Relative reactivity of thiamine monophosphate and thiamine diphosphate upon interaction with alkaline phosphatase

Reactivity of thiamin monophosphate (TMP) as calf intestinal alkaline phosphatase substrate in model transformations is lower comparing with thiamin diphosphate (TDP) reactivity. Under these conditions alkaline phosphatase catalyzes TDP, ADP and AMP hydrolysis approximately at same rate. It was shown that TDP competes with p-nitrophenyl phosphate more effectively than TMP for the binding in the active site. At pH 8.5 and 30 degrees C Km values are as follows: (5.2 +/- 1.6) x 10(-3) M for TMP and (3.0 +/- 0.8) x 10(-4) M for TDP. Under the same conditions the Vmax/Km value for TDP hydrolysis is 53 times higher than the one for corresponding reaction of TMP. It was suggested that positively charged thiazolium ion of TMP interacts with the nearest environment at the active center and by this way reduces enzyme activity.