Table II.
Comparison of the substrate specificity of SAM-dependent N-methyltransferases from various plant sources
| Plant | Substrate/Methylation Positiona
|
References | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| 7-mX/3N | 3-mX/1N | 1-mX/3N | Tb/1N | Tp/7N | Px/3N | X/3N | XR/7N | XMP/7N | ||
| % | ||||||||||
| Tea | ||||||||||
| Leaves (purified) | 100 | 17.6 | 4.2 | 26.8 | tr | 210 | tr | nd | nd | This study |
| Leaves (partially purified) | 100 | 14.0 | 20.0 | 21.4 | nd | 206 | – | 10.3 | – | Kato et al. (1996) |
| Leaves (crude) | 100 | tr | 4.5 | 25 | 2.5 | 250 | nd | nd | nd | Suzuki and Takahashi (1975) |
| Leaves (crude) | 100 | – | – | 20 | – | – | 16 | 56 | nd | Negishi et al. (1985) |
| Cocoa tea | ||||||||||
| Leaves (crude) | 100 | – | – | nd | – | nd | – | – | – | Ashihara et al. (1998) |
| Coffee | ||||||||||
| Endosperm (partially purified) | 100 | – | – | 185 | – | – | – | – | – | Mazzafera et al. (1994) |
| Fruits (crude) | 100 | – | 5.7 | 127 | 4.6 | 175 | nd | nd | – | Roberts and Waller (1979) |
| Leaves (crude)b | 100 | – | – | – | – | – | – | – | 104 | Schulthess et al. (1996) |
The relative activity is indicated as the percentage of the activity with 7-mX. CS activity with 7-mX (100%) in this work was 2.7 nkat mg−1 protein. nd, Not detected; tr, trace; –, not determined.
a
mX, Methylxanthine; Tb, theobromine; Tp, theophylline; Px, paraxanthine; X, xanthine; XR, xanthosine.
b
Km value of coffee leaf enzyme for 7-mX (0.4 mm) is lower than that for Tb (0.5 mm) (Mosli Waldhauser, 1997a).