scholarly journals Competition of two substrates for a single enzyme. A simple kinetic theorem exemplified by a hydroxy steroid dehydrogenase reaction

1969 ◽  
Vol 112 (3) ◽  
pp. 331-334 ◽  
Author(s):  
T. Pocklington ◽  
J. Jeffery
Keyword(s):  
Initial Rate ◽  
Maximum Velocity ◽  
Total Rate ◽  
Dehydrogenase Reaction ◽  
Rate Of Reaction ◽  
Nad Oxidoreductase ◽  
Lower Maximum ◽  
Hydroxy Steroid ◽  
Steroid Dehydrogenase ◽  
Single Enzyme

1. If two compounds are substrates for a single enzyme, and do not form any ternary complex with the enzyme or combine directly with each other, then the total initial rate of reaction for a mixture of the two compounds may be greater than the rate for either compound alone, or may lie between the rates for the compounds alone. It is the concentration of the compound with the higher maximum velocity that determines which applies, and there is one concentration of the compound of higher maximum velocity at which the total rate of reaction is independent of the presence or absence of the substrate of lower maximum velocity. The values concerned are derived. 2. An example is given of 5α-androstan-3-one and 5α-androstane-3,16-dione as substrates competing for a hydroxy steroid–NAD oxidoreductase (EC 1.1.1.53).

INHIBITION OF PLACENTAL 3β-HYDROXY-STEROID DEHYDROGENASE BY NATURALLY OCCURRING STEROIDS. A POTENTIAL MECHANISM REGULATING OESTROGEN SYNTHESIS FROM UNCONJUGATED PRECURSORS

1975 ◽  
Vol 79 (4) ◽  
pp. 740-748 ◽  
Author(s):  
John D. Townsley
Keyword(s):  
Regulatory Mechanism ◽  
Initial Rate ◽  
Inhibitory Effects ◽  
Alternative Substrates ◽  
Naturally Occurring ◽  
Potential Inhibitors ◽  
Hydroxy Steroid ◽  
Steroid Dehydrogenase ◽  
Endogenous Steroids

ABSTRACT We have proposed that inhibition of placental steroid 3-sulphatase by endogenous steroids may regulate oestrogen synthesis during human pregnancy. The possibility that an additional regulatory mechanism, involving the placental 3β-hydroxy-steroid dehydrogenase (SDH), may also be operative has now been examined. Inhibitory effects of naturally occurring steroids on SDH activity were determined from the reduction in initial rate of conversion of 3H-dehydroepiandrosterone to non-digitonin precipitable products by 10 000 × g supernatant from homogenates of human term placentae. The apparent Km for dehydroepiandrosterone was 0.33 × 10−6 m. Δ4-3-Oxo products of SDH action (4-androstene-3,17-dione, app. Ki = 0.60 × 10−6 m; progesterone, app. Ki = 1.5 × 10−6 m) were the most potent inhibitors and appeared to act non-competitively. Δ5-3β-Hydroxy alternative substrates were less inhibitory and in the case of pregnenolone (app. Ki = 4.5 × 10−6 m) behaved competitively. 11β-, 16α-, 17α- or 21-hydroxylation and epimerization of 3β- or 17β-hydroxyl functions of inhibitors decreased their activity. It is concluded that inhibition of both sulphatase and SDH by endogenous steroids may provide complementary methods of regulating placental oestrogen synthesis in vivo. The SDH mechanism may regulate oestrogen synthesis from unconjugated precursors, either formed within the placenta or derived from the circulation. The major potential inhibitors appear to be Δ4-3-ketones, acting non-competitively, and formed within the placenta. In the sulphatase mechanism alternative substrates of extraplacenta origin, acting competitively, are the major potential inhibitors controlling utilization of conjugated precursors.

scholarly journals 3-Hydroxy steroid dehydrogenase activities of cortisone reductase

1973 ◽  
Vol 135 (4) ◽  
pp. 881-888 ◽  
Author(s):  
William Gibb ◽  
Jonathan Jeffery
Keyword(s):  
Binding Site ◽  
Substrate Molecule ◽  
Ring Junction ◽  
Nad Oxidoreductase ◽  
Hydroxy Steroid ◽  
Steroid Dehydrogenase

The behaviour of various C19 and C18 steroids as substrates for crystalline preparations of cortisone reductase (EC 1.1.1.53) is described. 3α(Axial,3R)-, 3α(equatorial,3R)- and 3β(axial,3S)-hydroxy steroid–NAD oxidoreductase activities are demonstrated. Four pairs of the substrates differed only in the shape of the a/b ring junction, three pairs differed only in substitution at C-10, and four pairs differed only in substitution in ring d. The shape of the substrate molecule and certain substituents (e.g. 10β-methyl, 17β-hydroxy, 16-oxo or 17-oxo) altered substrate behaviour, but steroids differing considerably in shape nevertheless acted as substrates, suggesting the possibility of a large or flexible binding site. Km values varied about 10-fold, many being approx. 140μm. Vmax. values covered a greater range (about 200-fold) and the good substrates had high Vmax. values rather than low Km values.

ÜBER DEN ENZYMATISCHEN ABBAU VON D,L-3-METHOXY-4-HYDROXY-MANDELSÄURE ZU VANILLINSÄURE

1969 ◽  
Vol 61 (1) ◽  
pp. 104-110 ◽  
Author(s):  
Wilhelm Dirscherl ◽  
Betty Brisse
Keyword(s):  
Enzymatic Activity ◽  
Pseudomonas Fluorescens ◽  
Rat Liver ◽  
Mandelic Acid ◽  
Glyoxylic Acid ◽  
Type Iv ◽  
Quantitative Measurements ◽  
Nad Oxidoreductase ◽  
Hydroxy Steroid ◽  
Steroid Dehydrogenase

ABSTRACT The dehydrogenation of DL-3-methoxy-4-hydroxy-mandelic acid to 3-methoxy-4-hydroxyphenyl-glyoxylic acid by fractions of the rat liver, 20β-hydroxy-steroid-dehydrogenase (1. 1. 1.53, C = 20-dihydrocortisone: NAD oxidoreductase) and cells of pseudomonas fluorescens Type IV, as well as fractions obtained of this strain, was proved by paper chromatography of the incubation extracts. Quantitative measurements and kinetic research was only possible with cells of pseudomonas fluorescens and fractions obtained of them. The enzymatic activity is bound to the particles of the pseudomonas cells and the microsomes of the rat liver. Until now it was only possible to get just a small amount into solution.

A metabolic stability determination of Tetrahydrothiazolopyridine derivative a selective 11β-hydroxy steroid dehydrogenase type 1 (11β-hsd1) inhibitor

2017 ◽  
Vol 8 (3) ◽  
Author(s):  
MURUGESH KANDASAMY ◽  
MUHAMMED SALIHIN ◽  
MALLIKARJUNA RAO PICHIKA ◽  
SLAVKO KOMARNYTSKY ◽  
THIRUMURUGAN RATHINASABAPATHY
Keyword(s):  
Metabolic Stability ◽  
Hydroxy Steroid ◽  
Steroid Dehydrogenase

The mechanism of the oxidation of thiocyanate ion by peroxomonosulphate in aqueous solution. II. Kinetics of the reaction

1966 ◽  
Vol 19 (8) ◽  
pp. 1365 ◽  
Author(s):  
RH Smith ◽  
IR Wilson
Keyword(s):  
Aqueous Solution ◽  
Initial Rate ◽  
Stopped Flow ◽  
Thiocyanate Ion ◽  
First Order ◽  
Conductance Method ◽  
Rate Of Reaction ◽  
Kinetics Of

Initial rates of reaction for the above oxidation have been measured by a stopped-flow conductance method. Between pH 2 and 3.6, the initial rate of reaction, R, is given by the expression R{[HSO5-]+[SCN-]} = {kb+kc[H+]}[HSO5-]0[SCN-]20+ka[H+]-1[HSO5]20[SCN-]0 As pH increases, there is a transition to a pH-independent rate, first order in each thiocyanate and peroxomonosulphate concentrations.

DIFFICULTIES IN THE DIAGNOSIS OF THE ADRENOGENITAL SYNDROME IN INFANCY

PEDIATRICS ◽  
1972 ◽  
Vol 49 (2) ◽  
pp. 198-205
Author(s):  
C. H. Shackleton ◽  
F. L. Mitchell ◽  
J. W. Farquhar
Keyword(s):  
Hydroxylase Deficiency ◽  
Adrenogenital Syndrome ◽  
Steroid Excretion ◽  
21 Hydroxylase Deficiency ◽  
Hydroxy Steroid ◽  
Steroid Dehydrogenase

Pregnanetriol was not excreted by an infant (7 days old) who was later shown to have a defect in steroid 21-hydroxylase. However, the excretion of this compound increased during the following days (1.2 mg on the thirteenth day of life). A high excretion of 3β-hydroxy-Δ steroids was the most noticeable abnormality in steroid excretion noted on the seventh day of life (e.g., 3β, 16α-dihydroxy-5-pregnen-20-one, 15 mg; 3β, 21-dihydroxy-5-pregnen-20-one, 1.4 mg and 3β, 16α-dihydroxy-5-androsten-17-one, 7.4 mg). This high 3β-hydroxy-Δ steroid excretion results in difficulties in distinguishing a defect in 3β-hydroxy steroid dehydrogenase from a 21-hydroxylase deficiency. At the age of 14 months the principal steroids excreted were those predominant in other cases of 21-hydroxylase deficiency, viz. pregnanetriol and 5β-pregnane-3α, 17α, 20α-triol-11-one (11-oxo-pregnanetriol).

Bestimmung des Molekulargewichtes der 20-β-Hydroxy-steroid-dehydrogenase

1962 ◽  
Vol 17 (7) ◽  
pp. 432-436 ◽  
Author(s):  
Matatiahu Gehatia
Keyword(s):  
Molecular Weight ◽  
Diffusion Coefficient ◽  
Diffusion Coefficients ◽  
Specific Volume ◽  
Sedimentation Coefficient ◽  
Partial Specific Volume ◽  
Hydroxy Steroid ◽  
Steroid Dehydrogenase

The enzyme 20-β-Hydroxy-steroid-dehydrogenase obtained from the culture of Streptomyces hydrogenans and dissolved in 0.05 M Tris puffer, pH 7.3, has been investigated by means of a ultracentrifuge at 20 °C. The sedimentation- as well as the diffusion-coefficients obtained from various solutions at different concentrations were extrapolated to the concentration c = 0. The resulting zero-value for the sedimentation coefficient is s0 = 6.64 s and for the diffusion coefficient is D0 = 5.51 × 10-7 cm2/sec. Supposing the partial specific volume of the enzyme under consideration analogously to other similar proteins is V+=0.749 ml/g, the molecular weight has been estimated as M = 118 400.

The Reactions of Organometallic Compounds Involving Silicon. Reactions of Triphenyl-, Diphenylmethyl-, and Phenyldimethylsilyl-lithium with 9-Phenylfluorene in Tetrahydrofuran

1972 ◽  
Vol 50 (23) ◽  
pp. 3761-3766 ◽  
Author(s):  
M. A. Hamid
Keyword(s):  
Low Temperatures ◽  
Initial Rate ◽  
Organometallic Compounds ◽  
Initial Reactant ◽  
Thermodynamic Constants ◽  
Rate Of Reaction ◽  
Stop Flow

Triphenyl-, diphenylmethyl-, and phenyldimethylsilyl-lithium react very rapidly with 9-phenylfluorene in tetrahydrofuran (THF) but the reaction can be followed spectroscopically using the stop-flow technique at low temperatures. The order of reaction, found from the dependence of the initial rate of reaction on the initial reactant concentrations, is unity in both 9-phenylfluorene and the relevant organosilyl-lithium reagent. The thermodynamic constants of activation for the reaction between 9-phenylfluorene and triphenylsilyl-lithium (Ph3SiLi), diphenylmethylsilyl-lithium (Ph2MeSiLi), and phenyldimethylsilyl-lithium (PhMe2SiLi), respectively, are: ΔH≠ = 4.5, 4.0, and 4.0 kcal mol−1; ΔG≠ = 13.0, 12.5, and 12.5 kcal mol−1; ΔS≠ = −34.9, −34.8, and −34.9 cal mol−1 deg−1

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