Figure 4.

Fluorometric assay of GDP-ribosyl cyclase activity. Human chronic myelogenous leukemia K562 and K562/DOX cells (5×10⁶ cells) were incubated with 50 µM nicotinamide guanine dinucleotide for 1 h, and the fluorescence of cGDPR, the product of the enzymatic reaction, was monitored. The fluorescence intensity of the samples was converted to cGDPR concentration by comparison with cGDPR standards. K562/DOX, human chronic myelogenous leukemia K562 doxorubicin-resistant subline; GDP, guanosine diphosphate; cGDPR, cyclic GDP-ribose.