ALAS Activity Assay
d-AMINOLEVULINIC ACID SYNTHASE (ALAS)
Mouse liver homogenate
- Weigh out ~100mg liver.
- Add four volumes (400µL) of ice-cold 50mM potassium phosphate (KPi) pH7.4.
- Homogenize with ten up-and-down strokes in a 2-mL glass-Teflon (Potter- Elvehjem) tissue homogenizer in an ice bath.
- Store the resulting 20%w/v homogenate at −80 °C until needed.
Cultured cell homogenate
- Wash the cells with phosphate buffered saline (PBS) pH 7.4.
- Resuspend in about three pellet volumes of 50mM potassium phosphate (KPi) pH7.4.
- Sonicate (homogenize) while in an ice bath at the lowest practicable power setting for 3 cycles x 5 sec at 50% duty (2.5 sec on, 2.5 sec off).
Preparation of succinyl coenzyme A
- Prepare these three aqueous solutions and keep on ice:
- 23.85 mg/mL coenzyme A
- 10.5 mg/mL succinic anhydride, ground to fine powder
- 25 mg/mL NaHCO3.
- Mix equal volumes of these solutions and incubate on ice for 30 min.
- Use in the ALAS assay.
- The resulting ~10mM succinyl CoA solution could be also be aliquoted, stored for more than three months at −80 °C and thawed once, only when needed.
- Succinic anhydride must be dissolved in ice-cold water to minimize conversion to succinic acid before reacting with coenzyme A. Check that the final mixture reaction is maintained at pH 7–7.5 by the NaHCO3 solution.
ALAS assay
- Adjust enough of each sample to 5-10 mg protein per mL with 50mm potassium phosphate (KPi) pH7.4 to make 6 x 25-µL aliquots for three pairs of live and blank replicates. Blanks are heat inactivated for 10 min in a boiling water bath before the addition of the ALAS assay buffer, or incubated at 0 min at 37˚C after the ALAS assay buffer has been added.
- For each 25-µL of sample make 25µL of ALAS assay buffer by mixing 15.5µL 50mM KPi pH 7.4, 2.5µL 1M aqueous glycine pH~7, 2.5µL 10mM succinyl CoA, 0.5µL aqueous succinylacetone and 4.0µL 1mM aqueous pyridoxal 5'-phosphate.
- Add 25 μL ALAS assay buffer to each 25-μL aliquot of the protein adjusted sample.
- Incubate the resulting mixture at 37 °C for 30min.
- Add 450 µL of ice-cold water.
- Keep the diluted ALAS sample on ice or store frozen until the ALA can be derivatized.
Derivatization of d-aminolevulinic acid (ALA)
- Prepare derivatizing agent (DA) each sample by mixing water, 37% formaldehyde, ethanol and acetylacetone in a ratio of 107:5:15:23 by volume, respectively. Stir/vortex vigorously for 3 min or more until a clear colorless solution is obtained.
- Carry out all subsequent steps under minimal lighting.
- Mix 50-μL aliquot of the diluted ALAS assay sample (above) with 150 µL DA.
- Incubate in a covered heat block at about 100–103 °C for 5.0min.
- Cool immediately in an ice bath for 1 to 6h.
- Centrifuged for 10 min at 14,000 rpm in a microfuge at 4 °C.
- Collect the supernatant.
- Keep at 4°C until injection into the UPLC.
Ultra performance liquid chromatography (UPLC) quantitation
- Inject 10µL of supernatant containing the derivatized ALA into a Waters Acquity UPLC system which includes a binary solvent manager, sample manager, fluorescence detector, column heater and an Acquity UPLC BEH C18, 1.7 μM, 2.1 Å~ 100mm column.
- Set the fluorescence detector at 370nm excitation and 460nm emission.
- Keep the sample chamber dark and at 5 °C. Solvent A is 0.2% aqueous formic acid while Solvent B is 100% methanol.
- Set the flow rate at a constant 0.3 mL/min and the column at 50°C for the total run time of 12min.
- Use the following gradient schedule with the percent Solvent A at each step as follows:
- 0 min, 80%
- 6 min, 60%
- 7 min, 1%
- 9 min, 1%
- 9.5 min, 80%
- Set the gradient for solvent composition from 0 to 6 min as Waters Gradient 5 (convex with a higher slope at 0 min compared to that at 6 min), and that from 6 to 7 min at Waters Gradient 7, concave.
- Keep all other gradients in the method linear.
Standard curves
- Prepare pairs of sample aliquots at the same protein concentration as the unknowns to contain authentic ALA at seven different concentrations in a range similar to those as the sample unknowns, including 0.0µM.
- Inactivate one of each pair.
- Put these spiked samples through the ALAS activity assay protocol.
- Subtract the background UPLC peak readings at 0.0 µM was from those of the spiked samples and use these results to construct a standard curve that to determine the [ALA] in the 25-µL sample aliquots.
No Comments