Integration of Metabolism and Homeostasis

I. What is Metabolic Homeostasis?

• Carbohydrate, fat, and protein intake
• Their rates of oxidation
• Their rates of storage when present in excess
• Their rates of mobilization from storage when demand increases

II. Three Mechanisms of Intertissue Integration

1. Blood levels of nutrients/metabolites - the concentration of a compound in the blood affects the rate at which it is used or stored. For example, fatty acid concentration in blood is the major determinant of whether skeletal muscle uses fatty acids or glucose as fuel.
2. Hormones - intravascular messengers that carry information about the physiologic state and nutrient supply between their sites of synthesis and their target tissues.
3. Neural signals - the CNS controls tissue metabolism directly or through releasing hormones (e.g., activation of the sympathetic nervous system during stress triggers epinephrine release from the adrenal medulla).

III. The Special Role of Glucose

• Brain: ~150 g/day (the most glucose-dependent organ)
• Red blood cells, kidney medulla, exercising skeletal muscle: ~40 g/day combined

• Hyperglycemia: osmotic effects cause polyuria, polydipsia, protein glycation (forming HbA1c), nonenzymatic glycation of membranes and serum proteins, and increased risk of atherosclerosis
• Hypoglycemia: neurological deficits, coma, death in severe cases
• Unchecked high blood glucose and amino acids would exert a hyperosmolar effect causing progressive neurological deficits and eventual coma

IV. The Major Hormones of Metabolic Homeostasis

Insulin (Anabolic Hormone)

Glucagon (Catabolic/Fuel-Mobilizing Hormone)

• Stimulates glycogenolysis in liver → releases glucose into blood
• Stimulates gluconeogenesis from lactate, glycerol, and amino acids
• In conjunction with low insulin, mobilizes fatty acids from adipose triacylglycerols

Counterregulatory Hormones (Stress Hormones)

V. The Insulin:Glucagon Ratio

VI. Summary of Key Metabolic Principles

1. Blood nutrient levels act as direct signals - they auto-regulate their own metabolism (e.g., high fatty acids suppress glucose oxidation in muscle)
2. Hormones integrate whole-body responses - insulin and glucagon are released continuously in response to daily eating patterns; stress hormones are released acutely by neural signals
3. The CNS is both a major glucose consumer and a master regulator - it monitors blood glucose via the hypothalamus and coordinates hormonal/neural responses to maintain euglycemia
4. Consequences of failure - diabetes mellitus illustrates both sides: hyperglycemia from insulin deficiency/resistance leads to osmotic, glycation, and vascular pathology; iatrogenic or insulinoma-induced hypoglycemia produces neuroglycopenic symptoms

Electron Transport Chain (ETC)

Overview

Mitochondrial Location and Structure

• Impermeable to most small ions (H⁺, Na⁺, K⁺) and metabolites (ATP, ADP, pyruvate)
• Studded with cristae (infoldings) that greatly increase surface area
• Over half its protein content is directly involved in OXPHOS

The Four Complexes of the ETC

Complex I - NADH:CoQ Oxidoreductase (NADH Dehydrogenase)

• Accepts electrons from NADH → transfers them to FMN (flavin mononucleotide, derived from riboflavin)
• Electrons pass through Fe-S centers → reduce CoQ to CoQH₂
• Pumps 4 H⁺ from matrix to intermembrane space
• Inhibited by rotenone (pesticide), amytal (barbiturate), metformin (at high doses)

Complex II - Succinate Dehydrogenase

• Accepts electrons from succinate → fumarate oxidation, via FADH₂
• Transfers electrons through Fe-S proteins → to CoQ
• No proton pumping (no energy drop sufficient to pump H⁺)
• Inhibited by malonate (competitive)
• Also accepts electrons from glycerol-3-phosphate dehydrogenase and acyl CoA dehydrogenase (from β-oxidation)

Coenzyme Q (CoQ / Ubiquinone)

• A lipid-soluble quinone - freely diffuses within the IMM
• The mobile hub linking all flavoprotein dehydrogenases to the cytochromes
• Can accept 1 electron (semiquinone radical) or 2 electrons (CoQH₂)
• Transfers electrons from Complex I and II to Complex III

Complex III - Cytochrome bc₁ Complex

• Transfers electrons from CoQH₂ → cytochrome c via the Q cycle
• Contains cytochromes b, c₁ and Fe-S centers
• Pumps 4 H⁺ into intermembrane space
• Inhibited by antimycin A

Cytochrome c

• A small, water-soluble heme protein in the intermembrane space
• The mobile carrier shuttling single electrons from Complex III to Complex IV
• Fe³⁺ ↔ Fe²⁺ cycle with each electron transfer

Complex IV - Cytochrome c Oxidase

• The terminal oxidase - the only complex that reacts directly with O₂
• Contains cytochromes a and a₃, plus copper atoms (CuA and CuB)
• Collects 4 electrons from cytochrome c → reduces O₂ to 2 H₂O
• Pumps 2 H⁺ into intermembrane space
• Has a lower Km for O₂ than either myoglobin or hemoglobin - ensures O₂ is "pulled" all the way to Complex IV
• Inhibited by cyanide (CN⁻), CO, azide - all bind the heme iron of cytochrome a₃ and block electron transfer to O₂

Chemiosmosis and ATP Synthesis (Complex V)

1. Proton pumping: Complexes I, III, and IV pump H⁺ from the matrix to the intermembrane space. For each NADH oxidized, 10 H⁺ total are pumped (4 + 4 + 2).
2. Proton-motive force (PMF): The pumping creates both:
   • An electrical gradient (positive charges on cytosolic/IMS side)
   • A pH gradient (cytosolic side is more acidic)
   • Combined PMF drives H⁺ back through ATP synthase
3. ATP Synthase (Complex V):
   • Has two domains: F₀ (membrane-spanning H⁺ channel) and F₁ (catalytic sphere in matrix)
   • H⁺ re-entering through the F₀ c-ring drives its rotation
   • Rotation causes conformational changes in the three β-subunits of F₁ - binding ADP + Pi → phosphorylating → releasing ATP
   • One full rotation = 3 ATP synthesized
   • Inhibited by oligomycin (blocks H⁺ channel in F₀)
4. ATP/ADP antiporter: Newly synthesized ATP is transported out of the matrix in exchange for ADP (1 extra H⁺ cost per ATP exported)

Energy Yield

• NADH → O₂: ~53 kcal/mol (ΔG° = -52.6 kcal/mol)
• FADH₂ → O₂: ~41 kcal/mol

NADH Shuttles (Cytoplasmic NADH)

Inhibitors and Uncouplers

Note on uncouplers: When the H⁺ gradient is dissipated (e.g., by DNP), electron transport actually speeds up (no back-pressure) but ATP synthesis falls to zero - energy is entirely wasted as heat. DNP was historically used as a weight-loss drug; several deaths resulted.

Reactive Oxygen Species (ROS)

• Superoxide dismutase (SOD): O₂⁻ → H₂O₂
• Catalase: H₂O₂ → H₂O + O₂
• Glutathione peroxidase: H₂O₂ + GSH → H₂O

Clinical Correlates

• Ischemia/hypoxia: No O₂ → ETC stops → no PMF → no ATP → mitochondrial swelling → irreversible cell injury (necrosis)
• Cyanide poisoning: Inhibits Complex IV; all carriers upstream become fully reduced; O₂ cannot be used; cells die despite normal O₂ delivery
• Doxorubicin cardiotoxicity: Binds cardiolipin in the IMM; inhibits succinate oxidation, inactivates cytochrome oxidase, inhibits ATP synthase → decreased ATP in cardiomyocytes
• Mitochondrial myopathies: Mutations in mtDNA encoding ETC subunits; present with exercise intolerance, weakness, lactic acidosis, ragged red fibers on muscle biopsy
• AZT (zidovudine): Inhibits mitochondrial DNA polymerase γ → mtDNA depletion → severe mitochondrial myopathy

Digestion and Absorption of Carbohydrates

I. Major Dietary Carbohydrates

II. Digestion - Step by Step

A. Mouth - Salivary α-Amylase

• Salivary α-amylase (from parotid/submandibular glands) begins starch digestion
• Cleaves internal α-1,4-glycosidic bonds randomly → produces α-dextrins (shorter polysaccharides)
• Cannot cleave at branch points (α-1,6 bonds) or terminal bonds
• Inactivated in the stomach by HCl (low pH) - so salivary digestion is brief

B. Stomach

• No carbohydrate-digesting enzymes secreted
• Acid inactivates salivary amylase
• Sucrose and lactose pass through unchanged
• Mechanical breakdown continues

C. Small Intestine (Main Site of Digestion)

• Pancreatic α-amylase - continues cleaving α-1,4 bonds → produces:
  • Maltose (disaccharide)
  • Maltotriose (trisaccharide)
  • Limit dextrins (4-9 glucosyl residues with α-1,6 branch points)
• Bicarbonate - neutralizes gastric acid, restoring optimal pH for enzymatic activity

All brush border disaccharidases are integral membrane proteins anchored to the luminal membrane by a N-terminal hydrophobic domain. They are heavily glycosylated to protect against intestinal proteases.

• Duodenum: pancreatic amylase produces maltose, maltotriose, limit dextrins
• Jejunum: sucrase-isomaltase is most active here - handles sucrose and branched products
• Ileum: glucoamylase continues digesting any remaining oligomers (final opportunity)

III. Absorption of Monosaccharides

A. Na⁺-Dependent Active Transport (SGLT-1)

• Glucose and galactose are absorbed by SGLT-1 (Sodium-Glucose Linked Transporter 1) on the luminal/mucosal side of enterocytes
• SGLT-1 cotransports 1 glucose (or galactose) + 2 Na⁺ simultaneously into the cell
• The driving force is the low intracellular Na⁺ maintained by Na⁺/K⁺-ATPase on the serosal (basolateral) side - this is secondary active transport (indirectly ATP-dependent)
• Glucose and galactose compete for the same SGLT-1 transporter
• This allows glucose/galactose to be absorbed even against a concentration gradient

B. Facilitated Diffusion (GLUT Transporters)

• Fructose enters enterocytes via GLUT-5 on the luminal side - simple facilitated diffusion (no Na⁺ coupling)
• All three monosaccharides exit the cell on the serosal/basolateral side via GLUT-2 into the portal circulation
• Fructose and sugar alcohols are absorbed only down their concentration gradient - after a large intake, some remains in the lumen → bacterial fermentation → bloating, osmotic diarrhea

C. GLUT Transporter Summary

After absorption, monosaccharides travel via the hepatic portal vein to the liver, which is the first organ to process dietary glucose, galactose, and fructose.

IV. Fate of Undigested Carbohydrates: Dietary Fiber

• Gases: H₂, CO₂, CH₄ (methane) → flatulence
• Short-chain fatty acids (SCFAs):
  • Acetic acid (C2)
  • Propionic acid (C3)
  • Butyric acid (C4) - preferred fuel for colonic epithelial cells; also has antiproliferative activity (protective against colorectal cancer)
• SCFAs can be absorbed by colonic mucosa and provide ~10% of total daily calories

V. Glycemic Index

• High GI (near 1.0): glucose, galactose, lactose, maltose, most starches → rapid absorption, sharp insulin spike
• Low GI: fructose, sugar alcohols, resistant starch, soluble fiber (GI ~0 for non-starch polysaccharides)

VI. Clinical Correlates

Lactose Intolerance

• Most common carbohydrate malabsorption worldwide
• Congenital lactase deficiency - rare; presents in infancy with failure to thrive on breast milk or formula
• Adult-onset (primary) lactase deficiency - physiologic: lactase activity falls after weaning in most mammals and most humans. Persists into adulthood only in populations of North European origin and nomadic sub-Saharan/Arabian groups
• Mechanism: undigested lactose → bacterial fermentation → gas (CO₂, H₂) + lactic acid → osmotic diarrhea, bloating, abdominal cramps ~30-60 min after dairy ingestion

Sucrase-Isomaltase Deficiency

• Congenital deficiency common among the Inuit (Indigenous Arctic populations)
• Presents in infancy with sucrose intolerance: persistent diarrhea and failure to thrive when sucrose is introduced to the diet

Glucose-Galactose Malabsorption

• Rare autosomal recessive mutation in SGLT-1 gene
• Presents in neonates with severe osmotic diarrhea upon feeding (fructose is tolerated since it uses GLUT-5)

Hydrogen Breath Test

Diabetes and Dietary Fiber

Digestion and Absorption of Lipids

I. Overview

1. Mechanical and chemical emulsification (lipid droplets → smaller particles)
2. Enzymatic hydrolysis (lipases, phospholipase, esterase)
3. Micellar solubilization (bile salts form mixed micelles to transport products through the aqueous lumen)
4. Uptake into enterocytes, re-esterification, and chylomicron assembly
5. Secretion into lymphatics (not portal blood)

II. Digestion - Phase by Phase

A. Mouth

• Lingual lipase (from serous glands of tongue) begins hydrolysis of TAG at the sn-3 ester bond → 1,2-diacylglycerols + free fatty acids
• Limited contribution but relevant in neonates (where pancreatic lipase is immature)

B. Stomach

• Gastric lipase (from chief cells) continues hydrolysis at sn-3 bonds → 1,2-diacylglycerols + free fatty acids
• Free fatty acids and partial glycerides produced here act as emulsifying agents - they begin coating fat droplets, increasing the surface area for subsequent pancreatic enzymes
• Critical gastric function: slow, regulated emptying of chyme into the duodenum. CCK (cholecystokinin) is secreted when dietary lipids first appear in the small intestine and slows gastric emptying, allowing adequate time for pancreatic digestion

C. Small Intestine (Main Site of Digestion)

• CCK (from I cells of duodenum/jejunum in response to fat and protein):
  • Stimulates pancreatic enzyme secretion
  • Stimulates gallbladder contraction → bile into duodenum
• Secretin (from S cells in response to acid):
  • Stimulates pancreatic HCO₃⁻ secretion → neutralizes gastric acid
  • Optimal pH for pancreatic lipase is ~6; acid inactivates it

• Bile salts (synthesized in liver from cholesterol, stored in gallbladder) are amphipathic - hydrophilic face and hydrophobic face
• They coat fat droplets, reducing surface tension and breaking them into smaller droplets → emulsification greatly increases surface area for enzymatic attack
• Liver secretes ~2 g/day cholesterol and bile salts into bile; dietary cholesterol adds ~0.5 g/day

Key detail on colipase: Bile salts, while essential for emulsification, actually inhibit pancreatic lipase by displacing it from the lipid-water interface. Colipase (secreted as procolipase, activated by trypsin in the lumen) anchors at the interface and binds pancreatic lipase, restoring its activity. Without colipase, fat digestion would fail even in the presence of adequate lipase.

• 2-Monoacylglycerols (2-MAG)
• Free fatty acids (FFA) - long-chain (LCFA) and medium-chain (MCFA)
• Free cholesterol
• Lysolecithin (lysophospholipid)
• Glycerol (water-soluble; does not need micelles)

III. Micellar Solubilization

• Mixed micelles are cylindrical disk structures, ~4-6 nm diameter (50 Å)
• Exterior: hydrophilic face of bile salts (faces the aqueous lumen)
• Interior: hydrophobic lipid products, fat-soluble vitamins, cholesterol
• Micelles are water-soluble → can diffuse through the unstirred water layer adjacent to the brush border
• Most ingested lipid is absorbed by the mid-jejunum
• Bile salts are NOT absorbed with the lipids - they remain in the lumen, travel to the ileum, and are reabsorbed there into the enterohepatic circulation → returned to liver via portal vein → re-secreted in bile (recycled 6-10x/day)

IV. Uptake into Enterocytes and Intracellular Processing

Step 1: Entry across the Brush Border

• Lipid products diffuse passively from the micelle down their concentration gradient across the lipid bilayer of the brush border membrane
• The micelles per se do NOT enter the cell
• Glycerol and short/medium-chain fatty acids (≤12C) - water-soluble enough to pass directly into the portal blood without further processing

Step 2: Re-esterification in the Smooth Endoplasmic Reticulum (SER)

• 2-MAG pathway: 2-MAG + 2 LCFA (via acyl CoA intermediates) → TAG (main pathway in intestinal cells)
• Phosphatidic acid pathway: glycerol-3-phosphate → used when glycerol is available (from glycerol released inside the cell)
• Lysolecithin → re-esterified to phosphatidylcholine (lecithin) by lysolecithin acyltransferase, or synthesized de novo
• Cholesterol → re-esterified to cholesterol esters (by ACAT: acyl-CoA:cholesterol acyltransferase)

Step 3: Apolipoprotein Synthesis

• Apoproteins (apolipoproteins) are synthesized in the rough ER (RER)
• Key apolipoproteins for chylomicrons: Apo B-48 (intestine-specific, essential structural apoprotein), Apo A-I, Apo A-II, Apo A-IV
• Apolipoproteins migrate from RER to SER where they associate with lipid droplets (except Apo A-I, which joins in the Golgi)

Step 4-7: Chylomicron Assembly and Secretion

• Nascent chylomicrons form in the SER: core of TAG + cholesterol esters; surface of phospholipids + apolipoproteins
• Nascent chylomicrons travel to the Golgi (cis face) where apolipoproteins are glycosylated
• Transport vesicles bud off from the trans-Golgi and migrate to the basolateral membrane
• Exocytosis releases chylomicrons into the lamina propria

Step 8: Entry into Lymphatics (NOT portal blood)

• Chylomicrons (avg. diameter ~1000 Å = 100 nm) are too large to cross the fenestrae of blood capillaries
• They enter the lymphatic capillaries (lacteals) through larger interendothelial channels
• Lymph → lacteals → cisternae chyli → thoracic duct → left subclavian vein → systemic circulation

V. Fate of Different Chain-Length Fatty Acids

Clinical application: MCT (medium-chain triglyceride) oil is used therapeutically in patients with fat malabsorption (e.g., short bowel syndrome, lymphatic obstruction) because MCFAs bypass the normal chylomicron pathway entirely and go directly into portal blood.

VI. Enterohepatic Circulation of Bile Salts

• Bile salts secreted by the liver (~2g/day) are recycled 6-10 times per day
• After completing their role in micellar solubilization, bile salts remain in the intestinal lumen
• In the terminal ileum: actively reabsorbed by the Na⁺-dependent bile acid transporter (IBAT/SLC10A2)
• Portal vein → liver → re-conjugated and re-secreted into bile
• A small amount (~5%) escapes reabsorption and is lost in feces → must be replaced by new synthesis from cholesterol

VII. Clinical Correlates - Causes of Fat Malabsorption (Steatorrhea)

VIII. Absorption of Cholesterol

• Dietary cholesterol (~0.5 g/day) + biliary cholesterol (~2 g/day) enter the intestinal lumen
• Incorporated into mixed micelles → passively absorbed by enterocytes
• NPC1L1 (Niemann-Pick C1-Like 1 protein) on the brush border - the primary cholesterol transporter; ezetimibe blocks this transporter → used to lower blood cholesterol
• Inside the enterocyte: esterified by ACAT → incorporated into chylomicrons
• Only ~50% of dietary cholesterol is absorbed (vs. ~98% of dietary fat)

Renal Function Tests

Overview

1. Glomerular function tests (GFR, creatinine, BUN)
2. Tubular function tests (concentration/dilution, FENa, urine osmolality)
3. Urinalysis (dipstick + microscopy)
4. Newer biomarkers (Cystatin C, microalbumin, β₂-microglobulin)

I. Glomerular Function Tests

A. Glomerular Filtration Rate (GFR)

• GFR 50% of normal - kidney function begins to decline but patient usually remains asymptomatic; serum creatinine/BUN start rising
• GFR 30% of normal - moderate renal insufficiency; nocturia, anemia, decreased appetite, calcium/phosphorus abnormalities
• GFR 5-10% of normal - End-Stage Renal Disease (ESRD); uremia, acidemia, volume overload, neurologic/cardiac/respiratory manifestations → renal replacement therapy required

• Inulin clearance - the true gold standard (freely filtered, not secreted or reabsorbed), but requires intravenous infusion - impractical clinically
• Radionuclide methods (e.g., ¹²⁵I-iothalamate, ⁹⁹mTc-DTPA) - research and transplant evaluation

B. Serum Creatinine

• Produced at a constant daily rate (reflecting muscle mass)
• Freely filtered at the glomerulus
• Minimally secreted by the distal tubule (slightly overestimates GFR)
• Not significantly affected by dietary intake (unlike BUN)

• Adults: 0.8-1.2 mg/dL (Smith & Tanagho) / 0.5-1.5 mg/dL (Miller's)
• Young children: 0.4-0.8 mg/dL
• Pregnancy: 0.5-1.0 mg/dL (lower due to increased GFR)

• Serum creatinine remains within normal range until ~50% of renal function is lost - insensitive for early CKD
• A 50% reduction in GFR may cause only a doubling of creatinine (e.g., 1.0 → 2.0 mg/dL), which can be missed without a baseline value
• Falsely elevated by: Jaffe reaction interference (cephalosporins, ketones, glucose, fructose, ascorbic acid)
• Low muscle mass (elderly, malnourished, amputees) → falsely low creatinine despite reduced GFR

C. Estimated GFR (eGFR) Equations

CrCl (mL/min) = [(140 - Age) × Ideal Body Weight (kg)] / [Serum Creatinine (mg/dL) × 72]
× 0.85 (if female)

• Corrects for age (declining muscle mass) and sex
• Overestimates GFR in obese, edematous, or cachectic patients (IBW ≠ actual body weight in these groups)
• Does not account for extrarenal elimination or tubular secretion

eGFR = 186.3 × Cr⁻¹·¹⁵⁴ × Age⁻⁰·²⁰³ × 1.212 (if Black) × 0.742 (if female)

• Validated in adults with GFR <60 mL/min/1.73 m² (ages 18-70)
• Not reliable when GFR >60 mL/min/1.73 m² - tends to underestimate
• Not validated in pregnant women, children, or acutely ill hospitalized patients

• Most accurate equation currently available
• Reliable across both low and high GFR ranges
• Better predictor of ESRD risk, cardiovascular mortality, and all-cause mortality than MDRD
• Uses creatinine, age, sex, and race

D. Creatinine Clearance (24-hour Urine)

CrCl (mL/min) = [Urine Creatinine (mg/dL) × Urine Volume (mL)] / [Plasma Creatinine (mg/dL) × Collection Time (min)]

• Normal: 90-110 mL/min (uncorrected); 70-140 mL/min (corrected for body surface area)
• More accurate than serum creatinine alone but limited by collection errors (incomplete 24-h urine is the major pitfall)

E. Blood Urea Nitrogen (BUN)

• Freely filtered at the glomerulus
• Partially reabsorbed in the tubule (especially when flow is slow → higher BUN in dehydration)

• Dietary protein intake (high protein → high BUN)
• Hydration status (dehydration → high BUN)
• GI bleeding (blood digestion = protein load → high BUN)
• Liver disease (impaired urea synthesis → low BUN)
• Steroid use, tissue catabolism, exercise

F. BUN:Creatinine Ratio

II. Tubular Function Tests

A. Urine Specific Gravity

• Normal range: 1.002-1.030
• In renal failure: concentrating ability is progressively lost → specific gravity becomes fixed at 1.006-1.010 (isosthenuria)
• Diluting ability tends to be preserved until renal damage is extreme

B. Urine Osmolality

• Normal range: 50-1200 mOsm/kg (varies with hydration)
• Maximum concentration (after water deprivation): >800 mOsm/kg
• In prerenal AKI: urine osmolality >500 mOsm/kg (kidney conserving water)
• In intrinsic renal AKI (ATN): urine osmolality 300-400 mOsm/kg (isoosmotic = tubular dysfunction)

C. Fractional Excretion of Sodium (FENa)

FENa (%) = [Urine Na × Plasma Creatinine] / [Plasma Na × Urine Creatinine] × 100

Caveat: FENa may be misleadingly low (<1%) in contrast nephropathy, myoglobinuria, and early obstruction. In patients receiving diuretics, FE Urea (<35% = prerenal; >35% = renal) is more reliable.

D. Urine Concentration/Dilution Tests

• Water deprivation test: urine osmolality should rise to >800 mOsm/kg in normal subjects; failure indicates inability to concentrate → tubular dysfunction or AVP deficiency (diabetes insipidus)
• DDAVP (desmopressin) test: differentiates central from nephrogenic DI

III. Urinalysis

A. Dipstick (Chemical Analysis)

B. Proteinuria

• Normal: <150 mg/day (mostly Tamm-Horsfall protein from tubular cells)
• Microalbuminuria: 30-300 mg/day albumin = earliest marker of diabetic nephropathy and hypertensive nephropathy
• Significant proteinuria: >300 mg/day
• Nephrotic range proteinuria: >3.5 g/day

• Ratio of 0.3 ≈ 300 mg/g ≈ 300 mg/day
• Normal: <0.2

C. Urine Microscopy

IV. Newer / Emerging Biomarkers

Cystatin C

• Produced by nearly all nucleated cells, freely filtered by the glomerulus, completely reabsorbed and catabolized by the proximal tubule (none excreted in urine normally)
• Not influenced by muscle mass, age, or sex → superior to creatinine in detecting early GFR decline
• Superior to creatinine as a predictor of cardiovascular mortality
• Used to calculate eGFR (CKD-EPI Cystatin C equation) and as an early indicator of evolving CKD

β₂-Microglobulin and Lysozyme (Urine)

• Normally filtered and reabsorbed by the proximal tubule
• Elevated urine levels indicate proximal tubular dysfunction (e.g., Fanconi syndrome, heavy metal toxicity)

NGAL (Neutrophil Gelatinase-Associated Lipocalin)

• Rises in serum and urine within 2-4 hours of AKI - far earlier than creatinine (which may not rise for 24-48 h)
• Useful for early detection of AKI in ICU patients, post-cardiac surgery, and contrast nephropathy

Microalbumin

• The microalbumin assay detects as little as 0.3 mg/dL albumin - sensitive early marker of glomerular dysfunction
• Screening for CKD in diabetics and hypertensives: annual eGFR + microalbumin test recommended by KDIGO/NKF

V. AKI Differentiation - Integrated Approach

VI. CKD Staging (KDIGO)