Synthesis and study of the labeling of pregnenolone and progesterone specifically tritiated at the 17 position

The cationic charges on Arg347, Arg358 and Arg449 of human cytochrome P450c17 (CYP17) are essential for the enzyme's cytochrome b5-dependent acyl-carbon cleavage activities

CYP17 (17alpha-hydroxylase-17,20-lyase; also P450c17 or P450(17alpha)) catalyses the17alpha-hydroxylation of progestogens and the subsequent acyl-carbon cleavage of the 17alpha-hydroxylated products (lyase activity) in the biosynthesis of androgens. The enzyme also catalyses another type of acyl-carbon cleavage (direct cleavage activity) in which the 17alpha-hydroxylation reaction is by-passed. Human CYP17 is heavily dependent on the presence of the membrane form of cytochrome b(5) for both its lyase and direct cleavage activities. In the present study it was found that substitution of human CYP17 amino acids, Arg(347), Arg(358) and Arg(449), with non-cationic residues, yielded variants that were impaired in the two acyl-carbon bond cleavage activities, quantitatively to the same extent and these were reduced to between 3 and 4% of the wild-type protein. When the arginines were replaced by lysines, the sensitivity to cytochrome b(5) was restored and the acyl-carbon cleavage activities were recovered. All of the human mutant CYP17 proteins displayed wild-type hydroxylase activity, in the absence of cytochrome b(5). The results suggest that the bifurcated cationic charges at Arg(347), Arg(358) and Arg(449) make important contributions to the formation of catalytically competent CYP17.cytochrome b(5) complex. The results support our original proposal that the main role of cytochrome b(5) is to promote protein conformational changes which allow the iron-peroxo anion to form a tetrahedral adduct that fragments to produce the acyl-carbon cleavage products.

An analysis of the role of active site protic residues of cytochrome P-450s: mechanistic and mutational studies on 17alpha-hydroxylase-17,20-lyase (P-45017alpha also CYP17)

Certain cytochrome P-450s involved in the transformation of steroids catalyse not only the hydroxylation process associated with the group of enzymes, but also an acyl-carbon cleavage reaction. The hydroxylation occurs using an iron-monooxygen species while the acyl-carbon cleavage has been suggested to be promoted by an iron peroxide. In this paper we have studied the role of active site protic residues, Glu305 and Thr306, in modulating the two activities. For this purpose, the kinetic parameters for the hydroxylation reaction (pregnenolone-->17alpha-hydroxypregnenolone) and two different versions of acyl-carbon cleavage (17alpha-hydroxypregnenolone-->dehydroepiandrosterone and 3beta-hydroxyandrost-5-ene-17beta-carbaldehyde-->3beta-hydroxya ndrost -5,16-diene+androst-5-ene-3beta,17alpha-diol) were determined using the wild-type human CYP17 and its eight different single and double mutants. In addition the propensity of the proteins to undergo a subtle rearrangement converting the 450 nm active-form into an inactive counterpart absorbing at 420 nm, was monitored by measuring the t12 of the P-450-->P-420 conversion. The results are interpreted to draw the following conclusions. The functional groups of Glu305 and Thr306 do not directly participate in the two proton delivery steps required for hydroxylation but may be important participants for the provision of a net work of hydrogen bonds for 'activating' water that then acts as a proton donor. The loss of any one of these residues is, therefore, only partially debilitating. That the mutation of Thr306 impairs the hydroxylation reaction more than it does the acyl-carbon cleavage is consistent with the detailed mechanistic scheme considered in this paper. Furthermore attention is drawn to the fact that the mutation of Glu305 and Thr306 subtly perturbed the architecture of the active site, which affects the geometry of this region of the protein and therefore its catalytic properties.

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

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.

Synthesis of 16 alpha-3H androgen and estrogen substrates for 16 alpha-hydroxylase

The synthesis of 16 alpha-3H androgens and estrogens is described. 1-(3H)-Acetic acid in the presence of zinc dust reacts with 16 alpha-bromo-17-ketosteroids to produce 16 alpha-3H-17-ketosteroids. This chemical reaction was used to prepare 16 alpha-3H-dehydroepiandrosterone (I) and 16 alpha-3H-estrone acetate (XI) from 16 alpha-bromo-dehydroepiandrosterone (X) and from 16 alpha-bromo-estrone acetate (XII), respectively. Using appropriate microbiological techniques, it was possible to convert these radiolabelled substrates into 16 alpha-3H-androstenedione (II) and 16 alpha-3H-estradiol-17 beta (VII). 16 alpha-3H-Estrone (VI) was obtained by the chemical hydrolysis of 16 alpha-3H-estrone acetate. The label distribution as determined by microbiological 16 alpha-hydroxylations indicated a specific labelling of 77% for androgens and 65% for estrogens in the 16 alpha position. These substrates can be used for measuring the 16 alpha hydroxylase activity, an important step in the biosynthesis of estriol (VIII) and estetrol (IX).

New assay for pregnenolone and progesterone 17alpha- hydroxylase based on the specific substitution of a tritium situated on carbon 17

A new assay for the measurement of steroid 17alpha-hydroxylase activity in beef adrenals is described. This method is based on the biochemical mechanism of the enzymic reaction, i.e. the direct and stereospecific substitution of the proton located on the hydroxylated position. Progesterone or pregnenolone specifically labelled on the 17 position are solubilized in the incubation mixture with the help of Tween 80 and incubated under optimal conditions. The tritium enzymically released from the substrate is found in the medium as a molecule of water which is then distilled under reduced pressure and counted by liquid scintillation. The results obtained with this new method are comparable with those obtained with a conventional method using a 14C-labelled substrate.