Table 4. Inhibitory effects of UDP-xylose and UDP-glucuronic acid on GuUGD1–4.
| Inhibitor | Relative enzyme activity of isoforms (%) | |||
|---|---|---|---|---|
| GuUGD1 | GuUGD2 | GuUGD3 | GuUGD4 | |
| None | 100 | 100 | 100 | 100 |
| UDP-xylose (20 µM) | 80 | 106 | 92 | 63 |
| UDP-xylose (500 µM) | n.d. | n.d. | n.d. | 7 |
| UDP-glucuronic acid (20 µM) | 104 | 111 | 109 | 89 |
| UDP-glucuronic acid (1250 µM) | 95 | 103 | 109 | 34 |
Enzyme activity was calculated based on initial velocity within 10 min by monitoring the conversion of NAD+ to NADH, detected by the absorbance at 340 nm. All inhibitors were added just before the reaction was initiated. The concentration of the substrate UDP-glucose was twice the concentration of each Km calculated in the kinetic analyses. 100% corresponds to the activity indicated in the standard assay performed without an inhibitor. n.d., not detected.