JNTU B.Pharmacy III PHARMACOLOGY-I Previous Question Paper (February/March-2022 - R17)

Drug absorption is the process by which a drug enters the systemic circulation from its site of administration. It depends on physicochemical properties, formulation, and physiological factors. Mechanisms include passive diffusion, facilitated diffusion, active transport, and pinocytosis.

1. Passive Diffusion: The most common mechanism where drugs move from a region of higher concentration to lower concentration. It is non-saturable and does not require energy. Example: Lipid-soluble drugs like aspirin cross the lipid bilayer easily.

2. Active Transport: Requires carrier proteins and energy (ATP) to move drugs against a concentration gradient. It is saturable and subject to competitive inhibition. Example: Absorption of levodopa in the intestine.

3. Facilitated Diffusion: Uses carrier proteins but follows the concentration gradient without energy. Example: Glucose absorption via GLUT transporters.

4. Pinocytosis: The engulfment of large or lipid-insoluble molecules by the cell membrane. Example: Absorption of fat-soluble vitamins (A, D, E, K).

Enzyme inhibition occurs when a substance (inhibitor) binds to an enzyme and decreases its catalytic activity. This is a primary mechanism for drug action and drug-drug interactions.

1. Competitive Inhibition: The inhibitor structurally resembles the substrate and competes for the same active site on the enzyme. This can be overcome by increasing the substrate concentration. Example: Neostigmine competes with acetylcholine for the acetylcholinesterase enzyme.

2. Non-competitive Inhibition: The inhibitor binds to a site other than the active site (allosteric site), changing the enzyme's conformation and rendering it inactive regardless of substrate concentration. Example: Heavy metals like lead binding to sulfhydryl groups of enzymes.

3. Uncompetitive Inhibition: The inhibitor binds only to the enzyme-substrate complex, preventing the conversion of substrate to product. Example: Lithium inhibition of certain phosphatases.

4. Irreversible Inhibition: The inhibitor forms a strong covalent bond with the enzyme, permanently inactivating it. New enzyme synthesis is required to restore activity. Example: Aspirin irreversibly inhibits cyclooxygenase (COX) by acetylating the enzyme.