The 3-beta-HSD enzymatic system plays a crucial role in the biosynthesis of all classes of hormonal steroids. HSD VII is active against four 7-alpha-hydroxylated sterols. Does not metabolize several different C(19/21) steroids as substrates. Involved in bile acid synthesis. Plays a key role in cell positioning and movement in lymphoid tissues by mediating degradation of 7-alpha,25-dihydroxycholesterol (7-alpha,25-OHC): 7-alpha,25-OHC acts as a ligand for the G protein-coupled receptor GPR183/EBI2, a chemotactic receptor for a number of lymphoid cells. 7alpha-hydroxycholesterol + NAD(+) = 7alpha-hydroxycholest-4-en-3-one + H(+) + NADH 7alpha,25-dihydroxycholesterol + NAD(+) = 7alpha,25-dihydroxy-4-cholesten-3-one + H(+) + NADH (25R)-cholest-5-en-3beta,7alpha,26-triol + NAD(+) = (25R)-7alpha,26-dihydroxycholest-4-en-3-one + H(+) + NADH (24S)-7alpha-dihydroxycholesterol + NAD(+) = (24S)-7alpha,24-dihydroxycholest-4-en-3-one + H(+) + NADH Lipid metabolism; steroid biosynthesis. High levels in liver and lung, moderate levels in spleen, brain, heart, kidney, jejunum and testis. Up-regulated in 3Y1 cells upon growth arrest. Belongs to the 3-beta-HSD family.