2.20 Steroids and Prostaglandins

Introduction

Steroids and prostaglandins are two important groups of lipids that act as signaling molecules in the body. Steroids are lipids with a characteristic four-ring structure, while prostaglandins are derived from arachidonic acid and act as local hormones. Both play crucial roles in regulating various physiological processes.

Definition

Steroids: Steroids are lipids of high molecular weight which can be crystalline. A steroid nucleus consists of 17 carbon atoms arranged in four attached rings.

Prostaglandins: Prostaglandins exist in virtually every mammalian tissue, acting as local hormones. Prostaglandins are derived from arachidonic acid.

• Steroids = lipids with a four-ring structure that act as hormones.
• Prostaglandins = local hormones derived from arachidonic acid.

Core Concept

Steroids

• Steroids are lipids of high molecular weight.
• They can be crystalline.
• A steroid nucleus consists of 17 carbon atoms arranged in four attached rings.
• Three of the rings contain six carbon atoms, and the fourth contains five.
• The length and structure of the side chains that extend from these rings distinguish one steroid from another.
• These structures are synthesized from isoprene units.
• Cholesterol is a structural component of cell membrane.
• Cholesterol is the precursor of a large number of equally important steroids.

Prostaglandins

• Prostaglandins exist in virtually every mammalian tissue.
• They act as local hormones.
• Prostaglandins are derived from arachidonic acid.
• Their functions vary widely depending on the tissue.
• Some reduce blood pressure, whereas others raise it.
• In the immune system, various prostaglandins help to:
• Induce fever and inflammation
• Intensify the sensation of pain
• They also help to regulate the aggregation of platelets — an early step in the formation of blood clots.
• The ability of aspirin to reduce fever and decrease pain depends on the inhibition of prostaglandin synthesis.