Cell Fractionation and Centrifugation

Overview of Cell Fractionation

Cell fractionation is a specialized laboratory technique designed to break open cells and isolate specific cell components, known as organelles, while simultaneously preserving their individual biological functions. This procedural framework is divided into two primary stages:

• Homogenization: The physical act of breaking the cells open.

• Centrifugation: The process of separating the released cellular components based on physical properties.

Homogenization: Methods and Principles

Homogenization involves the physical disruption of cells using a device called a homogenizer or a blender. This process releases the internal contents of the cell, resulting in a liquid mixture termed the homogenate.

Physical Methods of Homogenization

• Mechanical Shearing: This involves grinding whole tissue using tools such as a mortar and pestle, blenders, Dounce homogenizers, or Teflon homogenization.

• Sonication: The application of high-frequency sound waves to shatter cell membranes.

• Osmotic Shock: Utilizing a hypo-osmotic buffer to cause cells to swell and explode.

• High Pressure: Using a French Press to force cells through a narrow valve, causing disruption.

Post-Homogenization Filtration

Following the creation of the raw homogenate, it must be filtered through a mesh or cheesecloth. This step is critical to remove the following:

• Remaining large, un-ruptured intact cells.

• Heavy fragments of connective cell walls.

Critical Pretreatment Conditions for Centrifugation

Before the filtered homogenate is placed into an ultra-centrifuge, three specific environmental conditions must be met to preserve organelle integrity:

• Cold Environment: Lowering the temperature decreases enzyme activity, which prevents the self-digestion (autolysis) of organelles.

• Isotonic Environment: The medium must maintain the same water potential as the cell contents to prevent osmotic bursting or shrinking.

• Buffered Environment: A constant pH (usually set at $7.4$) must be maintained to avoid the denaturation of cellular proteins.

Principles and Mathematical Foundations of Centrifugation

Cellular separation within an ultra-centrifuge is governed by physical properties including sedimentation velocity, the sedimentation coefficient ($S$ for Svedberg), and the diffusion coefficient.

Primary Centrifugation Methods

1. Sedimentation Velocity (Differential Centrifugation): Separation based on the speed at which particles settle.

2. Sedimentation Equilibrium (Isopycnic or Density Centrifugation): Separation based on buoyant density.

Formulas and Variables

• The Sedimentation Coefficient ($S$):     $S = \frac{1}{\omega^2 r} \times \frac{dr}{dt}$     Where:     * $\omega$ = angular velocity of the rotor in $\text{radians/sec}$ ($\text{calculated as } 0.10472 \times \text{RPM}$).     * $r$ = distance between the particle and the center of rotation ($mm$).     * $\frac{dr}{dt}$ = rate of movement of the particle.

• Conversion of RPM to Relative Centrifugal Force (RCF):     $RCF = 1.118 \times 10^{-5} \times (r) \times (\text{RPM})^2$     (Note: $r$ must be in centimeters for this specific conversion formula).

Differential Centrifugation Stages

Differential centrifugation relies on spinning the mixture at increasingly higher speeds to fractionate components by size and density. At each stage, the "pellet" (sedimented components) is collected, and the "supernatant" (remaining liquid) is subjected to the next speed tier.

General Fractionation Steps

• Low-Speed Centrifugation ($1,000 \times g$ for $10 \text{ minutes}$): Pellet contains whole cells, nuclei, and cytoskeletons.

• Medium-Speed Centrifugation ($20,000 \times g$ for $20 \text{ minutes}$): Pellet contains mitochondria, lysosomes, and peroxisomes.

• High-Speed Centrifugation ($80,000 \times g$ for $1 \text{ hour}$): Pellet contains microsomes and small vesicles.

• Very High-Speed Centrifugation ($150,000 \times g$ for $3 \text{ hours}$): Pellet contains ribosomes, viruses, and large macromolecules.

Detailed Fractionation Profiles (Rat Liver and General Components)

Crude Pellets from Rat Liver (P1 - P6)

• P1 ($1,000g \times 10m$): Nuclei, heavy mitochondria, plasma membrane (PM) sheets.

• P2 ($3,000g \times 10m$): Heavy mitochondria, PM fragments.

• P3 ($6,000g \times 10m$): Mitochondria, lysosomes, peroxisomes, intact Golgi.

• P4 ($10,000g \times 10m$): Mitochondria, lysosomes, peroxisomes, Golgi membranes.

• P5 ($20,000g \times 10m$): Lysosomes, peroxisomes, Golgi, large and dense vesicles (rough Endoplasmic Reticulum - rER).

• P6 ($100,000g \times 10m$): All ER vesicles, Plasma Membrane (PM), Golgi, endosomes.

Size and Sedimentation Properties Table (Subcellular Components)

Density Gradient Centrifugation

Density Gradient Centrifugation uses a medium—such as sucrose or Percoll—to create a density gradient within a centrifuge tube, where density is lowest at the top and highest at the bottom. This method allows organelles to settle at a specific level that corresponds to their own density.

• Mechanism: Sample is layered on top of a shallow gradient. Faster-sedimenting particles form bands further down the tube more quickly than smaller particles.

• Independence from Time: Particles separate solely based on buoyant density. Migration continues until the particle reaches a region where the surrounding medium's density matches its own, at which point further migration is prevented.

Collection and Analysis of Purified Fractions

Methods for Collecting Fractions

Proper collection ensures samples remain pure and intact:

1. Manual: Puncturing the sidewall of the centrifuge tube with a needle and withdrawing fractions via a syringe.

2. Machine (Gradient Uploader): Introducing a very dense, non-miscible medium into the bottom of the tube to push fractions upward for collection at the top.

3. Gravitational: If there is no pellet, fractions can be collected through a hole in the bottom of the tube.

Analytical Methods for Identification and Quantification

To ensure purified fractions can be used in downstream applications, they must be analyzed using the following methods:

1. Light or Electron Microscopy: To visualize the structure and purity of fractions.

2. Biochemical Assays: Determining the presence of specific "marker enzymes."

3. Antibody Assays (Western Blot): Assaying for a specific protein marker using antibodies.

4. Spectrophotometry (e.g., Bradford Assay): Determining the total protein concentration of the fraction.

5. Specific Activity Determination: Calculating the ratio of the activity of the enzyme of interest to the total protein concentration.