecessary. Also, that 18-OH-B may convert to either 18-DAL or SM for selective membrane transports, according to homeostatic requirements.