Steroid Biochemistry

Volume I

R. Hobkirk

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351093804
• Edition: 1
• Topic: Medicine
• Subtopic: Alternative & Complementary Medicine

An attempt has been made to attract contributions which illustrate the importance of certain enzymatic processes involved in steroid biosynthesis and metabolism and, in some cases, leading to steroidal action in target sites. Investigators actively engaged in research in such areas were invited to present their material in a manner which they considered fitting. It is hoped that as a result if this, the publication will possess sufficient depth to warrant approval. The blend of review material and experimental data originating in the author laboratories will, it is felt, make for useful reading.

Chapter 1

STEROID Δ⁴-REDUCTASES: THEIR PHYSIOLOGICAL ROLE AND SIGNIFICANCE

Albert F. Clark

TABLE OF CONTENTS

I.	Introduction
II.	Corticoids
III.	Aldosterone
IV.	Progesterone
V.	C19-Steroid Δ^4-Reductases
VI.	Summary
References

I. INTRODUCTION

In this review of the steroid Δ⁴-reductases, the enzymes which catalyze the reduction of the double bond between carbons 4 and 5 of the steroid molecule, only mammalian steroids and mammalian enzymes will be discussed. The literature in this area of steroid metabolism is very voluminous and it will not be possible to quote all references on the topic. The general aim of the review is to indicate the physiological importance of Δ⁴-reduction in the overall disposition of steroid hormones and, in appropriate situations, how it affects biological actions of the parent compounds. A few general comments on the biochemistry of the enzymes will be made in this introductory section.

With the exception of the estrogens, which have an aromatic ring A, the other biologically important, naturally occurring mammalian steroid hormones have in general the Δ⁴-3-ketone structure in ring A. This group includes the important glucocorticoids cortisol and corticosterone, the mineralocorticoid aldosterone, the androgen testosterone, and the progestin progesterone. Key biosynthetic precursors of these hormones (such as l l-deoxycortisol, deoxycorticosterone, 18-hydroxycorticosterone, and Δ⁴-androstenedione) also have the Δ⁴-3-ketone configuration in ring A. Δ⁴-3-Keto-intermediates are involved in the conversion of cholesterol to bile acids.

The reduction of the double bond between carbons 4 and 5 of steroid molecules can either be the first step in a sequence of reactions leading to water-soluble conjugates which are excreted or result in the formation of compounds of key biological importance. The reduction of the double bond between steroid carbons 4 and 5 involves the addition of two hydrogen atoms. Because the reduction leads to the formation of an assyrnetric carbon atom at carbon 5, either of two possible isomers can result. By standard steroid nomenclature these are the 5α- (hydrogen below plane of steroid molecule) and the 5β- (hydrogen above plane of steroid molecule) reduction products (see Figure 1). Both isomers are formed in mammals.

FIGURE 1. Steroid ring A reduction pathways yielding 5α- and 5β-metabolites.

As mentioned above, the enzymes catalyzing these reductions are called Δ⁴-reductases and can be either Δ⁴-5α-reductases or Δ⁴-5β-reductases, depending on the stereochemistry of the product at carbon 5. The Δ⁴-reductases catalyze an irreversible reduction as they are not capable of removing hydrogens from positions 4 and 5 of saturated steroids.

There have been two common approaches to the measurement of Δ⁴-reductase activity. Steroids with a conjugated double bond system in ring A (Δ⁴-3-ketone) have a characteristic absorption peak in the ultraviolet range in organic solvents at or close to 240 nm. This absorption peak disappears when the double bond between carbons 4 and 5 is reduced and, hence, this has provided the basis for many Δ⁴-reductase assays, particularly in the liver. The assay does not indicate in any way whether the product formed is a 5α- or a 5β-reduced steroid. However, in tissues other than liver, the rate of reduction is often not great enough to follow with changes in ultraviolet absorption spectra and ³H- or ¹⁴C-labeled steroids are utilized as substrates; the products are then isolated by standard chromatographic techniques and the rate of formation of the labeled products is used as the basis of assays for measuring Δ⁴-reductase activity.

The required cofactor for all these Δ⁴-reductases is NADPH. Bjorkhem¹ examined the mechanism and stereochemistry of rat liver microsomal Δ⁴-5α-reductase, utilizing 7α-hydroxycholest-4-en-3-one as substrate, and found the hydrogen to be preferentially transferred from the 4B position of NADPH. The evidence obtained suggested a cis or nonspecific addition of hydrogen rather than a trans addition. Bjorkhem and Danielsson² then investigated rat liver soluble Δ⁴-5β-reductase and found, using C_27^-, C_24^-, C_21^-, and C_19^--steroid substrates, that the 4A hydrogen of NADPH was transferred during the reduction reaction. Abul-Hajj³ confirmed the specificity of the 4B and 4A hydrogen transfer for the rat liver Δ⁴-5α-reductase and Δ⁴-5β-reductases, respectively, using testosterone as substrate.

When the details of Δ⁴-reductases for each of the hormone groups are examined, one notes that in general the liver contains both Δ⁴-5β-reductase and Δ⁴-5α-reductase activities while other tissues have only Δ⁴-5α-reductase activity. There are a few exceptions but in general there is no known biological significance to the existence of Δ⁴-5β-reductases is tissues other than the liver. For most steroids the liver is the principal site for their metabolism. There is considerable evidence that Δ⁴-reduction is the rate-limiting step in steroid metabolism and this is supported by the finding of low levels of 5α- and 5β-reduced 3-ketosteroids in the circulation. In other words, once the Δ⁴-reductases have performed their actions, other enzymes, in particular the 3-hydroxysteroid dehydrogenases, reduce the ring A saturated steroids to 3α- and 3β-hydroxysteroids, which in turn are then substrates for β-glucuronyl transferases and sulfokinases.

One other generalization for which examples will be noted later is that in the liver, Δ⁴-5β-reductases are soluble enzymes, i.e., they are found in the 105,000 × g cell homogenate supernatant, while the Δ⁴-5α-reductases are located in the microsomes or endoplasmic reticulum. In other tissues the Δ⁴-5α-reductases are also located in the microsomes and in the nuclei in some instances as well.

Details on the steroid Δ⁴-reductases will be discussed for the glucocorticoids (cortisol and corticosterone), aldosterone, progesterone, and C₁₉-steroids. Those involved in bile acid metabolism will not be discussed. There have been studies on the formation of Δ⁴-reduced steroid metabolites in many species. Many in vivo studies have been conducted in humans, but these can be in some ways considered to be indirect in that the urine metabolites are used as indicators of the extent and type of Δ⁴-reduction. These studies are complemented in many instances by in vitro experiments with tissues, tissue fractions, and purified enzymes from animals. Such a situation is of course by necessity since the appropriate human tissues are often not readily available, especially in experimental situations where changes in enzyme levels are being followed. However, one must also remember the possibility of species differences when results for humans and animals are being compared. Because of the many studies in the literature. on the use of urine steroid metabolite levels to indicate or follow changes in steroid secretion by the adrenal cortex or gonads, examples of such will be given in greater detail for the corticoids since Δ⁴-reduction is not believed to be greatly involved in their actions. Similar principles apply to the androgens and progestins with regard to steroid metabolites and hence fewer details will be given for them. However, the involvement of Δ⁴-5α-reduction in the action of progestins and androgens will be detailed more for the latter two groups of steroids because its importance has been better documented; this applies in particular to the androgens.

II. CORTICOIDS

This section will deal with some of the products secreted by the adrenal cortex, in particular, glucocorticoids and mineralocorticoids. Not all related steroids will be mentioned as in some instances they are precursors of the biologically important compounds and they are all metabolized similarly in ring A. Cortisol, being the principal glucocorticoid in humans, has been the subject of a number of studies with regard to its metabolism. Until very recently no physiological significance has been attached to the Δ⁴-reduction of cortisol other than its role in the pathway leading to metabolites completely reduced in ring A which can be conjugated with glucosiduronic acid and excreted in the urine. Hence, the Δ⁴-reducion of cortisol is not involved directly in the expression of corticoid activity. The effects will only be indirect in that altered rates of liver Δ⁴-reduction can lead to changes in the amount of hormone available for the expression of activity. Recently, a case of hypertension with evidence of increased mineralocorticoid production has been reported in which the abnormality found in steroid metabolism was an increased excretion of unconjugated dihydrocortisol. This has stimulated some studies on the possible biological significance of such metabolites. There is also some indirect evidence for d...

Table of contents

• Cover
• Title Page
• Copyright Page
• Table of Contents
• Chapter 1 Steroid Δ4 Reductases: Their Physiological Role and Significance
• Chapter 2 Steroid Carboxylic Acids
• Chapter 3 Biochemistry of Steroid 17-Hydroxysteroid Dehydrogenases
• Chapter 4 The Role of Steroid Sulfatase and Sulfotransferase Enzymes in the Metabolism of C21 and C19 Steroids
• Index