Cytochrome P450 in the mitochondria of the adrenal cortex functions in the monooxygenation reactions for the biosynthesis of various steroid hormones, such as cholesterol side chain cleavage, hydroxylation at 11β-position and that at 18-position of the steroid structure. The cytochrome is firmly associated with the mitochondrial membrane and therefore can be isolated only by the aid of ionic or non-ionic detergent. Recently, two cytochromes P450 each catalyzing a specified reaction have been purified to a homogeneous state, that is, P450scc having cholesterol side chain cleavage activity and P45011β having 11β-hydroxylation activity. The properties of these purified P450's as well as the other components of the monooxygenase system, adrenodoxin and adrenodoxin reductase, are, therefore, summarized and compared to those of P450 in the mitochondria) preparation in situ. Among many findings, both purified cytochromes P450 were revealed to be a low-spin type hemoprotein and their spin states were changed to a high-spin state by being complexed with the corresponding substrate. The binding of a substrate also facilitated the reduction of the cytochrome and appeared to increase the stability of the oxygenated form of cytochrome P450. These effects are important from the point of view that the primary role of the heme of cytochrome P450 is the activation of molecular oxygen. In addition, the results of our detailed kinetic studies on the transfer of electrons from adrenodoxin to cytochrome P450 in the reconstituted system have also been described Finally, the topology of adrenodoxin and the reductase were shown to be on the inner mitochondrial membrane by a peroxidase-labeled antibody method.

Cytochrome P450 in adrenocortical mitochondria.

The purpose of this chapter is twofold. First, we will attempt to summarize the existing knowledge on the wide variety of bacterial cytochrome P-450 hemeproteins that have been discovered. As such, this represents the first comprehensive review of the bacterial cytochromes P-450. It should become evident to the reader that these cytochromes comprise a class just as diverse, if not more so, than their eukaryotic counterparts. Perhaps the best known and most extensively characterized of these bacterial enzymes is P-450cam isolated from Pseudomonas putida. From the initial isolation in Gunsalus’s laboratory in 1967 and subsequent investigations in many laboratories, the numerous studies employing this cytochrome have shed much light on the mechanisms associated with the family of P-450-dependent monooxygenases as a whole. A second goal of this chapter is to review the detailed biophysical, biochemical, and molecular biological studies conducted with this system. It should be noted that “P-450cam” and “bacterial P-450” are not synonymous. The various P-450 molecules described herein are in all probability only a very small fraction of those present in the myriad of bacterial cells in the biosphere. In view of the variety of substrate transformations that this enzyme family is able to catalyze, some of which are energetically very difficult, their distribution in nature is no doubt ubiquitous. One need only look through the American Type Culture Collection Catalogue1 under such diverse genera as Pseudomonas, Rhodococcus, Bacillus, and Nocardia among others to see the potential roles of yet undiscovered P-450 monooxygenases in the degradation and transformation of a plethora of compounds. Even if only a small portion of the enzymes involved in these various biotransformations are P-450 isozymes, there would correspond many hundreds of P-450 hemeproteins with definable specificities.

Cytochrome P-450cam and Other Bacterial P-450 Enzymes

Substrate specificity of adrenocortical cytochrome P-450scc—I. Effect of structural modification of cholesterol side-chain on pregnenolone production

Reflections on sterol sidechain cleavage process catalyzed by cytochrome P450scc

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2876%2980999-4

Stereochemical specificity at carbon-20 and -22 of hydroxylated cholesterols for side-chain cleavage by adrenocortical cytochrome P-450scc.

Exclusion of 20(22)-dehydrocholesterol as an intermediate in the biosynthesis of pregnenolone in bovine adrenocortical mitochondrial acetone-dried powder preparations.

Chang Y. Byon

Papers

Exclusion of 20(22)-dehydrocholesterol as an intermediate in the biosynthesis of pregnenolone in bovine adrenocortical mitochondrial acetone-dried powder preparations.

Specificity of the cholesterol side-chain cleavage enzyme system of adrenal cortex.