Biotinyl-glucose-6-phosphate dehydrogenase preparation, kinetics, and modulation by avidin

The kinetics of free glucose-6-phosphate dehydrogenase (G-6-PDH), biotinylated G-6-PDH, and biotinylated G-6-PDH complexed with avidin were investigated. The kinetics of the free enzyme were consistent with a sequential rather than a ping-pong mechanism. The kinetics of the biotinylated enzyme were similar to that of the free enzyme, but the kinetic constants were different; theK m value for NADP was halved, whereas theK m for G-6-P decreased only slightly. In the presence of avidin, theK m of biotinylated G-6-PDH for G-6-P nearly doubled whereas theK m for NADP did not change significantly. Avidin complexed with biotinylated G-6-PDH inhibited the enzyme from acting. Based upon these reactions, it was possible to devise assays for either free biotin or free avidin using biotinylated G-6-PDH as the indicator enzyme. Concentrations of biotin between 40 and 60 mg/mL, or of 25-95 Μg/mL of avidin could be measured within 2 min through the use of biotinylated G-6-PDH.