Enzymes Responsible for Steroid Hydroxylation

The nature of the enzyme responsible for the hydroxylation reaction has long been unidentified it has been, however, recognized that the steroid 11 p-hydroxylase consists of at least three components which can be separated by ordinary purification procedures. This observation leads us to assume that the steroid hydroxylase has the nature of a multiple enzyme complex. 

Successful resolution and characterization of the components of steroid 11 p-hydroxylase have been achieved by us (26, 27, 60) and by Omura et al. (44,47) through the discovery of the flavoprotein, (NADPH-dehydrogenase), and a ferredoxin-like non-heme iron protein (adreno- 

NADPH - flavoprotein — non-heme iron protein - cytochrome P-450 — molecular oxygen. 

This electron transfer system functions to provide active reduced oxygen, which can be incorporated into the steroid molecule, and water. In addition to the above three protein components, a fourth protein has been postulated to be involved in this hydroxylation reaction (24), and it would serve to determine the position to be hydroxylated in the steroid molecule, although the particular nature of cytochrome P-450 is such that it causes interference in the separation of the postulated fourth protein from the heme protein. 

The same electron transfer system operates in the 22-, 20-, and 18-hydroxylations as well as in the 11 -hydroxylation. Further, the identical electron transfer system appears to be pertinent to the system of steroid hydroxylation in testis and ovary. 22-, 20-, 18-, and 11 p-hydroxy-lases are all distributed in the mitochondrial fraction of adrenal cortex, whereas 21- and 17-hydroxylases are located in the microsomal fraction. The electron transfer system in the microsomal steroid hydroxylation has not yet been established. 

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