Pyruvate kinase from Geobacillus stearothermophilus displays an unusual preference for Mn2+ in a cycling reaction

Previously, we developed a kinase cycling method using creatine kinase and pyruvate kinase (RMPK) both from rabbit muscle in the presence of an excess amount of ATP and IDP for the quantitative determination of substrate. To our surprise, the RMPK cycling reaction was 10-fold more efficient using Mn²⁺ rather than Mg²⁺. Here, we investigated PK from Geobacillus stearothermophilus (GSPK) as an alternative source of enzyme. Spectrophotometric real-time detection was accomplished by coupling the reaction to ADP-dependent glucokinase (ADP-GK) together with glucose-6-phosphate dehydrogenase (G6PD). The rate of increase in absorbance of NADH at 340 nm was monitored. GSPK displayed an even greater preference than RMPK for Mn²⁺ over Mg²⁺ in the cycling reaction with ATP and GDP or ATP and IDP. The much lower K_m values for the substrate in the presence of Mn²⁺ rather than Mg²⁺ are consistent with the results of the cycling reaction observed in this study.

Characterization and application of a novel nicotinamide mononucleotide adenylyltransferase from Thermus thermophilus HB8

Herein, we describe a novel enzymatic cycling method to measure nicotinamide mononucleotide (NMN) or nicotinic acid mononucleotide (NaMN), which are precursors of NAD biosynthesis. A gene encoding an NMN adenylyltransferase (NMNAT, EC 2.7.7.1) homologue was identified in Thermus thermophilus HB8. The gene from T. thermophilus (TtNMNAT) was engineered for expression in Escherichia coli and the recombinant enzyme found to be stable, retaining full activity after incubation for 45 min at 70°C. The K_m values for NMN and ATP were calculated to be 0.263 and 1.27 mM, respectively, with a V_max value of 60.3 μmoL/min/mg. TtNMNAT was successfully applied to the colorimetric NMN or NaMN assays, which employed (i) adenylation of NMN to NAD by TtNMNAT or adenylation of NaMN to deamido-NAD (NaAD) by TtNMNAT followed by amidation of NaAD to NAD by NAD synthetase (NADS, EC 6.3.1.5) and (ii) an NAD cycling reaction using 12α-hydroxysteroid dehydrogenase (12α-HSD, EC 1.1.1.176) and diaphorase (DI, EC 1.6.99.3) to accumulate reduced WST-8. This enzymatic cycling method enabled detection of 0.5 μM (12.2 nM in the reaction mixture) NMN or NaMN in an automatic clinical analyzer.