▁ster oid s ▁are ▁synth es ized . ▁Ster oid s ▁are ▁synth es ized ▁from ▁ch ol ester ol ▁in ▁their ▁respective ▁regions ▁of ▁the ▁ad ren al ▁cor tex . ▁The ▁process ▁is ▁controlled ▁by ▁ster oid ogen ic ▁ac ute ▁regul atory ▁protein ▁( St AR ) ▁which ▁s its ▁in ▁the ▁mit och ond rial ▁memb rane ▁and ▁regul ates ▁the ▁passage ▁of ▁ch ol ester ol . ▁ ▁This ▁is ▁the ▁rate - lim iting ▁step ▁of ▁ster oid ▁b ios yn thesis . ▁Once ▁St AR ▁has ▁transport ed ▁ch ol ester ol ▁into ▁the ▁mit och ond ria , ▁the ▁ch ol ester ol ▁mole c ule ▁under go es ▁a ▁string ▁of ▁ox id ation - redu ction ▁re actions ▁catal yz ed ▁by ▁a ▁series ▁of ▁en zym es ▁from ▁the ▁family ▁of ▁cy to chrome ▁P 4 5 0 ▁en zym es . ▁A ▁co enz yme ▁system ▁called ▁ad ren odox in ▁re duct ase ▁trans fers ▁electrons ▁to ▁the ▁P 4 5 0 ▁en zym e ▁which ▁initi ates ▁the ▁ox id ation - redu ction ▁re actions ▁that ▁transform ▁ch ol ester ol ▁into ▁the ▁ster oid ▁h orm ones . ▁Though ▁synth esis ▁is ▁initi ated ▁inside ▁mit och ond ria , ▁prec urs ors ▁are ▁sh utt led ▁to ▁the ▁end op las mic ▁ret icul um ▁for ▁processing ▁by ▁en zym es ▁present ▁in ▁the ▁end op las mic ▁ret icul um . ▁The ▁prec urs ors