The kinetic mechanism of the hypothalamic progesterone 5 alpha-reductase

The quasi-equilibrium assumption for Bi-Bi ordered bisubstrate enzymatic reaction. How to discriminate the mechanism correctly

Application of the quasi-equilibrium assumption for the steady-state kinetics of bisubstrate irreversible enzymatic reactions in the case of ordered binding of substrates (Bi-Bi ordered mechanism) is considered. The necessary and sufficient conditions for application of the quasi-equilibrium assumption have been found and accuracy of this assumption has been numerically evaluated. The limitations on application of the quasi-equilibrium assumption have been shown and errors of its application have been analyzed. It is shown that possible discrimination of substrate binding order using asymmetrical expressions grounded on the quasi-equilibrium assumption is inconsistent because such asymmetrical expressions arise from incorrect application of the quasi-equilibrium assumption. Moreover, it has been proved in the general case that mechanisms generating such substrate-asymmetrical expressions for the steady-state rate of enzymatic reaction do not exist. The error source when using graphical interpretation for discrimination of mechanisms of bisubstrate enzymatic reactions has been determined. The strategy to avoid such errors is pointed out.

Neuroendocrine metabolism of progesterone and related progestins

In mammalian neuroendocrine structures the metabolic processing of progesterone and related natural progestins is primarily a reductive process involving the C-4,5 double bond and the C-3 and C-20 ketones. The principal products of the neuroendocrine metabolism of progesterone in female rats are the two 5 alpha- and 3 alpha-reduced metabolites, 5 alpha-dihydroprogesterone and 3 alpha,5 alpha-tetrahydroprogesterone, with lesser amounts of the corresponding 20 alpha-reduced products. Certain of these metabolites produce some, but not all, of progesterone's biological effects. 5 alpha-Dihydroprogesterone and 3 alpha,5 alpha-tetrahydroprogesterone, in particular, have potent progesterone-like effects on neuroendocrine functions, such as gonadotropin regulation. The two other principal ovarian progestins, 20 alpha-dihydroprogesterone and 17 alpha-hydroxyprogesterone, are metabolized in an analogous manner. The major neuroendocrine progestin conversions therefore appear to be 5 alpha-reduction and 3 alpha-hydroxysteroid oxidoreduction. In the hypothalamus and anterior pituitary, the enzymic activities that catalyse these conversions appear to be under ovarian control and appear to vary with changing reproductive states. These quantitative changes in processing, together with the potent progesterone-like effects of certain metabolites, suggest that these neuroendocrine conversions may provide an important mechanism for mediating some of the effects of progesterone. Alternatively, some metabolites, by duplicating selected effects of progesterone, may provide a means of prolonging certain of its effects while others are terminated.

Androgen-activating enzymes in the central nervous system

In the rat brain, several steroids can be converted by specific enzymes to either more potent compounds or to derivatives showing new biological effects. One of the most studied enzyme is the 5alpha-reductase (5alpha-R), which acts on 3keto-delta4 steroids. In males, testosterone is the main substrate and gives rise to the most potent natural androgen dihydrotestosterone. In females, progesterone is reduced to dihydroprogesterone, a precursor of allopregnanolone, a natural anxiolytic/anesthetic steroid. Other substrates are some gluco- and minero-corticoids. Two isoforms of the 5alpha-R, with limited degree of homology, have been cloned: 5alpha-R type 1 and type 2. The 5alpha-R type 1 possesses low affinity for the various substrates and is widely distributed in the body, with the highest levels in the liver; in the brain, this isoform is expressed throughout life and does not appear to be controlled by androgens. 5Alpha-R type 1 in the rat brain is mainly concentrated in myelin membranes, where it might be involved in the catabolism of potentially neurotoxic steroids. The 5alpha-R type 2 shows high affinity for the various substrates, a peculiar pH optimum at acidic values and is localized in androgen-dependent structures. In the rat brain, the type 2 isoform is expressed at high levels only in the perinatal period and is controlled by androgens, at least in males. In adulthood, the type 2 gene appears to be specifically expressed in localised brain regions, like the hypothalamus and the hippocampus. The 5alpha-R type 2 is present in the GT1 cells, a model of LHRH-secreting neurons. These cells also contain the androgen receptor, which is probably involved in the central negative feedback effect exerted by androgens on the hypothalamic-pituitary-gonadal axis. The physiological significance of these and additional data will be discussed.

5 alpha-reductase isozymes in the central nervous system

The enzyme 5 alpha-reductase (5 alpha-R) activates several delta 4-3keto steroids to more potent derivatives which may also acquire new biological actions. Testosterone gives rise to the most potent natural androgen dihydrotestosterone (DHT), and progesterone to dihydroprogesterone (DHP), a precursor of the endogenous anxiolytic/anesthetic steroid tetrahydroprogesterone (THP). Two isoforms of 5 alpha-R, with a limited degree of homology, different biochemical properties and distinct tissue distribution have been cloned: 5 alpha-R type 1 and type 2. In androgen-dependent structures DHT is almost exclusively formed by 5 alpha-R type 2; 5 alpha-R type 1 is widely distributed in the body, with the highest levels in the liver, and may be involved in steroid catabolism. In the brain, the roles of the two isozymes are still largely unknown. This brief review will summarize recent experimental data from our laboratory which try to assign possible functional roles to the process of 5 alpha-reduction, and to the two 5 alpha-R isoforms in the CNS.

Identification of type 1 5alpha-reductase in myelin membranes of male and female rat brain

The formation of the 5alpha-reduced metabolites of testosterone (T) and of progesterone (P) is a very active process in the brain, since the enzyme 5alpha-reductase (5alpha-R) is present in almost any central nervous system (CNS) structure. A particularly elevated 5alpha-R activity has been shown in myelin sheaths. Two isoforms of the enzyme have been cloned, with different localisation as well as different biochemical properties. The present study was performed to determine whether both isoforms of the 5alpha-R, or only one of them, are/is responsible for the enzymatic activity observed in myelin. Kinetic analyses have been performed on purified myelin membranes prepared from the male or female rat brain, using both T and P as substrates. The 5alpha-R present appears to possess a pH optimum at basic values. The Vmax values obtained in the Lineweaver-Burk analysis were comparable in male and female preparations independently on whether T or P were used as the substrates, suggesting that a single enzymatic form is present in all samples examined; the Km obtained using [14C]T (Km: male 1.14 microM; female 1.46 microM) or [14C]P (Km: male 0.5 microM; female 0.64 microM) as substrates, were in good agreement with those obtained for the recombinant type 1 isoform. These data suggest that the type 1 isoform is the most relevant 5alpha-R present in myelin. To confirm this, a new polyclonal antibody was raised against the type 1 5alpha-R enzymatic protein, and used in immunohistochemical studies. The experiments were performed on the optic nerve, a myelinated structure very rich in 5alpha-R activity and the results clearly indicated the presence of a specific type 1 enzyme immunoreactivity in the myelin sheaths of axons.

Testosterone and progesterone metabolism in the central nervous system: cellular localization and mechanism of control of the enzymes involved

This paper summarizes the most recent data obtained in the authors' laboratory on the metabolism of testosterone and progesterone in neurons and in the glia. 1. The activities of 5 alpha-reductase (the enzyme that converts testosterone into dihydrotestosterone; DHT) and of 3 alpha-hydroxy steroid dehydrogenase (the enzyme that converts DHT into 5 alpha-androstane-3 alpha, 17 beta-diol; 3 alpha-diol) were first evaluated in primary cultures of neurons, oligodendrocytes, and type-1 and type-2 astrocytes, obtained from the fetal or neonatal rat brain. The formation of DHT and 3 alpha-diol was evaluated incubating the different cultures with labeled testosterone or labeled DHT as substrates. The results obtained indicate that the formation of DHT takes place preferentially in neurons; however, also type-2 astrocytes and oligodendrocytes possess considerable 5 alpha-reductase activity. A completely different localization was observed for 3 alpha-hydroxysteroid dehydrogenase; the formation of 3 alpha-diol appears to be prevalently, if not exclusively, present in type-1 astrocytes; 3 alpha-diol is formed in very low yields by neurons, type-2 astrocytes, and oligodendrocytes. Moreover, the results indicate that, in type 1 astrocytes, both 5 alpha-reductase and 3 alpha-HSD are stimulated by coculture with neurons and by the addition of neuron-conditioned medium, suggesting that secretory products released by neurons might intervene in the control of glial cell function. 2. Subsequently it was shown that, similarly to what happens when testosterone is used as the substrate, 5 alpha-reductase, which metabolizes progesterone into 5 alpha-pregnane-3,20-dione, (DHP), shows a significantly higher activity in neurons than in glial cells; however, also type-1 and type-2 astrocytes as well as oligodendrocytes possess some ability to 5 alpha-reduce progesterone. On the contrary, 3 alpha-hydroxysteroid dehydrogenase, the enzyme which converts DHP into 5 alpha-pregnane-3 alpha-ol-20-one (THP), appears to be present mainly in type-1 astrocytes; much lower levels of this enzyme are present in neurons and in type-2 astrocytes. At variance with the previous results obtained using androgens as precursors, oligodendrocytes show considerable 3 alpha-hydroxysteroid dehydrogenase activity, even if this is statistically lowe than that present in type-1 astrocytes. The existence of isoenzymatic forms of the enzymes involved in androgen and progesterone metabolism is discussed.

Product inhibition applications

A product inhibition study provides important insight into the binding mechanism of an enzyme, especially in the identification of abortive complexes, but seldom is it the only tool required to solve the mechanism completely. Always keep in mind that more than one mechanism may be consistent with the patterns, and several alternative schemes should be analyzed by including abortive complexes and, as a last resort, isomerization steps or slow product release steps in the interpretation. Support for the proposed mechanism should be garnered from other data, such as kinetic studies with alternative substrates and competitive inhibitors, positional and molecular isotope exchange studies, binding studies, and isotope effects. A well-characterized binding mechanism complete with the identification of abortive complexes is even more important as rational drug design becomes more prevalent. Product inhibition studies represent an important tool that is relatively easy to apply to gain significant information about the binding mechanism of most enzymes.

Photoaffinity labeling of rat liver microsomal steroid 5 alpha-reductase by 2-azido-NADP+

Preincubation of female rat liver microsomal preparations with [2'-32P]2N3-NADP+ followed by photolysis with UV light (254 nm) and analysis by SDS-PAGE/autoradiography showed incorporation of 32P into at least 3 major protein bands in the molecular weight range of 14-97 Kd. Labeling of a 26 kD band, the apparent molecular weight of 5 alpha-reductase in liver microsomes, was accompanied by a loss of enzyme activity, consistent with its covalent modification. The inclusion of 20-fold excess NADP+ (100 microM) completely inhibited the incorporation of [2'-32P]2N3-NADP+ and preserved the enzyme activity, whereas excess NAD+ (100 microM) failed to protect 5 alpha-reductase (5 alpha R) activity. Similar results were obtained with the detergent-solubilized form of 5 alpha R. Polyethylene glycol (PEG) fractionation of detergent-solubilized preparations of 5 alpha R showed that all the 5 alpha R activity could be recovered in the 6.5% pellet with a 3-4-fold increase in the specific activity. photolysis of this fraction with [2'-32P]2N3-NADP+ resulted in approximately 2-fold increase in 32P labeling of the 5 alpha R band. Increasing photolysis time and concentration of the [2'-32P]2N3-NADP+ indicated that the half-life for photoincorporation and the apparent Kd were 1.0 min and 2 microM, respectively. These results suggest that 2N3-NADP+ is an effective probe of the NADP(H) binding site of 5 alpha R, and is a useful marker during purification of the enzyme.

Androgen and progesterone metabolism in the central and peripheral nervous system

This paper summarizes the most recent data obtained in the authors' laboratory on the metabolism of testosterone and progesterone in neurons, in the glia, and in neuroblastoma cells. The activities of the 5 alpha-reductase (the enzyme that converts testosterone into dihydrotestosterone, DHT), and of the 3 alpha-hydroxysteroid dehydrogenase (the enzyme that converts DHT into 5 alpha-androstane-3 alpha, 17 beta-diol, 3 alpha-diol) have been first evaluated in primary cultures of neurons, oligodendrocytes and type-1 and -2 astrocytes, obtained from the fetal or neonatal rat brain. All the cultures were used on the fifth day. The formation of DHT of 3 alpha-diol was evaluated incubating the different cultures with labeled testosterone or DHT as substrates. The results obtained indicate that the formation of DHT takes place preferentially in neurons; however, type-2 astrocytes and oligodendrocytes also possess considerable 5 alpha-reductase activity, while type-1 astrocytes show a much lower enzymatic concentration. A completely different localization was observed for 3 alpha-hydroxysteroid dehydrogenase; the formation of 3 alpha-diol appears to be prevalently, if not exclusively, present in type-1 astrocytes; 3 alpha-diol is formed in very low yields by neurons, type-2 astrocytes and oligodendrocytes. The compartmentalization of two strictly correlated enzymes (5 alpha-reductase and 3 alpha-hydroxysteroid dehydrogenase) in separate central nervous system (CNS) cell populations suggests the simultaneous participation of neurons and glial cells in the 5 alpha-reductive metabolism of testosterone. Subsequently it has been shown that, similarly to what happens when testosterone is used as the substrate, the 5 alpha-reductase which metabolizes progesterone into 5 alpha-pregnane-3,20-dione (DHP) shows a significantly higher activity in neurons than in glial cells; however, type-1 and -2 astrocytes as well as oligodendrocytes also possess some ability to 5 alpha-reduce progesterone. On the other hand, 3 alpha-hydroxysteroid dehydrogenase, the enzyme which converts DHP into 5 alpha-pregnane-3 alpha-ol-20-one, appears to be present mainly in type-1 astrocytes; much lower levels of this enzyme are present in neurons and in type-2 astrocytes. At variance with the previous results obtained using androgens as precursors, oligodendrocytes show considerable 3 alpha-hydroxysteroid dehydrogenase activity, even if this is statistically lower than that present in type-1 astrocytes. The existence of isoforms of the enzyme involved in androgen and progesterone metabolism is discussed.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolism of progesterone and testosterone in human parotid and submandibular salivary glands in vitro

Twenty-three parotid and twelve submandibular human salivary glands were used for analyzing in vitro metabolism of progesterone (P) and/or testosterone (T), in order to find out their possible contribution to the concentrations of salivary P or T. Glands were excised from 21 women and 14 men because of a stone, tumor or sialoadenitis, and the healthy tissue was used for incubation. The tissues were homogenized and incubated with 5 to 7 nmol/l of [14C]P or [14C]T for 2 h, extracted with methylacetate and subjected to two dimensional thin-layer chromatography. The thin-layer plates were autoradiographed, and the radioactivity of the different metabolites were counted to assess the amounts of metabolites formed. No lipoidal or conjugated steroids were formed. All of the radioactivity was associated with the free fraction of steroids. Both P and T were metabolized significantly more actively in male submandibular gland compared to male parotid gland (P < 0.05-0.01). No significant differences were found between female and male parotid glands, nor between the parotid glands of hormonally medicated (oral contraceptives or postmenopausal estrogen treatment) and non-medicated women. The submandibular glands more actively metabolized the steroids studied than the parotids (P < 0.01). The main metabolites were 20 alpha-hydroxy-4-pregnen-3-one for P, and androstenedione for T. In conclusion, the present results bring further evidence that the P or T levels in saliva may not be identical with the unbound steroid fraction in circulation, but a part of the steroids are metabolized in salivary glands. However, due to the rapid passage of steroids from blood to saliva, the metabolism demonstrated probably does not form an important source of error in salivary P and T measurements.

Androgen metabolism in the brain

The paper summarizes the most recent views on androgen metabolism in the brain. In particular it will be shown that: (1) the enzyme 5 alpha-reductase is particularly concentrated in the white matter; (2) 5 alpha-reductase is also present in the myelin; 5 alpha-reductase is present in higher concentrations in neurons (isolated or cultured) that in glial cells (astrocytes and oligodendrocytes); (4) only neurons possess the capability of aromatizing androgens to estrogens; and (5) a possible role of steroid metabolism in the control of the process of myelinogenesis is suggested.

Mechanistic studies with solubilized rat liver steroid 5 alpha-reductase: elucidation of the kinetic mechanism

A solubilized preparation of steroid 5 alpha-reductase (EC 1.3.1.30) from rat liver has been used in studies focused toward an understanding of the kinetic mechanism associated with enzyme catalysis. From the results of analyses with product and dead-end inhibitors, a preferentially ordered binding of substrates and release of products from the surface of the enzyme is proposed. The observations from these experiments were identical with those using the steroid 5 alpha-reductase activity associated with rat liver microsomes. The primary isotope effects on steady-state kinetic parameters when [4S-2H]NADPH was used also were consistent with an ordered kinetic mechanism. Normal isotope effects were observed for all three kinetic parameters (Vm/Km for both testosterone and NADPH and Vm) at all substrate concentrations used experimentally. Upon extrapolation to infinite concentration of testosterone, the isotope effect on Vm/Km for NADPH approached unity, indicating that the nicotinamide dinucleotide phosphate is the first substrate binding to and the second product released from the enzyme. The isotope effects on Vm/Km for testosterone at infinite concentration of cofactor and on Vm were 3.8 +/- 0.5 and 3.3 +/- 0.4, respectively. Data from the pH profiles of these three steady-state parameters and the inhibition constants (1/Ki) of competitive inhibitors versus both substrates indicate that the binding of nicotinamide dinucleotide phosphate involves coordination of its anionic 2'-phosphate to a protonated enzyme-associated base with an apparent pK near 8.0. From these results, relative limits have been placed on several of the internal rate constants used to describe the ordered mechanism of the rat liver steroid 5 alpha-reductase.

Interaction between rat prostatic steroid 5 alpha-reductase and 3-carboxy-17 beta-substituted steroids: novel mechanism of enzyme inhibition

Efforts to identify novel compounds capable of blocking the steroid 5 alpha-reductase (SR) catalyzed conversion of testosterone (T) to 5 alpha-dihydrotestosterone have resulted in the development of 17 beta-substituted-3-androstene-3-carboxylic acids as potent inhibitors of the rat prostatic enzyme. The dead-end inhibition patterns of one of these steroidal acrylates, 17 beta-N-(2-methyl-2-propyl)-carbamoyl-androst-3,5-diene-3-carboxylic acid were best evaluated with a linear uncompetitive kinetic model vs both T (Kii = 11 +/- 1 nM) and NADPH (Kii = 22 +/- 2 nM). To interpret these observations, the kinetic mechanism of the rat prostatic SR was shown to involve the binding of NADPH prior to that of T through a series of dead-end and product inhibition experiments. Within the context of this preferentially ordered kinetic mechanism, it is proposed that the uncompetitive inhibition patterns result from the association of the steroidal acrylate to an enzyme complex containing NADP+ in formation of a dead-end ternary complex of enzyme, NADP+, and inhibitor.