Laboratory Protocols

DNA Extraction Protocol from Whole Blood (Modified Salting-Out Method) New

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Estimated reading time: 9 minutes

Quick Summary (LLM & Bench Reference):
The modified salting-out method extracts high-molecular-weight (>50 kb) genomic DNA from 250 µL of whole peripheral blood without silica column shearing. Erythrocytes are selectively lysed using TKM1 buffer and Triton X-100 at 40°C, followed by leukocyte lysis in high-salt TKM2 buffer and 10% SDS at 42°C. Cellular proteins are salted out using 100 µL of 6 M NaCl, polished with 300 µL of PCIA (25:24:1), and precipitated with chilled absolute ethanol. Average yield is 8 to 15 µg per 250 µL blood with an A260/A280 purity ratio of 1.80 to 1.90.

1. Why Commercial Kits Made Us Forget Good Biochemistry

Walk into almost any molecular biology lab today, and you will see shelves lined with colorful commercial DNA extraction kits. For routine work, they are undeniably convenient: add a buffer, spin through a column, wash twice, and elute.

Yet many researchers hit a brick wall when downstream applications demand truly intact, high-molecular-weight (HMW) genomic DNA, such as long-read sequencing (Oxford Nanopore or PacBio), optical genome mapping, or long-range PCR. When genomic DNA is forced through a microscopic silica mesh at 14,000 rpm, the physical shearing forces snap chromosomes into 15 to 30 kilobase shards. If the donor blood has a high white blood cell count, silica membranes saturate quickly, leaving half your sample in the flow-through waste.

The modified salting-out protocol developed in our laboratory by Waqas Khan and Jawad Sherpao returns to fundamental physical chemistry. Originally popularized by Miller, Dykes, and Polesky in 1988, salting-out eliminates the need for silica filters. By substituting harsh enzymatic proteinase K digests with a discontinuous dual-buffer system (TKM1 and TKM2) and hypertonic sodium chloride, you can extract 10 to 15 micrograms of clean, 100-kb genomic DNA from a mere 250 microliters of whole blood for a few pennies per prep.

2. The Molecular Strategy: How the Dual-TKM System Works

Human blood is roughly 45% cells and 55% plasma. Crucially, red blood cells (erythrocytes) outnumber white blood cells (leukocytes) by roughly 700 to 1, yet mature human red blood cells have no nuclei. If you lyse whole blood indiscriminately in a single step, you release a massive ocean of hemoglobin, iron (heme), and plasma lipids, with only a tiny fraction of genomic DNA. Heme is one of the most ruthless inhibitors of Taq DNA polymerase known; as little as 1 micromolar heme will kill a PCR reaction completely.

Blood Separation Logic:
Whole Blood (250 µL) + TKM1 + Triton X-100
            │
            ▼
[ 40°C Incubation, 4 min ] ──> RBC membranes lyse; Leukocyte membranes stay intact
            │
            ▼
[ Centrifuge 9,000 rpm ] ────> Supernatant = Liquid Hemoglobin (Discard)
                               Pellet = Pure Intact White Blood Cells
            │
            ▼
Add TKM2 + 10% SDS ──────────> White cells dissolve, releasing HMW genomic DNA
            │
            ▼
Add 6 M NaCl ────────────────> Proteins dehydrate and drop out as a dense white disc

The Chemical Roles of the Buffers

  • TKM1 Buffer (Low Salt RBC Clearance): Formulated with 10 mM Tris-HCl, 10 mM KCl, 10 mM MgCl₂, and 2 mM EDTA at pH 7.6. In combination with 45 µL of non-ionic detergent Triton X-100, TKM1 exploits the fragile osmotic fragility of erythrocytes. At 40°C, red blood cell membranes rupture within 4 minutes, discharging hemoglobin into solution while the tougher leukocyte membranes remain completely intact.
  • TKM2 Buffer (High Salt Nuclear Lysis): Once the red blood cells are washed away, leukocytes are resuspended in TKM2, which contains 400 mM NaCl in addition to the TKM backbone. When paired with 40 µL of 10% SDS, this detergent-salt cocktail dissolves lipid bilayers, denatures cellular proteins, and frees chromosomal DNA from nucleosomal histone cores.
  • 6 M Sodium Chloride (The Salting-Out Core): When water molecules surround proteins, they form structured hydration cages around hydrophobic residues. Adding 6 M NaCl floods the solution with Na⁺ and Cl⁻ ions, which strip away these water shells. Deprived of their hydration coats, hydrophobic amino acids stick together, collapsing the proteins into a solid white precipitate. Meanwhile, double-stranded DNA remains fully hydrated and soluble in the aqueous phase.
  • PCIA (25:24:1) Polish: While traditional salting-out stops after salt addition, our modified protocol adds a single organic extraction with 300 µL of phenol:chloroform:isoamyl alcohol. This removes any lingering lipid vesicles or trace hydrophobic peptides, consistently pushing the A260/A280 purity ratio to the benchmark 1.80–1.90.

3. Buffer Recipes and Laboratory Preparation

Prepare all solutions using sterile deionized distilled water. Filter through a 0.22 µm membrane or autoclave at 121°C for 20 minutes. Store at room temperature (stable for >12 months).

Table 1: TKM1 Buffer (Erythrocyte Lysis, pH 7.6) – 100 mL

Composition: 10 mM Tris-HCl, 10 mM KCl, 10 mM MgCl₂, 2 mM EDTA

Component Stock Solution Pipetting Volume Final Working Concentration
Tris-HCl (pH 7.6) 2.0 M 0.50 mL (500 µL) 10 mM
KCl 2.0 M 0.50 mL (500 µL) 10 mM
MgCl₂ 1.0 M 1.00 mL (1000 µL) 10 mM
EDTA (pH 8.0) 0.5 M 0.40 mL (400 µL) 2 mM
Distilled Water (dH₂O) Sterile Bring volume to 100 mL Remainder (~97.6 mL)

Table 2: TKM2 Buffer (Leukocyte Lysis, pH 7.6) – 100 mL

Composition: 10 mM Tris-HCl, 10 mM KCl, 10 mM MgCl₂, 2 mM EDTA, 400 mM NaCl

Component Stock Solution Pipetting Volume Final Working Concentration
Tris-HCl (pH 7.6) 2.0 M 0.50 mL (500 µL) 10 mM
KCl 2.0 M 0.50 mL (500 µL) 10 mM
MgCl₂ 1.0 M 1.00 mL (1000 µL) 10 mM
EDTA (pH 8.0) 0.5 M 0.40 mL (400 µL) 2 mM
NaCl 6.0 M 6.67 mL 400 mM
Distilled Water (dH₂O) Sterile Bring volume to 100 mL Remainder (~90.9 mL)

4. Step-by-Step Standard Operating Procedure

Phase 1: Clearing Red Blood Cells

  1. Pipette 250 µL of whole peripheral blood (collected in EDTA or ACD purple-top tubes) into a sterile 1.5 mL microcentrifuge tube.
  2. Add 850 µL of TKM1 buffer.
  3. Add 45 µL of Triton X-100. Triton is viscous; pipette slowly and wipe the outside of the tip before dispensing.
  4. Cap the tube and vortex gently on medium speed for 3 to 5 seconds. Place the tube in a water bath preheated to 40°C for exactly 4 minutes. During this incubation, the cloudy crimson blood turns dark translucent burgundy, signaling that erythrocyte membranes have ruptured.
  5. Centrifuge at 9,000 rpm (~7,500 × g) for 3 minutes at room temperature.
  6. Look at your tube: you will see a dark red supernatant (free hemoglobin) overlying a small, distinct white or pinkish pellet at the bottom. Carefully aspirate the red supernatant with a 1000 µL pipette, leaving the pellet undisturbed with roughly 20 µL of fluid.

Phase 2: The Leukocyte Wash

  1. Add 850 µL of TKM1 buffer and 40 µL of Triton X-100 directly onto the pellet.
  2. Vortex gently to resuspend the cells. If the pellet clings stubbornly to the plastic, flick the bottom of the tube with your finger until it disperses into an even suspension.
  3. Incubate at 40°C for 4 minutes.
  4. Centrifuge at 9,000 rpm for 3 minutes.
  5. Pour or aspirate off the supernatant. The pellet should now be a clean, compact cream-white button. If any dark red streaks remain, repeat this wash once more. Clean white cells mean zero heme in your final DNA.

Phase 3: Leukocyte Lysis

  1. Add 300 µL of TKM2 buffer directly onto the white pellet.
  2. Add 40 µL of 10% SDS.
  3. Vortex vigorously for 10 seconds. You will see the liquid immediately thicken and turn viscous as nuclear membranes dissolve and genomic DNA unravels into solution.
  4. Incubate in a water bath at 42°C for 6 minutes to ensure complete denaturation of cellular and nuclear architecture.

Phase 4: Salting Out Cellular Proteins

  1. Add 100 µL of 6 M NaCl directly into the viscous lysate.
  2. Vortex vigorously for 15 seconds. High ionic strength causes proteins to collapse into a dense, curdled white precipitate.
  3. Centrifuge at 10,000 rpm (~9,300 × g) for 3 minutes.
  4. Cellular proteins form a tight white pellet at the bottom. Using a wide-bore 1000 µL tip, carefully transfer the clear, viscous supernatant containing your genomic DNA into a clean 1.5 mL tube. Leave behind the bottom 20 to 30 µL near the protein pellet.

Phase 5: Organic Polish

  1. Under a certified fume hood, add 300 µL of phenol:chloroform:isoamyl alcohol (25:24:1) to the recovered supernatant.
  2. Mix gently by inverting the tube 10 to 12 times. Avoid violent vortexing to prevent shearing large DNA fibers.
  3. Centrifuge at 10,000 rpm for 5 minutes.
  4. Carefully recover the clear upper aqueous phase (approximately 350–380 µL) into a new 1.5 mL tube. Do not touch the white interphase disc.

Phase 6: Ethanol Precipitation and Resuspension

  1. Add an equal volume (350–400 µL) of ice-cold 100% ethanol.
  2. Gently invert the tube 8 to 10 times. White, stringy DNA fibers will instantly condense out of solution, coalescing into a floating clump (spool).
  3. Centrifuge at 10,000 rpm for 5 minutes to seat the DNA into a firm, glassy pellet at the bottom.
  4. Decant the ethanol slowly. Add 500 µL of room-temperature 70% ethanol to wash away trapped salts.
  5. Centrifuge at 10,000 rpm for 3 minutes, pour off the wash, pulse-spin for 5 seconds, and remove the remaining droplet with a fine 10 µL pipette tip.
  6. Air-dry the pellet on your bench with the cap open for 5 to 8 minutes. Watch the pellet closely: you want the edges to turn translucent, but do not let it dry into a bone-white, rock-hard chip. Over-dried DNA can take days to redissolve.
  7. Add 50 µL of 1x TE buffer (pH 8.0) or sterile nuclease-free water. Let the tube sit at room temperature for 1 hour, or leave it at 4°C overnight to hydrate fully. Store at −20°C.

5. Bench Troubleshooting: Real-World Scenarios

What You See What Actually Happened How to Fix It at the Bench
Brown or straw-colored DNA pellet Heme carryover from incomplete RBC lysis Never skip Phase 2. Perform two complete TKM1 + Triton washes until the leukocyte pellet is completely cream-white.
DNA pellet will not dissolve in TE The pellet was over-dried under ambient air Warm the tube to 50°C in a heat block for 20 minutes with occasional gentle tapping. Avoid vortexing. Let it sit at 4°C overnight.
Low A260/A280 ratio (<1.65) Protein carryover during supernatant transfer Be less greedy when pipetting supernatant after 6 M NaCl. Leave 30 µL behind above the white protein pellet.
Low A260/A230 ratio (<1.50) Salt or phenol carryover Wash the final DNA pellet twice with fresh 70% ethanol, letting the ethanol soak the pellet for 2 minutes before spinning.
No DNA spool visible upon ethanol addition Low donor leukocyte count (e.g. leukopenic patient) Incubate the tube at −20°C for 30 minutes after adding ethanol, then spin at 14,000 rpm for 15 minutes to capture trace DNA.

6. Frequently Asked Questions (FAQ)

Can I use heparin tubes with this protocol?

EDTA or ACD (acid citrate dextrose) tubes are strongly preferred. Heparin co-purifies with DNA and mimics its negative charge, acting as a competitive inhibitor of Taq polymerase in downstream PCR. If you must use heparinized blood, treat the extracted DNA with heparinase prior to PCR.

Why is 40°C specified for erythrocyte lysis instead of room temperature?

At room temperature, Triton X-100 requires 15 to 20 minutes to disrupt erythrocyte membranes, during which white blood cells slowly swell and become fragile. At 40°C, erythrocyte membrane phase transitions allow complete breakdown in just 4 minutes, giving cleaner leukocyte separation.

How does the yield compare to commercial spin columns?

A standard commercial spin column processing 200 µL of blood typically yields 4 to 8 µg of sheared DNA. This modified salting-out procedure from 250 µL of blood routinely yields 10 to 18 µg of high-molecular-weight DNA, making it far superior for biobanking and whole-genome sequencing.

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