4 fused rings
bile acids, steroid hormones, vitamin D
regulation of cholesterol
wthe enzyme is cholesterol-7-alpha-hydroxylase
cholic acid inhibits it, cholesterol activates it.
it's the rate limiting step.
desmolase with NAPDH and iscoproaldehyde
25-hydroxylase in the liver
1-hydroxylase in kidney (with PTH and Ca2+) in the kidney