Materials and Reagents

Eppendorf tubes (1.5-2.0 ml)
Cuvettes 1.5 ml (BRAND, catalog number: 759115 )
CO₂ tank
Chlorella sorokininana Shihira & Krauss, strain 211/8K (CCAP, Cambridge University) (Figure 2)
Liquid nitrogen
Milli-Q water
O-Acetyl-L-serine (OAS) (Sigma-Aldrich, catalog number: CDS020792 )
Dithiothreitol (DTT) (Sigma-Aldrich, catalog number: D9779 )
Sodium sulfide nonahydrate (Na₂S·9H₂O) (Sigma-Aldrich, catalog number: S2006 )
Trichloracetic acid (TCA) (Sigma-Aldrich, catalog number: 91228 )
Acetic acid (CH₃COOH) (Avantor Performance Materials, J.T. Baker^®, catalog number: 401424 )
Ethanol (EtOH) (Avantor Performance Materials, J.T. Baker^®, catalog number: 8007 )
Bio-Rad Protein Assay Dye Reagent Concentrate (Bio-Rad Laboratories, catalog number: 5000006 )
Potassium phosphate monobasic (KH₂PO₄) (Sigma-Aldrich, catalog number: P5655 )
Potassium phosphate dibasic (K₂HPO₄) (Sigma-Aldrich, catalog number: 04248 )
Note: This product has been discontinued.
Sodium chloride (NaCl) (Sigma-Aldrich, catalog number: S5886 )
Magnesium sulfate (MgSO₄) (Sigma-Aldrich, catalog number: M2643 )
Ethylenediaminetetraacetic acid ferric sodium salt (Fe-EDTA) (Sigma-Aldrich, catalog number: E6760 )
Calcium chloride (CaCl₂) (Sigma-Aldrich, catalog number: C5670 )
Potassium nitrate (KNO₃) (Sigma-Aldrich, catalog number: P8291 )
Copper(II) sulfate (CuSO₄) (Sigma-Aldrich, catalog number: 451657 )
Ammonium molybdate tetrahydrate, (NH₄)₆Mo₇O₂₄·4H₂O (Sigma-Aldrich, catalog number: M1019 )
Manganese(II) chloride (MnCl₂) (Sigma-Aldrich, catalog number: 13217 )
Note: This product has been discontinued.
Zinc sulfate heptahydrate (ZnSO₄·7H₂O) (Sigma-Aldrich, catalog number: Z0251 )
Boric acid (H₃BO₃) (Sigma-Aldrich, catalog number: B6768 )
Pyridoxal-phosphate (PLP) (Sigma-Aldrich, catalog number: P3657 )
Ninhydrin (Sigma-Aldrich, catalog number: N4876 )
Hydrochloric acid 37% (Avantor Performance Materials, J.T. Baker^®, catalog number: 6012 )
HEPES (Sigma-Aldrich, catalog number: H4034 )
Bovine serum albumin (BSA) (Sigma-Aldrich, catalog number: A7030 )
Basal medium (see Recipes)
Phosphate buffer (see Recipes)
Extraction buffer (see Recipes)
Ninhydrin solution (see Recipes)
1 M HEPES solution (see Recipes)

Equipment

Culture flask (WHEATON, catalog number: 356954 )
Fluorescent lamps (Philips Lighting, model: TL-D 30W/55 )
Bench centrifuge (Thermo Fisher Scientific, Thermo Scientific^TM, model: IEC CL30 )
French pressure cell press (AMINCO RESOURCES, model: FA-078 )
Superspeed centrifuge (Thermo Fisher Scientific, Thermo Scientific^TM, model: Sorvall RC-5C Plus )
Vortex mixer (Troemner, catalog number: 945302 )
Eppendorf ThermoMixer^® Comfort (Eppendorf, model: 5355 )
Eppendorf MiniSpin^® (Eppendorf, model: 5453000011 )
Thermo bath (Labortechnik medingen, model: MWB 5 )
Spectrophotometer (Cole-Parmer, JENWAY, model: 7315 )
Optical microscope (Esselte, Leitz, model: Leitz Laborlux K )

Software

SigmaPlot^® 12 software

Procedure

Algal culture condition
Grow Chlorella sorokiniana culture in batch in basal medium (see Recipes) at 35 °C and under continuous illumination (fluorescent lamps, 250 μmol photons m^-2 sec^-1). Bubble the culture with air containing 5% CO₂. Under these conditions, the algal growth rate constant (µ) is 3 d^-1.
Preparation of microalgae extracts
1. Harvest 200 ml of algal culture by centrifugation (4,500 x g for 10 min). Collect the cells during the exponential growth phase (culture OD₅₅₀ between 0.5 and 1.0).
2. Discard the supernatant and re-suspend the pellet in 5 ml of cold (4 °C) extraction buffer (see Recipes).
3. Lyse the cells by passing twice through a French pressure cell (1,100 psi).
  Note: Other methods and procedures can be used to lyse microalgae; among the most common methods, there are the use of magnetic stirrer, microwave radiation, ultrasonication and enzyme treatment (Dvoretskyet al., 2016; Farooq et al., 2016; Huang et al., 2016). The breaking of the cells can be observed with an optical microscope.
4. Centrifuge cell homogenate at 11,000 x g for 15 min at 4 °C. Use the resulting supernatant as crude extract (CE) and assay it for enzyme activity.
5. Keep the crude extract on ice or freeze the sample in liquid nitrogen and store at -80 °C for future assays.
  Note: Store the samples at -80 °C for a maximum period of six months. The frozen sample can be thawed at room temperature.
OASTL assay
1. Add the following solutions as shown in Table 1 to Eppendorf tubes (1.5-2.0 ml)
  
  Table 1. OASTL protocol assay
  
  Notes:
  1. For the preparation of the reaction mix, it is important to respect the order of the solutions as shown in Table 1.
  2. The volume of Milli-Q water in the sample depends on the amount of CE used, considering a final volume of the reaction mix of 100 µl.
  3. OAS (O-acetylserine) needs to be prepared fresh before use. DTT and Na₂S can be prepared, aliquoted and stored at -20 °C for six months.
2. After preparation of the reaction mix, vortex the samples and incubate the tubes in a thermomixer at 50 °C for 5 min.
3. Stop the reaction by adding 50 µl of 20% (v/v) trichloracetic acid (TCA). Spin at 13,000 x g and transfer the supernatants to clean tubes.
  Note: TCA can be stored at 4 °C for six months.
4. Add to the tubes 100 µl of glacial acetic acid and 200 µl of ninhydrin solution (see Recipes).
5. Incubate the samples at 100 °C for 5 min and then cool them in a Thermo bath at 10 °C for 5 min.
  Note: As an alternative to Thermo bath, the samples can be incubated in a thermomixer at 99 °C.
6. Add 550 µl of ethanol, vortex and read the absorbance spectrophotometrically at 560 nm.

Data analysis

To evaluate the range of reliable activity, it is necessary to make a calibration curve for L-cysteine using known concentrations of the amino acid (0.1-3.0 mM).
Note: We estimated valid an absorbance range at 560 nm between 0.5-1.5.
Enzymatic units were calculated using the following formula:
Enzymatic units (U) = A₅₆₀V₂V₀/εt₁V₁V_e
where, A₅₆₀ is the absorbance at 560 nm; V₂ is the final volume (ml) including ethanol; V₀ is the volume of the reaction mix incubated at 50 °C; ε is the OAS molar extinction coefficient (mM^-1 cm^-1); t₁ is the time (min) of incubation at 100 °C; V₁ is the volume of the reaction mix incubated at 50 °C included TCA; V_e is the crude extract volume used for the assay (ml).
Note: We estimated ε = 6.4 mM^-1 cm^-1.
OASTL activity was expressed in units that correspond to the formation of 1 µmol of cysteine min^-1. The OASTL activity in each sample should be correlated with the soluble protein content (mg ml^-1 extract) that was determined by the Bio-Rad Protein Assay based on the Bradford method (Bradford, 1976), using bovine serum albumin as the standard. The enzymatic unit correlated with the protein content is U mg^-1protein.
Note: The use of other methods for protein determination is possible.
Data of the mean ± SE of 3-6 independent experiments should be presented (Figure 3).
Experimental data analyses and graphs could be carried out using SigmaPlot^® 12 software (Carfagna et al., 2016).
Note: The use of other software to analyze data is possible.

Figure 3. OASTL activity in Chlorella sorokiniana. Effect of 24 h sulphur-starvation on enzymatic activity (Salbitani et al., 2014).

Recipes

Basal medium
13 mM KH₂PO₄
4.3 mM K₂HPO₄
0.35 mM NaCl
1.2 mM MgSO₄
0.35 mM Fe-EDTA
0.18 mM CaCl₂
5 mM KNO₃
Oligoelements: 0.31 mM CuSO₄, 0.12 mM (NH₄)₆Mo₇O₂₄, 14 mM MnCl₂, 0.76 mM ZnSO₄, 46 mM H₃BO₃
Adjust the pH to 6.5 and sterilize by autoclaving. The medium is stored at 2-8 °C, in the dark
Phosphate buffer, pH 7.5
To prepare 50 mM phosphate buffer pH 7.5, mix 0.94 ml of 1 M KH₂PO₄ with 4.06 ml of 1 M K₂HPO₄ and add distilled water up to a final volume of 100 ml
Note: Store the buffer at 2-8 °C for three months. The pH should be measured prior to use and adjusted, if necessary.
Extraction buffer
50 mM phosphate buffer pH 7.5
10 μM PLP (pyridoxal-phosphate)
1 mM DTT (dithiothreitol)
Note: Prepare fresh before use.
Ninhydrin solution
To prepare the ninhydrin solution, dissolve 0.12 g of ninhydrin in 3 ml of glacial acetic acid and 2 ml of hydrochloric acid 37%
Note: The ninhydrin solution can be stored in the dark at room temperature for five days.
1 M HEPES solution
Dissolve 11.9 g of HEPES in Milli-Q water up to a final volume of 50 ml
Adjust the pH to 7.20 with KOH
Note: Store the buffer at 2-8 °C for three months. The pH should be measured prior to use and adjusted, if necessary.

Acknowledgments

We thank the anonymous reviewers for helpful comments. This study was financed by Regione Campania (PON-Smart Generation and LR 5/2002).

References

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