FIGURE 3.

Abbreviated versions of the early steps in the neutral (left, in purple) and the cerebral 25-hydroxylase (right, in brown) pathways of bile acid biosynthesis. Pathway from 7-dehydrocholesterol and cholesterol to 3β,7β-diHCA(25R) are also shown as is the path to 25-hydroxyvitamin D₃ in red and black dashed boxes, respectively. Enzymes, metabolites, and reactions of the neutral pathway are in purple, those of the 25-hydroxylase pathway are in brown, while those generating 3β,7β-diHCA(25R) are in red. Enzymes/genes expressed in brain, and metabolites observed in CSF are in bold. Italics indicate that the named structure is one of a number of possibilities. Enzymes in italics are postulated catalysts. [O] indicates oxidation via non-enzymatic mechanism. Thick coloured arrows pointing upwards or downwards indicate significant positive or negative correlations even when the confounding variables are considered. Red triangles indicate significance ignoring confounding variables, in at least one of the two studies. The full stereochemistry and numbering system for cholesterol and 7-DHC is indicated. Abbreviated structures are shown for other sterols ignoring ring-stereochemistry.