Steroid Biochemistry

Volume II

R. Hobkirk

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351093811
• 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

ALTERNATE PATHWAYS OF STEROID BIOSYNTHESIS AND THE ORIGIN, METABOLISM, AND BIOLOGICAL EFFECTS OF RING B UNSATURATED ESTROGENS

B. R. Bhavnani and C. A. Woolever

TABLE OF CONTENTS

I. Introduction

II. Classical Pathway of Steroidogenesis

A. Source of Carbon Atoms of Cholesterol

B. Biosynthesis of Mevalonic Acid

C. Formation of Isopentenyl Pyrophosphate from Mevalonic Acid

D. Formation of Squalene from lsopentenyl Pyrophosphate

E. Cyclization of Squalene to Lanosterol

F. Conversion of Lanosterol to Cholesterol

G. Conversion of Cholesterol to Pregnenolone

III. Alternate Pathways of Steroidogenesis

A. Direct Conversion of Desmosterol to Steroid Hormones

B. Sesterterpene Pathway of Steroidogenesis

C. Biosynthesis of Ring B Unsaturated Estrogens

1. Metabolism of 7-³H-Dehydroepiandrosterone and 4-¹⁴C-Androstenedione Injected into the Umbilical Vein

2. Metabolism of 1-¹⁴C-Sodium Acetate and 7-³H-Cholesterol Injected into the Fetal Circulation

3. Metabolism of ¹⁴C-Squalene and ¹⁴C-Mevalonic Acid Injected into the Umbilical Circulation

4. Metabolism of 1-¹⁴C-Isopentenyl Pyrophosphate and (4,8,12)-¹⁴C-Farnesyl Pyrophosphate Injected into the Fetus

5. Theoretical Discussion on Alternate Pathways of Steroidogenesis

IV. Biologic Activity of Ring B Unsaturated Estrogens

V. Bioassays

A. General

B. Animal Bioassay Studies

1. Allen-Doisy Test

2. Uterotrophic Assay

3. Chick Oviduct Assay

C. Human Studies

1. Specific Responses

2. Other Responses in the Human

a. Hypothalamus and Pituitary

b. Clotting Mechanism

c. Metabolic Changes

d. Osteoporosis

e. Atherosclerosis

f. Neoplasms of the Reproductive Organs

D. The Metabolism of Equilin in Man

1. Intravenous Equilin Sulfate

2. Intravenous ³H-Equilin

VI. Concluding Remarks

Acknowledgments

References

I. INTRODUCTION

For the past 25 years, it has been generally accepted that cholesterol is the obligatory precursor in the biosynthesis of all steroid hormones. Details of the various steps for the synthesis of cholesterol from acetate and for its conversion to steroids were established mainly by in vitro incubation procedures using tissues such as the ovary, testis, and adrenal cortex. Although it became firmly established that acetate was the initial two carbon source of cholesterol, some experimentation suggested that cholesterol may not be the only precursor from which all steroid hormones are formed.

This chapter will discuss the evidence for the steroidogenic pathway which bypasses cholesterol. It will be referred to as the alternate pathway of steroidogenesis, while that which utilizes cholesterol as an obligatory intermediate will be termed the classical pathway of steroidogenesis. Although it is not the purpose of this chapter to discuss the classical pathway of biosynthesis and metabolism of steroid hormones, its salient features will be reviewed briefly to facilitate comparison with the alternate pathway, which will be discussed in detail. The metabolism and biological action of the ring B unsaturated estrogens will also be presented.

II. CLASSICAL PATHWAY OF STEROIDOGENESIS

The various steps in the classical pathway of steroidogenesis from acetate are acetate → mevalonic acid → isopentenyl pyrophosphate → 3,3-dimethyl allyl pyrophosphate → geranyl pyrophosphate → farnesyl pyrophosphate → squalene → lanosterol → cholesterol → pregnenolone → steroid hormones. The details are discussed below.

A. Source of Carbon Atoms of Cholesterol

Studies by Bloch, Lynen, Cornforth, Popják, and their colleagues on cholesterol biosynthesis from either methyl or carboxyl ¹⁴C-labeled acetate established that all 27 carbon atoms of cholesterol are derived from acetate. These investigations have been reviewed by Bloch.¹ The distribution of the 15 methyl and 12 carboxyl carbons in cholesterol is shown in Figure 1. This arrangement of the carbon atoms of acetate in cholesterol follows the “isoprene rule,” which was defined by Ruzicka as a working hypothesis as early as 1921.²

B. Biosynthesis of Mevalonic Acid

Three molecules of acetic acid in the form of acetyl-CoA (coenzyme A) thioester condense to form mevalonic acid (Figure 2). All of the intermediates involved are bound to coenzyme A, and the required enzymes are present in both the cytoplasmic particles and the soluble fraction of mammalian and avian liver^{3, 4, 5, 6} and in yeast.^{7, 8} The first step, catalyzed by a soluble enzyme β-keto-thiolase, results in the formation of acetoacetyl-CoA. In the presence of 3-hydroxy-3-methyl-glutaryl-CoA synthase (which is located in the mitochondria and the soluble fraction of liver^{9, 10} but not in the microsomal fraction as had been reported previously^{11, 12}), the acetoacetyl-CoA condenses with a third molecule of acetyl-CoA to give 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA), the immediate precursor of mevalonic acid. The reduction of this precursor to mevalonic acid is catalyzed by mevalonate: NADP⁺ oxidoreductase (EC 1.1.1.34), which is present in both yeast mitochondria¹³ and in the mammalian liver microsomes.^{14, 15} This microsomal system is thought to be the rate-limiting step in the biosynthesis of cholesterol in mammalian liver.^{12, 16, 17} Although this scheme is generally believed to be the major pathway of mevalonate biosynthesis, an alternate minor pathway involving malonyl-CoA has been described by Brodie et al.^{18, 19} The mechanisms involved in these transformations have been recently reviewed by Beytia and Porter.²⁰

FIGURE 1. Distribution of carboxyl(C) and meth...