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Modulation of the activity of human 17 alpha-hydroxylase-17,20-lyase (CYP17) by cytochrome b5: endocrinological and mechanistic implications.

ABSTRACT: Using NADPH-cytochrome P-450 reductase as electron donor the homogeneous pig 17 alpha-hydroxylase-17,20-lyase (CYP17) was shown to catalyse the conversion of delta 5, as well as delta 4, steroids (pregnenolone and progesterone respectively) predominantly into the corresponding 17 alpha-hydroxylated products. The latter were then cleaved by the lyase (desmolase) activity of the enzyme into androgens. Cytochrome b5 stimulated both these activities, but its most noticeable effect was on the formation of delta 16-steroids, which compulsorily required the presence of cytochrome b5. These results on the pig enzyme confirm the original findings [Nakajin, Takahashi, Shinoda and Hall (1985) Biochem. Biophys. Res. Commun. 132, 708-713]. The human CYP17 expressed in Escherichia coli [Imai, Globerman, Gertner, Kagawa and Waterman (1993) J. Biol. Chem. 268, 19681-19689] was also purified to homogeneity and was found to catalyse the hydroxylation of pregnenolone and progesterone without requiring cytochrome b5. Like the pig CYP17, the human CYP17 also catalysed the cytochrome b5-dependent direct cleavage of pregnenolone into the delta 5,16-steroid, but unlike it the human enzyme did not cleave progesterone at all. 17 alpha-Hydroxypregnenolone was, however, cleaved into the corresponding androgen but only in the presence of cytochrome b5. 17 alpha-Hydroxyprogesterone was a poor substrate for the human CYP17; although it was converted into androstenedione in the presence of cytochrome b5 its K(m) was 5 times higher and Vmax. 2.6 times lower than those for the hydroxylation of progesterone. The endocrinological and mechanistic implications of these results are discussed.

SUBMITTER: Lee-Robichaud P

PROVIDER: S-EPMC1136809 | biostudies-other | 1995 Jun

REPOSITORIES: biostudies-other

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Project description:Human CYP17 (P-450(17alpha), 17alpha-hydroxylase-17,20-lyase)-catalysed side-chain cleavage of 17alpha-hydroxyprogestogens into androgens is greatly dependent on the presence of cytochrome b5. The native form of cytochrome b5 is composed of a globular core, residues 1-98, followed by a membrane insertable C-terminal tail, residues 99-133. In the present study the abilities of five different forms of cytochrome b5 to support the side-chain cleavage activity of CYP17 were compared. The five derivatives were: the native pig cytochrome b5 (native pig), its genetically engineered rat counterpart (core-tail), the soluble core form of the latter (core), the core with the secretory signal sequence of alkaline phosphatase appended to its N-terminal (signal-core) and the latter containing the C-terminal tail of the native rat protein (signal-core-tail). When examined by Edman degradation and MS, the engineered proteins were shown to have the expected N-terminal amino acid sequences and molecular masses. The native pig was found to be acetylated at the N-terminal. The native pig and core-tail enzymes were equally efficient at enhancing the side-chain cleavage activity of human CYP17 and the signal-core-tail was 55% as efficient. The core and signal-core constructs were completely inactive in the aforementioned reaction. All the five derivatives were reduced to varying degrees by NADPH:cytochrome P-450 (NADPH-P450) reductase and the relative efficiencies of this reduction were reminiscent of the behaviour of these derivatives in supporting the side-chain cleavage reaction. In the side-chain cleavage assay, however, NADPH-P450 reductase was used in large excess so that the reduction of cytochrome b5 derivatives was not rate-limiting. The results highlight that productive interaction between cytochrome b5 and CYP17 is governed not only by the presence of a membrane insertable hydrophobic region on the cytochrome b5 but also by its defined spatial orientation at the C-terminal.

Project description:Membrane cytochrome b5 reductase is a pleiotropic oxidoreductase that uses primarily soluble reduced nicotinamide adenine dinucleotide (NADH) as an electron donor to reduce multiple biological acceptors localized in cellular membranes. Some of the biological acceptors of the reductase and coupled redox proteins might eventually transfer electrons to oxygen to form reactive oxygen species. Additionally, an inefficient electron transfer to redox acceptors can lead to electron uncoupling and superoxide anion formation by the reductase. Many efforts have been made to characterize the involved catalytic domains in the electron transfer from the reduced flavoprotein to its electron acceptors, such as cytochrome b5, through a detailed description of the flavin and NADH-binding sites. This information might help to understand better the processes and modifications involved in reactive oxygen formation by the cytochrome b5 reductase. Nevertheless, more than half a century since this enzyme was first purified, the one-electron transfer process toward potential electron acceptors of the reductase is still only partially understood. New advances in computational analysis of protein structures allow predicting the intramolecular protein dynamics, identifying potential functional sites, or evaluating the effects of microenvironment changes in protein structure and dynamics. We applied this approach to characterize further the roles of amino acid domains within cytochrome b5 reductase structure, part of the catalytic domain, and several sensors and structural domains involved in the interactions with cytochrome b5 and other electron acceptors. The computational analysis results allowed us to rationalize some of the available spectroscopic data regarding ligand-induced conformational changes leading to an increase in the flavin adenine dinucleotide (FAD) solvent-exposed surface, which has been previously correlated with the formation of complexes with electron acceptors.

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Modulation of the activity of human 17 alpha-hydroxylase-17,20-lyase (CYP17) by cytochrome b5: endocrinological and mechanistic implications.

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