Nitric Oxide Synthase

Nitric oxide is synthesized by the oxidation of L-arginine

[A] Lysine
[B] L-ornithine
[C] L-arginine
[D] Cysteine

Which of the following is FALSE

[A] small amount of NO produced by nNOS and eNOS are noted in physiological state
[B] NO produced by eNOS plays a protective role in vascular endothelium and vascular function
[C] iNOS will produce a large amount of NO in normal state
[D] Neutrophils express iNOS

Nitric oxide is synthesized by the oxidation of L-arginine by nitric oxide synthase (NOS) expressed in cardiac myocytes.

Nitric Oxide Synthase (NOS)

🔹 Overview

• Nitric Oxide Synthase (NOS) is an enzyme responsible for producing Nitric Oxide (NO) from L-arginine.
• NO is a vital signaling molecule in the cardiovascular, nervous, and immune systems.
• The reaction also produces citrulline as a by-product.

🔎 Isoforms of NOS

There are three main isoforms of NOS, each encoded by different genes:

1. nNOS (NOS1)
   • Location: Neuronal tissue, skeletal muscle, gastrointestinal tract.
   • Function: Neurotransmission, synaptic plasticity, smooth muscle relaxation.
2. iNOS (NOS2)
   • Location: Macrophages, neutrophils, hepatocytes, vascular smooth muscle.
   • Function: Produces large amounts of NO during inflammation and immune defense.
   • Regulation: Induced by cytokines (e.g., TNF-α, IL-1, IFN-γ).
3. eNOS (NOS3)
   • Location: Endothelial cells.
   • Function: Maintains vascular tone, prevents platelet aggregation, protects against atherosclerosis.

⚙️ Reaction Catalyzed

L-Arginine + NADPH + O₂ → Nitric Oxide (NO) + L-Citrulline + NADP⁺

• Requires cofactors:
  • FAD, FMN, NADPH, Heme, Tetrahydrobiopterin (BH₄), Ca²⁺/Calmodulin

📌 Regulation of NOS

• nNOS & eNOS are calcium/calmodulin-dependent → activated when intracellular Ca²⁺ rises.
• iNOS is calcium-independent once induced (produces sustained high-output NO).
• Inhibitors: Asymmetric dimethylarginine (ADMA), L-NAME (experimental drug).

🩺 Clinical Significance

• eNOS dysfunction → contributes to hypertension, atherosclerosis, diabetes, heart failure.
• iNOS overactivation → septic shock, tissue damage from excessive NO.
• nNOS abnormalities → neurodegenerative diseases (Parkinson’s, Alzheimer’s).
• Drugs targeting NO pathway:
  • Nitroglycerin, Sodium nitroprusside → donate NO → vasodilation.
  • Phosphodiesterase-5 inhibitors (e.g., sildenafil) → prolong NO effect by preventing cGMP breakdown.

NOS can be divided into three subtypes

• – Neuronal nitric oxide synthase (nNOS)
• – Inducible nitric oxide synthase (iNOS)
• – Endothelial nitric oxide synthase (eNOS)

• iNOS -Inducible nitric oxide synthase

iNOS are not responsible for producing NO in normal physiological state.

iNOS will produce a large amount of NO when an inflammatory response occurs

In humans, neutrophils also express iNOS.

Nitric Oxide Synthase (NOS),,NOS isoforms (nNOS, iNOS, eNOS),,Nitric oxide pathway,,Endothelial nitric oxide synthase,,Neuronal nitric oxide synthase,,Inducible nitric oxide synthase,, ,Nitric oxide and vascular function,,Role of NO in cardiovascular system,,NOS cofactors (BH4, FAD, FMN, NADPH, heme),,Calcium/calmodulin-dependent NOS,,Cytokine-induced iNOS,,ADMA nitric oxide synthase inhibitor,,NO and cGMP signaling,,NOS dysfunction in hypertension,,Septic shock nitric oxide,,NO donors (nitroglycerin, sodium nitroprusside),,PDE5 inhibitors and nitric oxide (sildenafil),, ,Mechanism of nitric oxide synthase,,Difference between eNOS, iNOS, nNOS,,Clinical significance of nitric oxide synthase,,Endothelial dysfunction and nitric oxide,,NOS in neurological diseases,,Nitric oxide in immune defense,,Nitric oxide in learning and memory,,Nitric oxide synthase gene locations,,NOS inhibitors and activators,,Nitric oxide in cardiovascular, nervous, and immune systems