Cholesterol Systems in Insects and Animals

Jacqueline Dupont

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351087551
• Edition: 1
• Topic: Medicine
• Subtopic: Veterinary Medicine

The reviews in this collection are unique in their intent to provide a basis for understanding of the subject. They include historical, descriptive, and comparative information which is not always presented in state of the science reviews. Cholesterol is viewed in each chapter as part of a system structural, kinetic, or metabolic. The complex nature of the place of cholesterol in living systems is illustrated in each chapter.

Chapter 1

STEROLS AND INSECTS

Henry W. Kircher

TABLE OF CONTENTS

I.	Introduction
II.	Absence of Sterol Synthesis by Insects
III.	Utilization of Sterols by Insects
	A. Common Sterols
	1. Cholesterol
	2. Cholestanol, the “Sparing Sterol” Concept, and Ovarian Transfer of Sterols to Progeny
	3. 7-Dehydrocholesterol
	4. Ergosterol
	5. Sitosterol, Stigmasterol, and Campesterol
	6. Cholesteryl Esters
	7. Desmosterol
	8. Zymosterol
	B. Less Common Sterols
	C. Sterol Precursors, Nor-sterols and Metabolites
	D. Summary
IV.	Status and Function of Sterols in Insects
	A. Dietary Uptake and Tissue Distribution of Sterols
	1. Uptake
	2. Tissue Sterols
	B. Sterols in Reproduction
	1. Ovarian Development
	2. Oogenesis and Egg Laying
	3. Embryogenesis and Hatchability
	4. Maturation of Progeny
	5. Male Fertility
	C. Sterols for Defensive Purposes
	D. Summary
V.	Metabolism
	A. Hydrolysis, Esterification and Conjugation
	1. Hydrolysis
	2. Esterification
	3. Conjugation
	B. Introduction of Double Bonds
	1. Desaturation of Cholesterol to 7-Dehydrocholesterol
	2. Desaturation of Cholestanol
	3. Desaturation of Other Sterols
	C. Saturation of Double Bonds
	1. Δ^5,7 to Δ^5-Sterols
	2. Desmosterol (Δ^5,24) to Cholesterol (Δ^5)
	3. 22-Dehydrodesmosterol to Cholesterol
	4. Cholestanone to Cholestanol
	5. The Mexican Bean Beetle, a Special Case
	D. Dealkylation at C-24 in the Phytosterol Side Chain
	1. Removal of the C24-Ethyl Group
	2. Removal of the C24-Methyl Group
	E. Truncations and Removal of the Side Chain
	F. Summary
VI.	Ecdysone
	A. Structure
	B. Biosynthesis
	C. Sites of Ecdysone Biogynthesis
	D. Mechanism of Action
	E. Biological Activity
	F. Ecdysteroid Involvement in Reproduction
	G. Catabolism and Excretion
	H. Phytoecdysteroids
	I. Summary
Appendix 1
Appendix 2
Glossary of Sterol Names
Glossary of Other Names
References

I. INTRODUCTION

Most animals are insects. Because of their small size, they are usually not as apparent as other organisms, but in numbers, insects are estimated to comprise 70 to 80% of all animal species and perhaps exist in more species than all other animal and plant species combined. Their habitats are extremely diverse, ranging from aquatic species in pools at the edge of glaciers to carnivores buried in the flesh of mammals.

Insects arose from some terrestrial arthropod about 250 to 300 million years ago. Their general form and life processes have remained largely unaltered for the last 150 million years. Fossil records of cockroaches show that little morphological change occurred in some species over this period. Many climatological and geological changes took place during this time; the present ubiquity of cockroaches is a good reflection of their past adaptive potential.

Insects are one of the few multicellular organisms that still use Homo sapiens as prey. Insects can see, hear, feel, smell, taste, eat, digest, excrete, reproduce bisexually, fly, and walk upside down. Some are viviparous and parthenogenic. They go through several discrete life stages — egg, embryo, larva, and pupa — during which the nymph, maggot, grub, or caterpillar tissues differentiate into wings, legs, gonads, eyes, antennae, and sex organs. Finally, the reproductive adult form emerges to start a new life cycle.

The principal aspect of the life of insects that is of interest here, however, is their inability to synthesize cholesterol. The tissues of all other animals higher than nematodes in the evolutionary scale and of all plants beyond certain bacteria can assemble isoprene units to squalene and cyclize the latter to lanosterol or cycloartenol. These two tetracyclic triterpenes are then further metabolized by living systems to hundreds of naturally occurring sterol molecules: phytosterols, sapogenins, cardenolides, steroid alkaloids, sex hormones, corticoids, and bile acids. Insects, although able to metabolize ingested sterols, cannot synthesize them de novo.

The discovery that insects require a dietary sterol for growth and maturation took place in the middle 1930s. Hobson¹ and Van’t Hoog,² working with a blow fly and a fruit fly, found that the addition of cholesterol to organic solvent extracted media ingredients restored the nutritional value of the diets and allowed the insects to go through their full life cycle. In the absence of the sterol, the flies died during the early larval stages.

During the next two decades, numerous other insects were tested with essentially the same results.³ A sterol free diet inhibited growth and usually caused larval death. All common sterols with an intact skeleton and a 3β-hydroxy group were utilized to varying degrees. Some sterol esters were used, others were not. Hydroxylation of the B ring at C-6 or C-7 or truncation of the side chain to cholane, pregnane, or androstane derivatives prevented their use. By 1956, the following observations were generally accepted:

1. Insects required an exogenous source of sterol. It is principally supplied by the diet, but may also come from intestinal microorganisms.

2. Cholesterol can be universally used.

3. Plant eating insects can also use phytosterols, e.g., ergosterol, sitosterol, stigmasterol; carnivorous species, such as the hide beetle, appeared to require cholesterol.

4. The ability to use various sterols and the dietary concentrations required for optimal growth varies between species.

Results reported by the early workers on sterol utilization should be viewed with care. The criteria used in most of the studies were limited to rates of larval development, the size of the insect, and its ability to pupate. Reproductive capacity was rarely measured. The purity of phytosterols was also difficult to assess, and often the sources of the sterols and their physical constants were...

Table of contents

• Cover
• Title Page
• Copyright Page
• Table of Contents
• Chapter 1 Sterols and Insects
• Chapter 2 Cholesterol and Membranes
• Chapter 3 Cholesterol Transport
• Chapter 4 Cholesterol Catabolism and Bile Acid Metabolism
• Chapter 5 Cholesterol Balance and Whole Body Kinetics
• Index