Subcellular Distribution and Developmental Expression of Cholesterol Ester Hydrolases in Fetal Rat Brain Cultures

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Abstract
Cholesterol ester hydrolase activities previously have been identified in brain and linked to the production of myelin, which has very low levels of esterified cholesterol. We have studied two cholesterol ester hydrolase activities (termed the pH 6.0 and pH 7.2 activities) in cultures derived from 19- to 21-day-old dissociated fetal rat brains and in developing rat brain. In vivo the levels of both the pH 6.0 and pH 7.2 activities began to increase by about 10 postnatal days, reached maximal levels at 20 days (20 and 1.5 nmol/h/mg protein, respectively), and thereafter remained nearly constant (pH 6.0) or decreased somewhat before becoming constant (pH 7.2). In contrast, in the cultures the pH 6.0 cholesterol ester hydrolase activity was low until 21 days in culture (DIC; 20 nmol/h/mg protein), increased to a peak activity at 31 DIC (60 nmol/h/mg protein), remained high for 24 days, and finally decreased (18 nmol/h/mg protein at 63 DIC); the pH 7.2 cholesterol ester hydrolase activity was very low until 20 DIC, increased to a peak activity at 31 days (3 nmol/h/mg protein), and thereafter decreased to a lower level (2 nmol/h/mg protein) that was maintained for about 24 days before decreasing (0.7 nmol/h/mg protein at 63 DIC). Therefore, (a) the time courses of appearance of both cholesterol ester hydrolase activities were delayed by 10–14 days relative to that seen in vivo, and (b) the specific activities observed in the cultures were transiently two- to three-fold higher than in rat brain, but then declined to levels characteristic of whole brain homogenates. Subcellular fractionation of the cultures demonstrated that the pH 7.2 cholesterol ester hydrolase activity, along with myelin basic protein and 2′,3′-cyclic nucleotide-3′-phosphohydrolase activity, was enriched in a membrane fraction collected at an interface between 0.32 M and 0.9 M sucrose; the pH 6.0 cholesterol ester hydrolase activity, in contrast, was enriched in the microsomal fraction.

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