Table 3.

CTP is formed reversibly when microsomes are incubated with [³H]CDP and either F-P-P or GG-P-P

Addition to incubation mixture, 100 μM	[3H]CDP converted to [3H]CTP, pmol/mg
None	<0.1
F-P	<0.1
F-P-P	580
GG-P-P	739

Reaction mixtures contained 25 mM Tris⋅HCl (pH 7.4), tobacco microsomes (15 μg protein), 5 mM MgCl₂, 1 mM EDTA, 10 mM sodium orthovanadate, 20.8 μM [³H]CDP (480 cpm/pmol), and 100 μM F-P, F-P-P, or GG-P-P, where indicated, in a total volume of 0.01 ml. After an incubation for 15 min at 30°C, an aliquot (1 μL) was removed and applied to a polyethyleneimine-cellulose chromatogram. The chromatogram was developed with 0.3 M sodium phosphate (pH 3.5), and the radioactive zones corresponding to [³H]CDP and [³H]CTP were located by scanning with a Bioscan Imaging System. The radioactive zones were scraped from the plate and eluted with 1 ml of 1% SDS, and the amount of [³H]CDP converted to [³H]CTP was determined by scintillation spectrometry.