Recombinant Mouse Heparan Sulfate 6-O-Sulfotransferase-3/HS6ST3 Activity Assay

Figure 1. Representative diagram of substrate and product migration.

1. Dilute recombinant mouse HS6ST3 (R&D Systems, Catalog # 5406-ST) to 2.67 µg/mL, 2 µg/mL,
   1.33 µg/mL, and 0.67 µg/mL in Assay Buffer 2. Dilute recombinant human Syndecan-4 (R&D Systems, Catalog # 2918-SD) to make a 1 mg/mL stock solution in PBS.
2. Combine 15 µL 1 mM PAPS, 15 µL ³⁵ S-PAPS, 150 µL 1 mg/mL rhSyndecan-4, and 45 µL deionized water (substrate mixture sufficient for ~15 reactions).
3. Start the reactions by combining 15 µL of enzyme at each dilution with 15 µL of the substrate mixture.
4. Include a 0 ng control containing 15 µL of the assay buffer and 15 µL of the substrate mixture.
5. Incubate at 37° C for 20 minutes.
6. Add 15 µL 5X SDS gel loading buffer to each reaction (stops reaction and prepares for electrophoresis). Mix.
7. Load 22.5 µL of each reaction onto a gel. Leave empty space between each sample loaded.
8. Run electrophoresis at 200 volts for 30 minutes. Note: The loading buffer should run no more than halfway into the gel.
9. Transfer the gel onto blotting paper and dry on a gel dryer for 1 hour or until completely dry.
10. Affix two autorad markers to the blotting paper containing the dried gel.
11. In a darkroom, expose the blotting paper with the dried gel to X-ray film. Place in a film cassette and expose overnight.
12. Develop the film.
13. Align the image of the autorad markers on the X-ray film with the markers on the blot.
14. Determine where the assay product, the unreacted PAPS, and the free sulfate (if any) migrated on the gel (Figure 1). For the 0 ng control, identify the empty region where the product would have migrated if any had been produced.
15. Cut out the regions in each lane of the gel that contain the product and the unreacted PAPS.
16. Place each excised region into a separate liquid scintillation vial.
17. Add 5 mL liquid scintillation fluid to each liquid scintillation vial.
18. Use a liquid scintillation counter to count the amount of ³⁵ S in each vial.
19. Enzyme Activity calculation:

    Activity (pmolse/min) =	S x	Ci	X	1
    		Ct		t

    Where: S = applied donor substrate (pmoles) = 1,000 pmoles in this protocol
    C_i = incorporated radioisotope (counts from product)
    C_t = total applied radioisotope (counts from product and unreacted PAPS)
    t = reaction time (min)
    
    Plot activity vs. enzyme (µg) for specific activity. Specific activity = slope of the line.

Final Assay Conditions/reaction

PAPS: 1,000 pmoles (the amount of 35S-PAPS in the assay is negligible)
rmHS6ST3: 0.04 µg, 0.03 µg, 0.02 µg, 0.01 µg, 0 µg
rhSyndecan-4: 0.33 mg/mL