Table I.

Substrate specificities of the recombinant N-methyltransferases

SubstrateProductRelative Activity
CaXMT1CaMXMT1CaMXMT2CaDXMT1
% (pmol min−1 nmol protein−1)
XMP
XR7mXR100 (98)
X
1mX
3mX
7mXTb100 (66)100 (63)1.0
PxCf5.05.3100 (42)
TbCf3.8
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.