Table I.
Substrate specificities of the recombinant N-methyltransferases
| Substrate | Product | Relative Activity
|
|||
|---|---|---|---|---|---|
| CaXMT1 | CaMXMT1 | CaMXMT2 | CaDXMT1 | ||
| % (pmol min−1 nmol protein−1) | |||||
| XMP | – | – | – | – | |
| XR | 7mXR | 100 (98) | – | – | – |
| X | – | – | – | – | |
| 1mX | – | – | – | – | |
| 3mX | – | – | – | – | |
| 7mX | Tb | – | 100 (66) | 100 (63) | 1.0 |
| Px | Cf | – | 5.0 | 5.3 | 100 (42) |
| Tb | Cf | – | – | – | 3.8 |
| Tp | – | – | – | – | |
Αll substrates were tested at a 500-μM concentration, and methyltransferase activity was determined by measuring the radioactivity of the transferred 14C-labeled methyl group from AdoMet. The relative activity of each recombinant protein with the most preferred substrate was set at 100%. The no. in parentheses indicates the observed value of each activity. Values are the averages of three independent measurements. –, Not detected; XR, xanthosine; X, xanthine; 1mX, 1-methylxanthine; 3mX, 3-methylxanthine; 7mX, 7-methylxanthine; Px, paraxanthine; Tb, theobromine; Tp, theophylline; Cf, caffeine.