A cytochrome P450 monooxygenase involved in the metabolism of endogenous cholesterol and its oxygenated derivatives (oxysterols) (By similarity). Mechanistically, uses molecular oxygen inserting one oxygen atom into a substrate, and reducing the second into a water molecule, with two electrons provided by NADPH via cytochrome P450 reductase (CPR; NADPH-ferrihemoprotein reductase) (By similarity). Functions as a critical regulatory enzyme of bile acid biosynthesis and cholesterol homeostasis. Catalyzes the hydroxylation of carbon hydrogen bond at 7-alpha position of cholesterol, a rate-limiting step in cholesterol catabolism and bile acid biosynthesis (Probable). 7-alpha hydroxylates several oxysterols, including 4beta-hydroxycholesterol and 24-hydroxycholesterol. Catalyzes the oxidation of the 7,8 double bond of 7-dehydrocholesterol and lathosterol with direct and predominant formation of the 7-keto derivatives (By similarity). cholesterol + O2 + reduced [NADPH--hemoprotein reductase] = 7alpha-hydroxycholesterol + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] 4beta-hydroxycholesterol + O2 + reduced [NADPH--hemoprotein reductase] = 4beta,7alpha-dihydroxycholesterol + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] lathosterol + O2 + reduced [NADPH--hemoprotein reductase] = 7alpha,8alpha-epoxy-5alpha-cholestan-3beta-ol + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] lathosterol + O2 + reduced [NADPH--hemoprotein reductase] = 5alpha-cholestan-7-oxo-3beta-ol + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] 7-dehydrocholesterol + O2 + reduced [NADPH--hemoprotein reductase] = 7-oxocholesterol + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] (24S)-hydroxycholesterol + O2 + reduced [NADPH--hemoprotein reductase] = (24S)-7alpha-dihydroxycholesterol + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] (24R)-hydroxycholesterol + O2 + reduced [NADPH--hemoprotein reductase] = (24R)-7alpha-dihydroxycholesterol + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] Lipid metabolism; bile acid biosynthesis. Steroid metabolism; cholesterol degradation. Up-regulated by fasting, returns to ground state upon feeding. Up-regulated by experimentally induced diabetes. Down-regulated by insulin treatment. Belongs to the cytochrome P450 family.