SI ▁occurs ▁in ▁animal ▁t issues ▁concerned ▁with ▁ster oid ▁h orm one ▁b ios yn thesis , ▁such ▁as ▁the ▁ad ren al , ▁test is , ▁and ▁ov ary . ▁K SI ▁in ▁Com am omas ▁test oster oni ▁is ▁used ▁in ▁the ▁de grad ation ▁path way ▁of ▁ster oid s , ▁allowing ▁this ▁b acter ia ▁to ▁util ize ▁ster oid s ▁containing ▁a ▁double ▁bond ▁at ▁ Δ 5 , ▁such ▁as ▁test oster one , ▁as ▁its ▁sole ▁source ▁of ▁carbon . ▁In ▁m amm als , ▁transfer ▁of ▁a ▁double ▁bond ▁at ▁ Δ 5 ▁to ▁ Δ 4 ▁is ▁catal yz ed ▁by ▁ 3 - β - h ydro xy - Δ 5 - st ero id ▁de h ydro gen ase ▁at ▁the ▁same ▁time ▁as ▁the ▁de h ydro x yl ation ▁of ▁ 3 - β - h ydro x yl ▁group ▁to ▁k et one ▁group , ▁while ▁in ▁C . ▁test oster oni ▁and ▁P . ▁put ida , ▁ Δ 5 , 3 - ket ost ero id ▁is omer ase ▁just ▁trans fers ▁a ▁double ▁bond ▁at ▁ Δ 5 ▁of ▁ 3 - ket ost ero id ▁to ▁ Δ 4 . ▁ ▁A ▁ Δ 5 - 3 - ket ost ero id ▁is omer ase - dis rupted ▁mut ant ▁of ▁stra in ▁T A 4 4 1 ▁can ▁grow ▁on ▁de h ydro ep i andro ster one , ▁which ▁has ▁a ▁double