Description
NAD Kinase (NADK, EC 2.7.1.23) is widely present in animals, plants, microorganisms, and cultured cells. It is the only known enzyme that catalyzes the phosphorylation of NAD⁺ to NADP⁺ in vivo. It can utilize ATP or inorganic polyphosphate [poly(P)] as a phosphoryl donor to catalyze the phosphorylation of NAD(H), generating NADP(H). Therefore, NADK plays a crucial role in synthesizing NADP(H) and regulating the balance between NAD(H) and NADP(H).Assay PrincipleNADK catalyzes the phosphorylation of NAD⁺ to generate NADP⁺. The generated NADP⁺ is then reduced to NADPH by Glucose-6-Phosphate Dehydrogenase (G6PDH). The rate of increase in NADPH, measured by the rise in absorbance at 340 nm, reflects the activity of NADK.Component50TStorageExtraction Buffer50 mL2-8℃Reagent 125 mL2-8℃Reagent 250 mL2-8℃Reagent 31EA-20℃Reagent 41EA-20℃Required Materials and Equipment (Not Provided)UV spectrophotometer, benchtop centrifuge, adjustable pipettes, 1 ml quartz cuvette, mortar and pestle, ice, and distilled water.Sample Preparation1.Bacteria, Cells, or Tissues:Bacteria or Cultured Cells: Collect cells by centrifugation and discard the supernatant. Add Extraction Buffer at a ratio of 1 ml per 5-10 million cells (e.g., 1 ml for 5 million cells). Sonicate on ice (20% power or 200W, pulse 3s on/10s off, repeat 30 times). Centrifuge at 8000 g, 4°C for 10 min. Collect the supernatant and keep it on ice.Tissues: Homogenize tissue on ice in Extraction Buffer at a ratio of 1:5-10 (w/v) (e.g., 0.1 g tissue in 1 ml buffer). Centrifuge the homogenate at 8000 g, 4°C for 10 min. Collect the supernatant and keep it on ice.2.Serum (or Plasma) Samples: Assay directly.Assay Procedure:1.Preheat the spectrophotometer for at least 30 min. Set wavelength to 340 nm. Zero with distilled water.2.Pre-warm Reagent 1 and Reagent 2 at 37°C (for mammalian samples) or 25°C (for other species) for at least 15 min.3.Working Solution I Preparation: Add 12 mL of Reagent 1 to the contents of Reagent 3. Mix thoroughly. Aliquot and store unused portions at -20°C. Avoid repeated freeze-thaw cycles.Working Solution II Preparation: Add 45 mL of Reagent 2 to the contents of Reagent 4. Mix thoroughly. Aliquot and store unused portions at -20°C. Avoid repeated freeze-thaw cycles.4.Assay Setup:ReagentTest Tube (µL)Control Tube (µL)Sample100100Working Solution I400Reagent 1400Mix thoroughly. Incubate at 37°C (mammalian) or 25°C (other species) for 15 min.Immediately boil for 2 min (tighten caps to prevent evaporation).Cool on ice.Centrifuge at 10,000 g, 25°C for 10 min. Collect the supernatant.5.Detection:ReagentVolume (µL)Supernatant (from step 4)200Working Solution II800Add reagents to a new tube or cuvette. Mix thoroughly after addition.Let the reaction stand at room temperature for 15 min.Measure the absorbance at 340 nm.Calculate ΔA = ATest - AControl.NADK Activity Calculation:General Parameters:VTotal (Total reaction volume for detection step) = 5 × 10⁻⁴ L (0.5 mL = 500 µL)ε (NADPH molar extinction coefficient) = 6.22 × 10³ L/mol/cmd (Cuvette light path) = 1 cmVSample (Sample volume in initial reaction) = 0.1 mL (100 µL)VSample Total (Total extraction volume) = 1 mLT (Reaction time for NADK enzyme step) = 15 minCpr (Sample protein concentration, mg/mL)W (Sample mass, g)500 (Cell/Bacteria count in millions for example calculation: 5 million)1. For Serum (Plasma):Definition: One unit of activity is defined as the amount of enzyme that generates 1 nmol of NADP⁺ per minute per ml of serum.Calculation:NADK Activity (nmol/min/ml) = [ΔA × VTotal ÷ (ε × d) × 10⁹] ÷ VSample ÷ TSimplified Formula: NADK (nmol/min/ml) = 53.59 × ΔA2. For Tissues, Bacteria, or Cells:a. Based on Sample Protein Concentration:Definition: One unit of activity is defined as the amount of enzyme that generates 1 nmol of NADP⁺ per minute per mg of protein.Calculation:NADK Activity (nmol/min/mg prot) = [ΔA × VTotal ÷ (ε × d) × 10⁹] ÷ (VSample × Cpr) ÷ TSimplified Formula: NADK (nmol/min/mg prot) = 53.59 × ΔA ÷ Cprb. Based on Sample Fresh Weight:Definition: One unit of activity is defined as the amount of enzyme that generates 1 nmol of NADP⁺ per minute per gram of fresh tissue.Calculation:NADK Activity (nmol/min/g fresh weight) = [ΔA × VTotal ÷ (ε × d) × 10⁹] ÷ (W × VSample / VSample Total) ÷ TSimplified Formula: NADK (nmol/min/g fresh weight) = 53.59 × ΔA ÷ Wc. Based on Bacterial or Cell Density:Definition: One unit of activity is defined as the amount of enzyme that generates 1 nmol of NADP⁺ per minute per 10⁴ cells.Calculation (example for 5 million cells in 1 ml extract):NADK Activity (nmol/min/10⁴ cell) = [ΔA × VTotal ÷ (ε × d) × 10⁹] ÷ (500 × VSample / VSample Total) ÷ TSimplified Formula: NADK (nmol/min/10⁴ cell) = 0.107 × ΔAPrecautionsBefore formal assay, it is essential to perform a pilot test with 2-3 samples expected to have significant differences in activity