Figure 6.

Expression of cholesterol 24-hydroxylase as a function of age. (A) Ontogeny of serum and brain 24S-hydroxycholesterol in mice. Sterol levels were measured by isotope dilution GC-MS in sera and tissues from animals of the indicated ages. For days 3–30, each point represents a separate measurement in pooled sera from multiple animals (n = 3–10). For days 42–300, each point represents a measurement in a single animal. Serum levels (●), brain levels (○). (B) Ontogeny of 24-hydroxylase protein expression in murine brain. Aliquots (30 μg) of membrane protein prepared from animals of the indicated ages were immunoblotted with an antipeptide antibody directed against 24-hydroxylase. Antigen-antibody complexes were detected by enhanced chemiluminescence. Size standards are indicated on the left. The lane marked + is a control containing lysate (5 μg protein) from 293 cells transiently transfected with a murine 24-hydroxylase expression plasmid (pCMV-m24). (C) Ontogeny of 24-hydroxylase protein expression in human brain. Aliquots of total cell protein (50 μg) were prepared from individuals of the indicated ages and analyzed by immunoblotting as described in B. Size standards are indicated on the left. The lane marked + is a control containing lysate (1 μg protein) from 293 cells transiently transfected with a human 24-hydroxylase expression plasmid (pCMV-h24) mixed with 50 μg of total cell protein from a frontal lobe specimen of a 24-year-old individual.