Binding characteristics of 125I‐labeled Bolton‐Hunter substance P (125I‐BHSP), a radioactive analogue of substance P, were studied with a crude syn‐aptosomal fraction (P2) of the rat brain. A specific binding of 125I‐BHSP that was saturable, reversible, and temperature dependent was demonstrated. Under the conditions of the binding assay (20°C, 5 min), nonspecific binding represented no more than 25% of total binding, and in contrast to [3H]dopamine 125I‐BHSP was not taken up within synaptosomes by a ouabain‐sensitive process. Scatchard analysis indicated the existence of a single population of noninteracting sites with a high affinity (KD, 470 pM) and a low‐density (Bmax, 13 fmol/mg protein). Substance P and different substance P analogues were tested for their competitive potencies in inhibiting 125I‐BHSP binding to synaptosomes. (Tyr8) substance P, substance P, and BHSP strongly inhibited 125I‐BHSP specific binding. Among various tachykinins, physalaemin was the most potent (one fourth the potency of substance P). When substance P C‐terminal fragments were tested for their ability to compete with 125I‐BHSP binding, a good relationship was found between competitive activity and peptide length. N‐terminal fragments of substance P were ineffective. However, (nor‐Leu11)substance P and the methyl ester of substance P1‐11 slightly reduced 125I‐BHSP binding. (D‐Pro2, D‐Phe7, D‐Trp9) substance P and (D‐Pro2, D‐Trp7,9)substance P, two substance P antagonists, were also tested. (D‐Pro2, D‐Trp7,9) substance P was the only one to inhibit 125I‐BHSP binding even slightly. Results were compared with those previously obtained concerning 125I‐BHSP binding to mesencephalic embryonic cells in primary cultures using similar conditions for the binding assay.

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The last step of aldosterone biosynthesis, an 1 1p-hydroxylation followed by two 18-hydroxyIations, are catalyzed, in the bovine system, by the same enzyme, the cytochrome P-45Oll, (deoxycorticosterone (DOC) -corticosterone -18-hydroxycorticosterone -aldosterone). The 1 lp-and 18-hydroxylase activities were studied separately with a reconstituted enzymic system, using 1 l-de~xy Steroid Biochem. 33, 119-1241, were characterized for both activities (llp-and 18-hydroxylase). The value of reversible K, for the 18-hydroxylation (Ki = 5 pM for 18-vinylprogesterone and 30 pM for 18-ethynylprogesterone) is lower than that for the 1lp-hydroxylation (30 pM and 100-150 pM, respectively); the former inhibitor is stronger than the latter for both steps.The binding of substrates and inhibitors to the active site was also examined by difference absorption spectroscopy. 18-Vinylprogesterone gave rise to a type I spectrum with a K, value of 35 pM close to that of progesterone, while 18-ethynylprogesterone showed a reverse type I spectrum with a much higher K, value (140 pM). Based on these results, a hypothetical model, involving a conformational change of the enzyme for the second step, is proposed.Keywords: cytochrome ; P-45OI1,; mechanism-based inhibitors ; aldosterone biosynthesis ; 18-vinylprogesterone.Aldosterone overproduction leads to oedematous diseases and hypertension (Corvol et al., 1990). These disorders are clinically treated with spirolactones which act as antagonists at the receptor level. We developed a new approach namely by focusing on the inhibition of cytochrome P-4501,, (P-450,,,), the last enzyme in aldosterone biosynthesis. For this purpose, progesterone derivatives designed as mechanism-based inhibitors of P-4501,, were synthesized in our laboratory (Viger et al., 1988).The most potent inhibitors, tested with rat adrenal cell-free extracts, were 18-vinylprogesterone (1 8-VP) and 18-ethynylprogesterone (18-EP) (Fig. 1) which completely inhibit aldosterone production at 0.8 pM and 8 pM, respectively (Viger et al., 1989). In this study the inhibitions by 18-VP and 18-EP were studied more accurately using a bovine reconstituted enzymic P-450,,,, P-450,,, and P-450,,,,, cytochromes P-450 1 lP-18-hydroxylase, P-450 cholesterol side-chain cleavage, P-450 17a-hydro~ylase/C,,,~, lyase and P-450 aromatase.Enzyme. Steroid 1 ID-monoxygenase; cytochrome P-450, ,8 (EC 1.14.15.4). system including P-45011,, adrenodoxin and adrenodoxin reductase.P-450,,, purified from bovine adrenal cortex can catalyze the 1 lp-hydroxylation of 11 -deoxycorticosterone, the 18-hydroxylation of corticosterone as well as aldosterone formation (Fig. 2) (Wada et al., 1985;Yanagibashi et al., 1986). However two distinct forms of P-45Ol,,, encoded by two different genes exist in the bovine adrenal cortex (for a review, see Muller, 1993). Indeed two bovine P-45011, cDNAs, pc P-450 118-3 and pc P-450 llp-2 were cloned (Moroashi et al., 1987;Kirita et al., 1988). Expression into COS-7 cells revealed that both enzymes were capable of catalyzin...

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