Farnesyl pyrophosphate synthase: Real-time kinetics and inhibition by nitrogen-containing bisphosphonates in a scintillation assay

A mix-and-read FlashPlate (R) (PerkinElmer, Waltham, MA) assay for the enzyme farnesyl pyrophosphate (FPP) synthase (FPPS) was developed to rapidly measure both steps in the synthesis of FPP from dimethylallyl pyrophosphate (DMAPP). The assay used either DMAPP or geranyl pyrophosphate (GPP) and [H-3]isopentenyl pyrophosphate ([H-3]IPP) as substrates, and measured the FPPS-catalyzed conversion of these into [H]FPP or [H-3]GPP by capturing the products onto a phospholipid-coated scintillating microtiter plate and monitoring the product formation in a chargecoupled device imager. The Michaelis-Menten parameters-k(cat) GPP (38/min), K-m IPP (0.6 mu M), and K-m GPP (0.7 mu M)-were consistent with previous studies using difficult phase separation techniques. The 50% inhibitory concentrations of various nitrogen-containing bisphosphonates (N-BPs) were determined and were also consistent with prior literature. Without precedent, weaker inhibition (5,mu M) of the non-N-BPs was also detected. In preincubation studies, the potency of the N-BPs, and specifically zoledronate, increased slowly over time by 100-fold. This potency shift was reversed significantly by the inclusion of GPP with zoledronate. Zoledronate was uncompetitive with respect to IPP. Thus, these studies were consistent with prior structural and thermodynamicthen-nodynami studies, and suggest a rapid formation of a lower-affinity complex between zoledronate and the GPP binding site, followed by the formation of a very tight complex of zoledronate and enzyme, which excludes further binding of GPP. Furthen-nore, one of the substrates from the first step in the catalytic cycle, DMAPP, was identified as a 1 mu M inhibitor of the second step of the catalysis, suggesting that the FPP two-step synthesis is regulated by DMAPP.