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MBBS 1st Year Biochemistry Practical - Important Topics Notes

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SECTION 1: QUALITATIVE TESTS FOR CARBOHYDRATES

A. General Tests (detect all sugars)

• Reagent: Alpha-naphthol in alcohol + conc. H₂SO₄
• Principle: H₂SO₄ hydrolyzes carbohydrates to monosaccharides, dehydrates them to furfural/hydroxymethylfurfural, which reacts with alpha-naphthol to give a purple ring
• Result: Purple/violet ring at junction of two liquids = POSITIVE
• Note: All carbohydrates give positive; proteins give false positive (triose formation)

• Reagents: Fehling's A (copper sulfate) + Fehling's B (sodium potassium tartrate + NaOH)
• Principle: Reducing sugars reduce Cu²⁺ (blue) to Cu⁺ (brick-red cuprous oxide precipitate)
• Result: Brick-red precipitate = POSITIVE (reducing sugar present)
• Positive: Glucose, fructose, maltose, lactose
• Negative: Sucrose (non-reducing), starch

• Reagent: Sodium citrate + sodium carbonate + copper sulfate
• Colors (semi-quantitative): Green < Yellow < Orange < Red/Brick-red (increasing sugar)
• Advantage: Stable single reagent; more sensitive

B. Specific Tests

• Reagent: Copper acetate in acetic acid
• Result: Monosaccharides give red precipitate within 2-3 min; disaccharides take longer (>10 min)

• Reagent: Resorcinol + conc. HCl
• Principle: Ketoses are dehydrated faster than aldoses; fructose → cherry-red color
• Result: Cherry red = ketose (fructose); delayed/faint = aldose (glucose)

• Reagent: Phenylhydrazine + sodium acetate + glacial acetic acid (+ heat)
• Crystals:
  • Glucose → needle-shaped crystals
  • Fructose → same as glucose (both form glucosazone)
  • Maltose → sunflower/hedgehog crystals
  • Lactose → powder puff/cotton ball crystals

• Reagent: Iodine in potassium iodide solution
• Result: Starch = blue-black; glycogen = reddish-brown; cellulose = no change

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SECTION 2: QUALITATIVE TESTS FOR PROTEINS

A. Colour Reactions

• Reagent: NaOH + dilute CuSO₄
• Principle: Copper ions form a complex with peptide bonds (2+ peptide bonds needed)
• Result: Violet/purple = POSITIVE
• Negative: Amino acids and dipeptides (no biuret reaction)

• Reagent: 0.1% ninhydrin
• Result: Purple/blue-violet color = POSITIVE (alpha-amino acids)
• Exception: Proline and hydroxyproline give yellow color (secondary amines)

• Reagent: Conc. HNO₃ (+ heat, then NaOH)
• Principle: Nitration of aromatic rings (Phe, Tyr, Trp)
• Result: Yellow color (xanthoproteic acid), turns orange on adding NaOH = POSITIVE

• Reagent: Millon's reagent (mercuric nitrate + nitrous acid)
• Result: Brick-red/rose color = POSITIVE (tyrosine present)
• Note: Gives false positive with urea

• Reagent: Glyoxylic acid + conc. H₂SO₄
• Result: Violet ring at interface = POSITIVE

• Reagent: NaOH + lead acetate (heat)
• Result: Black precipitate of lead sulphide = POSITIVE (cysteine, cystine, methionine)

B. Precipitation Reactions

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SECTION 3: URINE ANALYSIS

A. Physical Examination

B. Chemical Tests for Abnormal Constituents

• Benedict's Test - Brick-red precipitate (positive when blood glucose >180 mg/dL = renal threshold)
• Dipstick - Glucose oxidase method (specific for glucose)
• Causes: Diabetes mellitus, renal glycosuria, gestational diabetes

• Heat & Acetic Acid Test: Heat urine → white cloudiness; add acetic acid → persists = protein
• Sulphosalicylic Acid Test: White precipitate with SSA = protein present
• Dipstick: Detects albumin (least 30 mg/dL)
• Normal: <150 mg/day; Nephrotic syndrome: >3.5 g/day

• Rothera's Test: Sodium nitroprusside + NH₃ → purple/magenta ring = acetoacetate/acetone
• Gerhardt's Test: FeCl₃ → wine red = acetoacetate (not acetone)
• Causes: Uncontrolled DM, starvation, prolonged vomiting

• Fouchet's Test: BaCl₂ precipitates bilirubin → add Fouchet's reagent → green/blue color
• Gmelin's Test: Conc. HNO₃ → color spectrum (yellow → red → violet → blue) = positive
• Causes: Obstructive jaundice, hepatocellular jaundice (conjugated bilirubin only)

• Ehrlich's Test: DMAB reagent (p-dimethylaminobenzaldehyde) → pink/red color
• Normal: Traces (0.2-1.0 EU/day); Increased: hepatocellular damage, hemolysis

• Benzidine Test (or Guaiac): Blue color = hemoglobin present
• Dipstick: Peroxidase activity of hemoglobin

• Hay's Sulphur Test: Sulfur powder sprinkled on urine - sinks with bile salts (reduce surface tension)
• Foam Test: Yellow/green foam on shaking

• Creatinine + picric acid in alkaline medium → orange-red complex
• Normal urine creatinine: 1-2 g/day

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SECTION 4: BLOOD ANALYSIS / QUANTITATIVE ESTIMATIONS

A. Blood Glucose Estimation

• Principle: Glucose reduces alkaline cupric ions to cuprous ions → with phosphomolybdic acid → blue color (molybdenum blue); measured at 420 nm
• Normal fasting blood glucose: 70-110 mg/dL
• Diabetes diagnosis: FBS >126 mg/dL (two readings) or random >200 mg/dL

• Glucose → gluconic acid + H₂O₂ (by glucose oxidase)
• H₂O₂ + chromogen → colored complex (by peroxidase)
• More specific, no interference from other sugars

B. Blood Urea Estimation

• Principle: Urea + diacetyl monoxime → yellow color in acidic conditions (Fearon reaction)
• Normal: 20-40 mg/dL (blood urea); BUN = 10-20 mg/dL
• Clinical significance: Elevated in renal failure (azotemia), dehydration, high protein diet

• Urea → NH₃ + CO₂ (by urease) → NH₃ measured with Nessler's reagent (yellow-brown) or Berthelot's reagent

C. Serum Creatinine

• Jaffe's Reaction: Creatinine + alkaline picrate → orange-red (measured at 520 nm)
• Normal: 0.6-1.2 mg/dL (males); 0.5-1.0 mg/dL (females)
• Elevated in renal failure, rhabdomyolysis

D. Total Protein Estimation

• Protein + biuret reagent → violet complex (measured at 540 nm)
• Normal serum total protein: 6.0-8.0 g/dL
• Albumin: 3.5-5.0 g/dL; Globulins: 2.0-3.5 g/dL; A/G ratio: 1.2-2.0

E. Serum Bilirubin (van den Bergh Reaction)

• Direct (conjugated): Bilirubin + diazo reagent → purple/pink azo pigment (without methanol)
• Indirect (unconjugated): Requires methanol (accelerator) to react
• Normal: Total <1.0 mg/dL; Direct <0.3 mg/dL; Indirect <0.8 mg/dL
• Jaundice visible when total >2.5-3.0 mg/dL

F. Serum Alkaline Phosphatase (ALP)

• Principle: ALP hydrolyzes p-nitrophenyl phosphate → p-nitrophenol (yellow color at 405 nm) + phosphate
• Normal: 30-120 IU/L
• Elevated in: Obstructive jaundice, bone disease (Paget's), liver disease

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SECTION 5: ENZYMES - PRACTICAL ASPECTS

Key Enzyme Assay Concepts

1. Temperature - Optimum ~37°C for most human enzymes; denaturation above 60°C
2. pH - Pepsin works best at pH 2; trypsin at pH 8; ALP at pH 9-10
3. Substrate concentration - Michaelis-Menten kinetics (Km, Vmax)
4. Enzyme concentration - Activity proportional to enzyme concentration
5. Inhibitors - Competitive, non-competitive, uncompetitive

• Plot 1/[S] vs 1/V
• X-intercept = -1/Km; Y-intercept = 1/Vmax
• Competitive inhibitor: increases Km (same Vmax); non-competitive: decreases Vmax (same Km)

Clinically Important Enzyme Estimations in Practicals

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SECTION 6: LIPIDS

A. Qualitative Tests for Lipids

• Fats dissolve in organic solvents (chloroform, ether, benzene); insoluble in water

• Lipids + Sudan III stain → orange-red color (fat droplets stain red)

• Fat + KHSO₄ (heat) → acrolein (pungent smell) = glycerol present = fat confirmed

• Fat leaves a translucent (greasy) spot on paper = positive for fat

B. Quantitative Lipid Estimations

• Cholesterol + acetic anhydride + conc. H₂SO₄ → green/emerald color → measured at 620 nm
• Normal Total Cholesterol: <200 mg/dL (desirable); 200-239 borderline; >240 high
• LDL <100 mg/dL (optimal); HDL >40 mg/dL (males), >50 (females)

• GPO-PAP method: Triglycerides → glycerol + fatty acids (lipase) → glycerol-3-phosphate → H₂O₂ → colorimetric measurement
• Normal: <150 mg/dL

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SECTION 7: LABORATORY SAFETY & QUALITY CONTROL

Safety Rules

• Always wear gloves and lab coat when handling reagents
• Concentrated acids (H₂SO₄, HNO₃): add acid to water, never water to acid
• Dispose biohazardous samples (blood, urine) in designated containers
• Reagents labeled and stored appropriately

Quality Control Concepts

• Accuracy vs Precision: Accuracy = closeness to true value; Precision = reproducibility
• Normal range: Mean ± 2 SD covers 95% of normal population
• Levey-Jennings chart: Plot daily QC values; Westgard rules detect errors
• Internal QC vs External QC (EQAS)

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QUICK REFERENCE: REAGENTS AND EXPECTED RESULTS

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EXAM TIPS FOR VIVA / SPOTTERS

1. Always state the principle before describing the procedure
2. Know the positive control for each test (e.g., glucose solution for Benedict's)
3. For urine tests - state whether it is a test for conjugated or unconjugated bilirubin
4. Know the renal threshold for glucose = 180 mg/dL
5. For Osazone test - know crystal shapes (needle = glucose/fructose; sunflower = maltose; powder-puff = lactose)
6. Biuret test needs minimum 2 peptide bonds - dipeptides do NOT give this reaction
7. Ninhydrin gives yellow with proline/hydroxyproline, not purple
8. Rothera's test is more sensitive than Gerhardt's for ketones (Rothera detects acetone + acetoacetate; Gerhardt only detects acetoacetate)
9. The van den Bergh direct reaction tests conjugated bilirubin (water-soluble); indirect = unconjugated
10. For enzyme assays - always mention measurement at specific wavelength and mention the substrate used

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