Organization of androgen biosynthesis in the testis

Steroidogenesis in the fetal testis and its susceptibility to disruption by exogenous compounds

Masculinization depends on adequate production of testosterone by the fetal testis within a specific "masculinization programming window." Disorders resulting from subtle deficiencies in this process are common in humans, and environmental exposures/lifestyle could contribute causally because common therapeutic and environmental compounds can affect steroidogenesis. This evidence derives mainly from rodent studies, but because there are major species differences in regulation of steroidogenesis in the fetal testis, this may not always be a guide to potential effects in the human. In addition to direct study of the effects of compounds on steroidogenesis, information also derives from study of masculinization disorders that result from mutations in genes in pathways regulating steroidogenesis. This review addresses this issue by critically reviewing the comparative timing of production and regulation of steroidogenesis in the fetal testis of humans and of rodents and its susceptibility to disruption; where there is limited information for the fetus, evidence from effects on steroidogenesis in the adult testis is considered. There are a number of fundamental regulatory differences between the human and rodent fetal testis, most notably in the importance of paracrine vs. endocrine drives during masculinization such that inactivating LH receptor mutations block masculinization in humans but not in rodents. Other large differences involve the steroidogenic response to estrogens and GnRH analogs and possibly phthalates, whereas for other compounds there may be differences in sensitivity to disruption (ketoconazole). This comparison identifies steroidogenic targets that are either vulnerable (mitochondrial cholesterol transport, CYP11A, CYP17) or not (cholesterol uptake) to chemical interference.

Characterization of the hypothalamic-pituitary-gonadal axis in estrogen receptor (ER) Null mice reveals hypergonadism and endocrine sex reversal in females lacking ERalpha but not ERbeta

To determine the role of each estrogen receptor (ER) form (ERalpha, ERbeta) in mediating the estrogen actions necessary to maintain proper function of the hypothalamic-pituitary-gonadal axis, we have characterized the hypothalamic-pituitary-gonadal axis in female ER knockout (ERKO) mice. Evaluation of pituitary function included gene expression assays for Gnrhr, Cga, Lhb, Fshb, and Prl. Evaluation of ovarian steroidogenic capacity included gene expression assays for the components necessary for estradiol synthesis: i.e. Star, Cyp11a, Cyp17, Cyp19, Hsd3b1, and Hsd17b1. These data were corroborated by assessing plasma levels of the respective peptide and steroid hormones. alphaERKO and alphabetaERKO females exhibited increased pituitary Cga and Lhb expression and increased plasma LH levels, whereas both were normal in betaERKO. Pituitary Fshb expression and plasma FSH were normal in all three ERKOs. In the ovary, all three ERKOs exhibited normal expression of Star, Cyp11a, and Hsd3b1. In contrast, Cyp17 and Cyp19 expression were elevated in alphaERKO but normal in betaERKO and alphabetaERKO. Plasma steroid levels in each ERKO mirrored the steroidogenic enzyme expression, with only the alphaERKO exhibiting elevated androstenedione and estradiol. Elevated plasma testosterone in alphaERKO and alphabetaERKO females was attributable to aberrant expression of Hsd17b3 in the ovary, representing a form of endocrine sex reversal, as this enzyme is unique to the testes. Enhanced steroidogenic capacity in alphaERKO ovaries was erased by treatment with a GnRH antagonist, indicating these phenotypes to be the indirect result of excess LH stimulation that follows the loss of ERalpha in the hypothalamic-pituitary axis. Overall, these findings indicate that ERalpha, but not ERbeta, is indispensable to the negative-feedback effects of estradiol that maintain proper LH secretion from the pituitary. The subsequent hypergonadism is illustrated as increased Cyp17, Cyp19, Hsd17b1, and ectopic Hsd17b3 expression in the ovary.

Altered molecular dynamics and antioxidant status in the spermatozoa in testosterone-induced oligospermia in mouse

Though supraphysiological doses testosterone (T) and its derivatives are known to suppress spermatogenesis in mammals by interfering with the hypothalamus-pituitary axis leading to oligozoospermia, no study has been performed to evaluate the integrity of the sperm cells produced by such individuals. In T-induced oligozoospermia in the mouse, the spermatozoa showed suppressed zona-binding ability though the motility and viability remained unchanged. In order to assess whether this decreased zona-binding ability is due to perturbations in the mechanical properties of the sperm membranes, we attempted to examine the molecular dynamics employing a lipophilic spin label (16-doxyl stearate) and a protein-binding label (Mal-Net) in two sets of independent experiments. The results showed that the rotational freedom of lipophilic molecules reduced significantly within the first week of T-treatment. During weeks 1 through 4, the protein rotation was found to be retarded significantly. We observed a sharp increase in the ascorbyl radical associated with the cauda epididymal spermatozoa and epididymal fluid of testosterone-treated mice. Moreover, the glutathione (GSH) content in the spermatozoa and the epididymal fluid increased significantly after testosterone-treatment. Further, there was a elevation in the superoxide dismutase (SOD) activity and suppression in the superoxide anion radical generated by the cauda epididymal spermatozoa of testosterone-treated animals. A change in the mechanical properties of a bilayer could modify both the mechanical properties and the function of incorporated proteins. In many instances, a liquid-crystalline bilayer is necessary for protein function. It is likely that the change in the physical properties of sperm membranes might cause the inhibition of enzymes associated with spermatozoa after T-treatment. The alterations in the sperm membrane structure and the antioxidant potentials of both the spermatozoa and the cauda epididymal fluid could also account for the decrease in the zona-binding index of the spermatozoa in T-treated animals. Thus, this study demonstrates for the first time that supraphysiological doses of testosterone could modify the mechano-dynamic properties of sperm membranes and could perturb the redox status of both spermatozoa and the epididymal fluid.

A study of androgen biosynthesis by the human testis in vitro

The biosynthetic pathways in human testicular tissue have been studied extensively in our laboratory without the use of radioisotopes. Experiments were conducted with normal testicular tissue from patients undergoing orchiectomy for prostatic cancer. These studies have shown that the preferred pathway of testosterone biosynthesis is influenced by the nature and concentration of cofactor added to the incubation medium. Four enzymes are involved in the transformation of pregnenolone to testosterone, that is, 3 beta-hydroxysteroid dehydrogenase, 17 alpha-hydroxylase, C17-C20 lyase and 17 beta-hydroxysteroid oxidoreductase. Our studies show that the 4-ene pathway predominates in the biosynthesis of testosterone from pregnenolone. Analysis of several samples of human testicular vein blood supports the contention that 4-androsten-3,17-dione is the immediate precursor of testosterone.

Identification of phospholipids capable of modulating the activities of some enzymes involved in androgen and 16-androstene biosynthesis in the immature pig testis

Testicular steroidogenic enzymes in the microsomal fraction from immature pigs were investigated for the effects of phospholipids of known structure on androgen and 16-androstene biosynthesis. Untreated (control) microsomes metabolized pregnenolone to 17-hydroxypregnenolone, DHA and small quantities of progesterone, 17-hydroxyprogesterone, androstenedione and testosterone; and to 5,16-androstadien-3 beta-ol (andien-beta) and 4,16-androstadienone (dienone) in the 16-androstene pathway. Phosphatidyl(P)-serine, P-glycerol, P-ethanolamine, P-inositol, P-choline and phosphatidic acid did not significantly alter the 17-hydroxylase/C-17,20 lyase or "andien-beta-synthetase" activities. Thus, the C21 side-chain cleavage reactions appeared not to be dependent upon phospholipids for optimal activity. The conversion of pregnenolone to 4-ene steroids (progesterone, 17-hydroxyprogesterone, androstenedione and testosterone) was inhibited by dilinoleoyl-phosphatidyl-choline, but other phospholipids tested were without effect. On the other hand, the conversion of andien-beta to dienone was inhibited by P-serine, P-inositol and P-cholines with short saturated or long polyunsaturated acyl chains. Therefore, the presence of these phospholipids in pregnenolone incubations had different consequences for 3 beta-hydroxysteroid dehydrogenase-isomerase activities. It is concluded that substrate specific 3 beta-HSD-isomerases exist for androgen and 16-androstene biosynthesis and that phospholipids may play an intrinsic role in their catalytic activity.

The interference of polychlorinated biphenyls (Aroclor 1254) with membrane regulation of the activities of cytochromes P-450C21 and P-450(17) alpha,lyase in guinea-pig adrenal microsomes

Regulation of cytochromes P-450 21-hydroxylase (P-450C21) and P-450 17 alpha-hydroxylase/C17,20-lyase (P-450(17) alpha,lyase) activities and impairment of this regulation by Aroclor 1254 was studied in guinea-pig adrenal microsomes. In a membrane depleted system, a decrease in the normally predominant, P-450C21 activity and an increase in P-450(17) alpha,lyase activities was observed. The same deviations were observed in intact microsomes with increase in the reaction temperature (0-40 degrees C). Breaks in Arrhenius plots for activities of P-450C21 and P-450(17) alpha,lyase correlate with transition temperatures reported for the microsomal membrane. These results point to: (1) preference of a gel state membrane for catalytic expression of P-450C21 suggesting a clustered organization of this P-450 species with reductase; (2) preference of a fluid membrane for lyase activity suggesting a random collision mechanism for reduction of P-450(17) alpha,lyase. Aroclor 1254 introduced to reaction mixtures containing intact microsomes elicited basically the same changes as caused by depletion of the microsomal membrane or by increase in the incubation temperature. Lack of effect of Aroclor 1254 on P-450C21 and P-450(17) alpha,lyase activities in the membrane depleted system demonstrates that its interference with monooxygenase activities is mediated by the microsomal membrane. The similarities between altered cytochrome P-450 mediated activities in the presence of Aroclor 1254 and the deviations observed in the membrane depleted system or upon increase in the incubation temperature may suggest that this chemical exerts its impacts by influencing membrane fluidity.

Phospholipases modulate immature pig testicular androgen and 16-androstene biosynthetic pathways in vitro

The role of membrane phospholipids in porcine testicular androgen and 16-androstene biosynthesis was examined by monitoring the effects of phospholipase treatments on the activities of the steroid transforming enzymes. Untreated (control) microsomes from immature pig testes converted pregnenolone to 17-hydroxypregnenolone and DHA to 5,16-androstadien-3 beta-ol (andien-beta) and 4,16-androstadien-3-one (dienone) in the 16-androstene pathway, these metabolites accounting for most (65%) of the pregnenolone converted. The 4-ene steroids in the androgen pathway (progesterone, 17-hydroxyprogesterone, androstenedione and testosterone) totalled less than 10% of the pregnenolone metabolites. No estrogens or 5 alpha-reduced metabolites were detected. Treatment with phospholipase A2 or C, decreased the conversion of pregnenolone to 4-ene-3-oxo steroids but did not decrease the quantities of 5-ene-3 beta-hydroxysteroids. Confirmation of these findings was obtained by measuring the individual enzymatic steps. Phospholipases A2 and C significantly reduced the conversion of DHA to androstenedione and andien-beta to dienone but did not affect 17-hydroxylase or 'andien-beta-synthetase'. However, when the C-17, 20 lyase step was measured alone, phospholipase C decreased the quantity of androstenedione produced indicating that the side-chain cleavage reaction may involve a lipid component. The different effects of phospholipases on these enzymes suggests that pregnenolone metabolism may be regulated by alterations in the membrane microenvironment.

Thermodynamics and modulation of progesterone microcompartmentation and hydrophobic interaction with cytochrome P450XVII based on quantification of local ligand concentrations in a complex multi-component system

An approach towards the determination of hydrophobic ligand distribution in endoplasmic reticulum membrane suspensions, and of hydrophobic ligand interaction with membrane-anchored proteins, based on calculations of local ligand pools, is presented. Rat testicular microsomes containing cytochrome P450XVII (P450XVII) were used as the model system and considered as consisting of three compartments, i.e. membrane lipid phase, aqueous phase and the ligand-binding protein, P450XVII. Combinations of spectrophotometry, ultracentrifugation and equilibrium dialysis were used to quantify progesterone concentrations in each of the three compartments, as well as partition coefficients, Kp. Since the substrate-access channel of P450XVII is likely to face the membrane-lipid phase, corrected spectral dissociation constants, Ks(corr), were calculated on the basis of free, i.e. not enzyme-bound, progesterone concentrations in the membrane compartment. Modulation of individual components and construction of more complex systems demonstrated the validity of this concept for analysis of multicompartment systems. Although ligand distribution was considerably affected by both ligand and membrane concentrations, Kp and Ks(corr) values were found to be independent of both parameters; Kp values amounted to 1920 and 3120, and Ks(corr) values amounted to 260 microM and 96 microM at 4 degrees C and 25 degrees C, respectively. Thermodynamic parameters delta H, delta S and delta G were calculated from Van't Hoff plots for progesterone partition into the membrane compartment, and for progesterone binding to P450XVII. Both of these processes were entropy dominated, and free energy changes amounted to about -18 kJ/mol for Kp and -20 kJ/mol for Ks(corr). Modification of P450XVII by gonadotropin-induced down-regulation, and by addition of a competitive inhibitor (estradiol) had no effect on progesterone partition. Consideration of Kp = 310 for estradiol allowed the determination of a corrected K1 = 3.09 mM. Modification of the membrane-lipid phase by detergents affected progesterone-P450XVII interaction solely by modulation of Kp; modification of the aqueous phase by addition of bovine serum albumin as a fourth compartment acted solely via additional steroid attraction. This model system therefore stresses the relevance of the local environment of membrane-bound enzymes or receptors for quantification of their interaction with substrates or ligands.

A microcompartmentation analysis of intermediate leakage response to substrate excess in a membrane-bound bifunctional enzyme: local control of hydroxyprogesterone channeling efficiency during cytochrome P450XVII-catalysed androgen biosynthesis

Evidence is presented for the first time that the cytochrome P450XVII-catalysed androgen formation from progesterone (P) in rat testicular microsomal membranes represents a metabolic sequence that exhibits the ability of intrinsic regulation of intermediate transfer and product formation efficiency. Exposure of this system, which catalyses a hydroxylation and oxidative cleavage reaction sequence, to increasing P concentration results in a decreased specific retention of the putative intermediate, 17 alpha-hydroxyprogesterone (HP) in the membrane compartment, and in a decreased HP conversion to androgens in favour of increasing HP transfer into the extramembrane space. This behaviour results in a decreased ratio of product vs. intermediate formation rates, which is interpreted as a partial "uncoupling" of the normal hydroxylation and cleavage reaction sequence catalysed by P450XVII. A similar pattern can likewise be observed in isolated testicular Leydig cells after exposure to increasing P concentrations under more physiological continuous-flow conditions. Further calculations indirectly indicate that the specific retention of HP in the membrane compartment can partially be attributed to its specific association with the P450XVII during catalysis. The results strongly suggest the existence of a local "channel" that becomes more leaky and therefore less effective if loaded with high influx rates. This pattern may be related to significant but incomplete competition of exogenously entering P and endogenously formed and transiently bound HP for oxygen attack at the P450XVII active site.

Effects of prolonged cyclosporine-A treatment on the Leydig cells of the rat testis

The effects of a prolonged (30-day) treatment with daily therapeutical doses of cyclosporin A (CAS) (20 mg/kg) on testicular Leydig cells were studied in adult rats. CSA administration provoked a significant decrease in both basal and human chorionic gonadotropin (hCG)-stimulated testosterone concentration in the peripheral blood without affecting the volume of the testes or the interstitial space. However, there was conspicuous atrophy of the Leydig cells, due mainly to a decrease in mitochondria and smooth endoplasmic reticulum, the organelles containing the enzymes of testosterone synthesis. Lipid droplets, in which cholesterol is stored, were notably increased. The nuclear volume and the surface area per cell of rough endoplasmic reticulum fell significantly in Leydig cells of CAS-treated animals. In light of these findings, it is concluded that CSA inhibits the growth and steroidogenic capacity of rat Leydig cells, probably by depressing their protein synthesis. Whether the mechanism underlying the action of CSA on Leydig cells is only indirect, by blockade of hypophyseal gonadotropin release, or also direct is unsettled and requires further investigation.

Specific accumulation of 17 alpha-hydroxyprogesterone in microsomal membranes during the process of cytochrome P-450(C-17)-catalysed androgen biosynthesis. A dynamic study of intermediate formation and turnover

A complete dynamic analysis of cytochrome P-450(C-17)-catalysed androgen biosynthesis from a single dose of progesterone and 17 alpha-hydroxyprogesterone in a double-label double-substrate experiment was performed in order to elucidate the controversial intermediacy of 17 alpha-hydroxyprogesterone. Label distribution within the steroid fractions as well as in the membrane and buffer compartments yields direct evidence that the endogenously formed 17 alpha-hydroxyprogesterone (which is in an 'intermediate state') accumulates to a higher degree in microsomal membranes than does the exogenously added 17 alpha-hydroxyprogesterone (which is in a 'substrate state') under certain conditions. It is also demonstrated that endogenously formed 17 alpha-hydroxyprogesterone may partly leave the membrane compartment (in terms of a 'leakage' or 'overflow' phenomenon) and is then able to equilibrate with the pool of exogenously added 17 alpha-hydroxyprogesterone. Since only the label distribution in the membrane-associated (but not always in the aqueous) 17 alpha-hydroxyprogesterone pool corresponds to the label distribution in the androgen fraction, it is concluded that only the membrane-associated 17 alpha-hydroxyprogesterone pool is directly accessible to cytochrome P-450(C-17)-catalysed conversion into androgens.

The annual testicular cycle in the turtle, Chrysemys picta: a histochemical and electron microscopic study

This work is a study of testicular function in Chrysemys picta using changes in ultrastructure and steroid histochemistry as indices of Leydig and Sertoli cell activity. The cytological features of these cells are described in reference to four periods of tubular development. Leydig and Sertoli cells show distinct changes in morphological appearance during the seasonal cycle. Leydig cells are hypertrophic with an active 3 beta-hydroxysteroid dehydrogenase (HSD) and abundant smooth endoplasmic reticulum (SER) in early spring when androgen levels are high and animals mate and atropic in mid-summer when spermatogenesis is proceeding. Leydig cell atrophy is associated with a reduction in the volume of cytoplasm and SER. Leydig cells become active again in the fall showing a return toward the spring condition, with an increase in 3 beta-HSD activity. In contrast, although Sertoli cells show variations in abundance of organelles and inclusions during the annual cycle, no obvious degenerative changes could be seen and SER is always present. 3 beta-HSD enzyme activity in Sertoli cells is weak or absent in spring but intense during summer. Taken together, these observations suggest that Sertoli and Leydig cell functions are asynchronous.

Comparison of components of the testis interstitium with testosterone secretion in hamster, rat, and guinea pig testes perfused in vitro

Components of the testis and cytoplasmic organelles in Leydig cells were quantified with morphometric techniques in hamster, rat, and guinea pig. Testosterone secretory capacity per gram of testis and per Leydig cell in response to luteinizing hormone (LH) (100 ng/ml) stimulation was determined in these three species from testes perfused in vitro. Numerous correlations were measured among structures, and between structures and testosterone secretion, to provide structural evidence of intratesticular control of Leydig cell function. Testosterone secretion per gm testis and per Leydig cell was significantly different in the three species: highest in the guinea pig, intermediate in the rat, and lowest in the hamster. The volume of seminiferous tubules per gm testis was negatively correlated, and the volumes of interstitium, Leydig cells, and lymphatic space per gm testis were positively correlated with testosterone secretion. No correlations were observed between volumes of blood vessels, elongated spindleshaped cells, or macrophages per gm testes and testosterone secretion. The average volume of a Leydig cell and the volume and surface area of smooth endoplasmic reticulum (SER) and peroxisomes per Leydig cell were positively correlated, and the volume of lysosomes and surface area of inner mitochondrial membrane per Leydig cell were negatively correlated with testosterone secretion. No correlations were observed between volume and surface area of rough endoplasmic reticulum (RER), Golgi apparatus, and lipid, and volume of ribosomes, cytoplasmic matrix, and the nucleus with testosterone secretion per Leydig cell. These results suggest that Leydig cell size is more important than number of Leydig cells in explaining the difference in testosterone-secreting capacity among the three species, and that this increase in average volume of a Leydig cell is associated specifically with increased volume and surface area of SER and peroxisomes. An important unresolved question is what is the role of peroxisomes in Leydig cell steroidogenesis.

Production of testosterone from progesterone by rat testicular microsomes without release of the intermediates 17 alpha-hydroxyprogesterone and androstenedione

It has been shown that during the in vitro conversion of progesterone to androstenedione, 17 alpha-hydroxyprogesterone is not an obligatory intermediate which equilibrates with freely diffusible steroids in the incubation medium. Recently a cytochrome P-450 was purified that catalyzed, in addition to hydroxylase/lyase activities, reduction of androstenedione to testosterone. In order to determine whether progesterone could be transformed to testosterone without both intermediates (17 alpha-hydroxyprogesterone and androstenedione) being equilibrated with steroids in the medium, several double-label double-substrate experiments were performed. When rat microsomes were incubated with an equimolar mixture of [14C]progesterone and 17 alpha-hydroxy[3H]progesterone, androstenedione was isolated with a 11-fold higher 14C/3H ratio than 17 alpha-hydroxyprogesterone, indicating that androstenedione could not be produced from free, diffusible 17 alpha-hydroxyprogesterone. Incubation of an equimolar mixture of 17 alpha-hydroxy[3H]progesterone and [14C]androstenedione with testicular microsomes resulted in the incorporation of 3-4-fold more 17 alpha-hydroxyprogesterone into testosterone than of androstenedione, although the latter is the immediate precursor of testosterone. In an experiment in which equimolar concentrations of [3H]progesterone and [14C]androstenedione were incubated with testicular microsomes, the large pool of progesterone inhibited competitively lyase activity, but still the label of progesterone was incorporated into testosterone to the same extent as that of androstenedione. These results indicate that testosterone can be produced by immature rat testicular microsomes from added progesterone on an organized unit without the intermediates equilibrating with the incubation medium.

Differential effects of combinations of phospholipase A2 and phospholipase C on the activity of rat epididymal nuclear and microsomal 4-ene steroid 5 alpha-reductase

Epididymal 4-ene steroid 5 alpha-reductase converts testosterone to 5 alpha-dihydrotestosterone. The enzyme is localized to the nuclear and microsomal fractions, and the activity can be altered by modifying the phospholipids in the membrane environment. To investigate the membrane dependence of 4-ene steroid 5 alpha-reductase, we have treated nuclear and microsomal membranes with combinations of phospholipase A2 and phospholipase C, and examined the effects on 4-ene steroid 5 alpha-reductase activity. Sequential addition of phospholipase A2 and phospholipase C to the nuclear fraction, reduced the 4-ene steroid 5 alpha-reductase activity to approx 25% of the control level. Neither the nature of the phospholipase, nor the sequence of addition altered the inhibition. When both phospholipases were added simultaneously, nuclear 4-ene steroid 5 alpha-reductase activity was inhibited in a linear fashion, and in tests for cooperativity, the effects of phospholipase A2 and phospholipase C were clearly additive. The microsomal enzyme responded differently to sequential phospholipase treatments; if phospholipase A2 was followed by phospholipase C, or phospholipase C followed by phospholipase A2, the 4-ene steroid 5 alpha-reductase activity was, respectively, 13 and 27% of the control. In contrast, sequential addition of the same phospholipase reduced the activity of 4-ene steroid 5 alpha-reductase to approx 40% of the control level. Furthermore, simultaneous addition of phospholipase A2 and phospholipase C to the microsomal fraction, resulted in non-linearity of 4-ene steroid 5 alpha-reductase activity with time, whereas when added individually, linearity of 4-ene steroid 5 alpha-reductase was maintained. Consequently, it was not possible to test for cooperative effects of phospholipases on the microsomal 4-ene steroid 5 alpha-reductase. These findings suggest that for the nuclear 4-ene steroid 5 alpha-reductase, the polar and non-polar regions of the membrane environment have similar functions, which are most likely involved in the maintenance of the structural integrity of the enzyme. For the microsomal enzyme, the polar and non-polar regions of the membrane appear to have different functions, not only for the maintenance of enzyme integrity, but also in the mechanism at the active site.

Product inhibition of steroid-17 alpha-monooxygenase by endogenous 17 alpha-hydroxyprogesterone in microsomes and isolated Leydig cells from rat testis

Use of integrated rate equations for analysis of progesterone metabolism by isolated Leydig cells and microsomes from rat testis in presence of several progesterone concentrations within several periods reveals competitive product inhibition by endogenously formed 17 alpha-hydroxyprogesterone (Kpm = 0.1 microM) of steroid-17 alpha-monooxygenase activity (Ksm = 0.8 microM). The discrepancy between this very low interaction constant of endogenous 17 alpha-hydroxyprogesterone with the steroid-17 alpha-monooxygenase, and the respective values (from the literature) for exogenous 17 alpha-hydroxyprogesterone which are about 50-fold higher, may be explained by accumulation of endogenous 17 alpha-hydroxyprogesterone at the catalytic site of the steroid-17 alpha-monooxygenase. This mechanism may be important for intratesticular regulation of androgen biosynthesis from precursor steroids.

Estimation of kinetic parameters of androgen-synthesizing enzyme activities in superfused Leydig cells from rat testes: difference between endogenous and exogenous substrates

Kinetic parameters of 3 beta-hydroxysteroid dehydrogenase/isomerase, steroid-17 alpha-monooxygenase, and steroid-17,20-lyase activities were estimated under steady-state conditions. Purified Leydig cells from rat testes were superfused with pregnenolone, progesterone, or 17 alpha-hydroxyprogesterone. The Km values for both the monooxygenase- and the lyase-catalyzed reactions were by factors of five to ten higher if analyzed with the exogenously added substrate (0.98 and 0.65 microM, respectively) than if calculated from endogenous substrate derived from a precursor (0.10 and 0.13 microM, respectively). This discrepancy may be explained by different substrate partition between the intra- and extracellular spaces and by different substrate concentration at the active site of the respective enzyme, depending on whether the actual substrate is of exogenous or endogenous source.

Steroid secretion by mouse testes perfused in vitro

We determined the concentration of steroids in blood plasma of CBF1 male mice and the steroidogenic potential of the mouse testis. Steroid secretion rates are based on measuring selected C18, C19, and C21 steroids in the venous effluent of testes perfused with a defined medium containing luteinizing hormone. Steroids were isolated by thin-layer and/or column chromatography and quantified by radioimmunoassay. These include pregnenolone, 17 alpha-hydroxypregnenolone, dehydroepiandrosterone, androstenediol, progesterone, 17 alpha-hydroxyprogesterone, androstenedione, testosterone, dihydrotestosterone, 3 alpha-androstanediol, and 3 beta-androstanediol. Testosterone is the primary steroid secreted by mouse testes perfused in vitro and is the chief androgen present in blood plasma. Pregnenolone, an obligatory intermediate in steroid synthesis, is converted to testosterone via two separate steroidogenic pathways in approximately equal proportions. This is unlike other species in which testosterone biosynthesis proceeds preferentially via either the delta 4 or the delta 5 pathway. Our results, taken together, provide the first comprehensive assessment of Leydig cell steroidogenic activity in the mouse, demonstrate putative enzymatic pathways subserving androgen biosynthesis, and establish the predominant steroids in the peripheral circulation of adult mice.

Distribution of progesterone-binding cytochrome P450 and steroid-17 alpha-hydroxylase/C-17,20-lyase within different compartments of the rat testis

Progesterone-binding capacity of cytochrome P450 and rate of progesterone consumption by cytochrome P450-dependent hydroxylases are highest in the Leydig cell fraction but are also detectable in the non-Leydig cell fraction of interstitial cells as well as in seminiferous tubules from rat testis. The Leydig cell compartment, however, contributes only to a minor extent to the total progesterone binding and hydroxylation within the whole testis.

Relation of membrane property of microsomes to androgen biosynthesis

Spin-labelled fatty acids I(12,3) and I(1,14) were incorporated into microsomal membrane of cryptorchid mouse testis and Leydig cell tumor as well as liver. The freedom of motion of spin of I(12,3) was more restricted in testis microsome than in liver. At the lower temperatures, the freedom of motion of spin in the tumor microsomes was similar to that in the testis, but at higher temperature (20-50 degrees C) was much greater. Plotting of the empirical parameter, h0/h-1, calculated by the spectra of I(1,14), against the reciprocal of the absolute temperature clearly showed two inflection points in both liver and testis microsomes, one at 19 decrees C and the other at 30 degrees C. On the other hand, tumor microsomes lacked these break points and permitted spin to move more freely. These results suggest that tumor microsomes contain the increased fluidity. The importance of membrane fluidity in relation to steroid biosynthesis was also discussed.

Testicular steroidogenesis in the rhesus monkey (Macaca mulatta

Studies were undertaken to investigate testicular steroidogenesis in the Rhesus monkey Macaca mulatta. Testicular fragments (50 mg) were incubated for 3 hr with pregnenolone-7-3H or with progesterone-7-3H. The major metabolite of pregnenolone was progesterone (70.1%), with a lesser conversion to 17-hydroxyprogesterone (1.6%), androstenedione (3.3%), and testosterone (7.2%). The delta-5 intermediates 17-hydroxypregnenolone (4.6%) and dehydroepiandrosterone (8.6%) were also identified in the pregnenolone incubates. A majority of the progesterone substrate was not metabolized by the testicular fragments (80.1%), while some conversion to 17-hydroxyprogesterone (3.4%), androstenedione (4.8%), and testosterone (11.7%) occurred in the incubates. These results suggest that testicular fragments from the Rhesus monkey may convert pregnenolone to testosterone through both the delta-4 and the delta-5 pathways.

Effects of trypsin and phospholipases A2 and C on enzyme organization in testis microsomes

Pregnenolone and progesterone concentrated in the microsomal fraction of cryptorchid mouse testis compared with mitochondria and cytosol. While the concentrating mechanisms had high capacity and low association constants the effect did not seem to be due to nonspecific solubility in the lipid components since 17-hydroxyprogesterone, dehydroepiandrosterone, androstenedione and testosterone did not show differential concentration. Also digestion with phospholipases A2 and C to the point where most of the phospholipids were specifically split, only lowered the differential binding of pregnenolone and progesterone by less than half. Trypsin had a greater effect, short digestion at 0 degrees C lowering the specific binding to 35-40% and decreasing the steroid dehydrogenases to a similar extent. The members of the mixed function oxidase system in the testis microsomes were particularly sensitive to trypsin, cytochrome P-450 and, as a consequence, 17alpha-hydroxylase and 17, 20-lyase activity being eliminated under tha same conditions while liver microsomal cytochrome P-450 was hardly affected. Bonds split by trypsin seem to play a more important role in the hydroxylase activity of testis microsomes than in the hepatic system.