Steroid Synthesis in the Smooth ER

Steroid Synthesis in the Smooth Endoplasmic Reticulum

  • The smooth endoplasmic reticulum (smooth ER) plays a crucial role in steroid synthesis, but only in specific cells.

  • These cells are primarily those involved in synthesizing cholesterol and steroid hormones.

  • Not all cells have this function; it is limited to those actively producing cholesterol/steroid hormones.

  • Approximately 80% of steroid synthesis in the body occurs in liver cells or intestinal cells.

  • The primary product in these cells is cholesterol.

Cholesterol Synthesis: A Multi-Step Process

  • Cholesterol synthesis involves 31 steps.

  • The rate-limiting step: HMG CoA reductase

HMG CoA Reductase

  • HMG CoA is converted to Mivalonate via the enzyme HMG CoA reductase.

  • This step is crucial because the body regulates the amount of HMG CoA reductase to control cholesterol production.

  • Other steps in the synthesis may occur constitutively (always made).

  • This process occurs in liver cells.

Importance of Cholesterol

  • Cholesterol serves as the precursor (starting material) for all steroid hormones.

  • Steroid hormones are derived from cholesterol through enzymatic conversion.

  • The specific enzymes present determine the type of steroid hormone produced.

  • Enzyme A converts cholesterol to intermediate X, which then can be converted by different enzymes into different steroid hormones.

Major Families of Steroid Hormones

  • Three main families:

    • Glucocorticoids

    • Mineralocorticoids

    • Gonadocorticoids

Glucocorticoids

  • Regulate glucose levels in the body.

  • Cortisol is the most abundant glucocorticoid in humans and is often called the "stress hormone."

  • Effects of cortisol:

    • Increases glucose levels.

    • Suppresses the immune system.

Mineralocorticoids

  • Control mineral levels in the body.

  • Aldosterone is the most abundant mineralocorticoid in humans.

  • Function of aldosterone:

    • Regulates water levels by controlling sodium movement in the kidneys.

    • Reabsorption of sodium into the bloodstream causes water to follow, retaining water in the body.

Gonadocorticoids

  • Regulate the gonads (testes and ovaries).

  • Include estrogens and testosterone.

  • Produced mainly in testes and ovaries upon activation.

  • Also produced in the adrenal gland.

  • After activation of the testes/ovaries, the amount of gonadocorticoids produced far exceeds adrenal gland production.

Steroid Production Overview

  • Steroids are modifications of cholesterol.

  • Conversion from one steroid to another depends on the specific enzymes present.

Statins and Cholesterol Control

  • Statins are drugs prescribed for high cholesterol levels.

  • Mechanism of action: regulate HMG CoA reductase to reduce cholesterol synthesis.

  • By inhibiting this rate-limiting step, statins decrease overall cholesterol production.

Back to Membranes

  • The lecture will now transition back to the discussion of membranes; the endoplasmic reticulum